tracing iron-fueled microbial carbon production within the hydrothermal...

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Geochimica et Cosmochimica Acta 75 (2011) 5526–5539

Tracing iron-fueled microbial carbon production withinthe hydrothermal plume at the Loihi seamount

Sarah A. Bennett a, Roberta L. Hansman b, Alex L. Sessions b, Ko-ichi. Nakamura c,Katrina J. Edwards a,⇑

a University of Southern California, Los Angeles, CA 90089, USAb California Institute of Technology, Pasadena, CA 91125, USA

c National Institute of Advanced Industrial Sciences and Technology, Tsukuba, Ibaraki 305-8567, Japan

Received 29 November 2010; accepted in revised form 23 June 2011; available online 8 July 2011

Abstract

The Loihi hydrothermal plume provides an opportunity to investigate iron (Fe) oxidation and microbial processes in asystem that is truly Fe dominated and distinct from mid-ocean ridge spreading centers. The lack of hydrogen sulfide withinthe Loihi hydrothermal fluids and the presence of an oxygen minimum zone at this submarine volcano’s summit, results in aprolonged presence of reduced Fe within the dispersing non-buoyant plume. In this study, we have investigated the potentialfor microbial carbon fixation within the Loihi plume. We sampled for both particulate and dissolved organic carbon in hydro-thermal fluids, microbial mats growing around vents, and the dispersing plume, and carried out stable carbon isotope analysison the particulate fraction. The d13C values of the microbial mats ranged from �23& to �28&, and are distinct from those ofdeep-ocean particulate organic carbon (POC). The mats and hydrothermal fluids were also elevated in dissolved organic car-bon (DOC) compared to background seawater. Within the hydrothermal plume, DOC and POC concentrations were elevatedand the isotopic composition of POC within the plume suggests mixing between background seawater POC and a 13C-depleted hydrothermal component. The combination of both DOC and POC increasing in the dispersing plume that cannotsolely be the result of entrainment and DOC adsorption, provides strong evidence for in-situ microbial productivity bychemolithoautotrophs, including a likelihood for iron-oxidizing microorganisms.� 2011 Elsevier Ltd. All rights reserved.

1. INTRODUCTION

Deep-sea hydrothermal systems are a source of chemi-cally reduced fluids to the deep ocean (Elderfield andSchultz, 1996; German and Von Damm, 2004). As hydro-thermal fluids mix with oxygenated seawater, the resultingredox disequilibria can provide energy for substantialmicrobial primary productivity (McCollom, 2000). Due tothe elevated temperature and therefore lower density ofthe hydrothermal fluids, hydrothermal fluids rise into the

0016-7037/$ - see front matter � 2011 Elsevier Ltd. All rights reserved.

doi:10.1016/j.gca.2011.06.039

⇑ Corresponding author. Address: University of Southern Cali-fornia, Department of Biological Sciences, 3616 Trousdale Park-way, Los Angeles, CA 90089-0371, USA. Tel.: +1 213 821 4390.

E-mail address: [email protected] (K.J. Edwards).

water column, mix and become diluted with surroundingseawater, forming a hydrothermal plume. When fluids havereached the same density as the surrounding seawater, theplume advects horizontally away from the source, carriedby local currents. Plumes represent a chemically- andmicrobially-rich hydrothermal environment, which can betransported 100s of kilometers into the deep ocean (Luptonand Craig, 1981). In a typical mid-ocean ridge (MOR)hydrothermal system, elevated concentrations of H2, meth-ane, ammonium, reduced sulfur, iron (Fe) and manganeseprovide potential energy sources to support microbialgrowth (McCollom, 2000; Cowen et al., 2002; Lam et al.,2008; Dick et al., 2009; Sylvan et al., 2009).

The Loihi hydrothermal system is a less typical hydro-thermal environment. The Loihi Seamount is a submerged

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Microbial carbon production within the Loihi hydrothermal plume 5527

(summit 1000 mbsl) volcano 25 miles SE of the Big Islandof Hawaii (Karl et al., 1988). Hydrothermal venting at Loi-hi is driven by magmatic degassing, resulting in hydrother-mal fluids atypical of a basalt hosted MOR setting. Thevent fluids at Loihi are highly enriched in carbon dioxide,methane, ammonium, phosphate, Fe and manganese, andare low in pH and temperature (<100 �C) (Sedwick et al.,1992; Wheat et al., 2000; Glazer and Rouxel, 2009). Thelack of a macrobenthic population also sets Loihi apartfrom MOR hydrothermal systems and instead, the reducedchemicals support an abundant prokaryotic population,dominated by Fe oxidation (Moyer et al., 1995, 1998).

Until recently the role of Fe cycling microorganisms hasbeen overlooked in typical hydrothermal settings, in partbecause of the low energy yield associated with Fe oxida-tion (Edwards et al., 2003), but more simply because theyhave historically been considered a difficult class of micro-organisms to work with (e.g. Wolfe, 1964). Oxidation orreduction of Fe results in only 0.0004% of the calculated to-tal biomass within the hydrothermal plume (McCollom,2000; Bennett et al., 2011), with H2 and reduced sulfur pro-viding a much greater amount of metabolic energy forchemoautotrophs to utilize. However, it has been calculatedthat the large flux of Fe delivered to the Loihi hydrothermalsystem could potentially result in 20 kg of biomass per dayas a result of the large abundance of Fe-oxidizing bacteriafound at this site (Edwards et al., 2004). This is equivalentto 7.3 � 106 g of biomass per yr, which is 0.0004% of the to-tal biomass estimated for all hydrothermal plumes through-out the ocean (McCollom, 2000). Therefore as a result of Feoxidation, the predicted biomass produced just at the Loihisystem is equivalent to the fraction of biomass producedcollectively at hydrothermal systems throughout the ocean.This highlights the importance of Fe oxidation at volcanicseamounts as a source of energy to chemosynthetic pro-karyotic communities.

Microbial Fe oxidation results in the exudation of Ferich biominerals and at least 60% of Fe oxidation at Loihiis a result of Fe-oxidizing microorganisms, as can be visual-ized as thick microbial mats that line the seafloor (Karlet al., 1989; Emerson and Moyer, 2002). Fe oxidizers withinthe f-proteobacteria subclass are one of the predominantphylotypes identified at Loihi, along with members of thee-proteobacteria, relating to sulfur cycling bacteria (Moyeret al., 1995; Emerson and Moyer, 1997; Edwards et al.,2004). During colonization experiments, short term experi-ments demonstrate solely the presence of chemoautotrophicbacteria (Rassa et al., 2009) and it was hypothesized thatmicrobial complexity within long term incubations wouldbe a result of a wider array of chemoautotrophic metabolicpotential as well as the establishment of mixotrophic andheterotrophic populations.

Fe-oxidizing microorganisms are also predicted to bepresent in the hydrothermal plume overlying the LoihiSummit. The summit of Loihi intersects the oxygen mini-mum zone (Glazer and Rouxel, 2009). The lack of hydro-gen sulfide within the Loihi fluids along with the low pHand low oxygen, results in elevated concentrations of dis-solved reduced Fe within the dispersing hydrothermalplume (Malahoff et al., 2006). These high Fe(II) concentra-

tions provide an energy source for Fe-oxidizing microor-ganisms, which may result in a direct input of labileorganic carbon into the water column. Elevated Fe concen-trations as a result of venting from the Loihi seamount havebeen detected at the Hawaii Ocean Time Series station –ALOHA, 473 km NE of the Loihi Seamount (Boyleet al., 2005)

In typical MOR hydrothermal settings, Fe-oxidizingbacteria may play an important role in biogeochemical pro-cesses and the detection of biogenically formed Fe oxideprecipitates in hydrothermal plumes and seafloor depositsconfirm their presence (Karl et al., 1989; Wirsen et al.,1993). Organically complexed Fe(III) and the presence ofthermodynamically unstable Fe(II) detected within MORhydrothermal plumes (Bennett et al., 2008; Toner et al.,2009), that may be the result of Fe-oxidizing bacteria, arepredicted to have important consequences for the oceanicFe cycle (Tagliabue et al., 2010).

The Loihi Seamount therefore provides a unique loca-tion to investigate Fe-oxidizing microorganisms in a dy-namic environment, in the virtual absence of the typicalMOR plume microorganisms such as the sulfur oxidizers(Sylvan et al., 2009). In this study, we focus on organic car-bon at the Loihi seamount, both particulate and dissolved,in hydrothermal fluids, microbial mats and the dispersingplume in order to investigate microbial productivity.

2. SAMPLE COLLECTION

The samples for this study were collected in October2009 around the summit of the Loihi Seamount at approx-imately 1300 m water depth (R/V Kilo Moana (KM09-23))(Fig. 1). This was the final leg of the NSF-funded Iron-Oxidizing Microbial Observatory project (FeMO) led byEdwards, Moyer, Emerson, Tebo and Staudigel. Hydro-thermal fluids were collected from Pele’s Pit and microbialmat samples were collected from Pele’s Pit, The Pit ofDeath and along the southern slope of the Loihi mound.Further details on sample locations are described in theresults.

2.1. Sampling

2.1.1. Hydrothermal fluids

End-member hydrothermal fluids were collected with750 ml titanium samplers using the ROV Jason-II. The noz-zle of the titanium syringe was inserted directly into thevent orifice and the bottle was triggered on visualizationof fluid flow through an indicator weep-hole anterior tothe sample chamber. Shipboard treatment of the sampleswas conducted as reported in Glazer and Rouxel (2009)with the exception of DOC and POC, which is described be-low. Prior to deployment, the samplers were thoroughlywashed with ultra pure deionised water (>18.2 MX cm�1)to minimize organic contaminants. On board the ship, thetitanium syringe was placed in a beaker of ultra pure deion-ised water and the bottle was triggered, sucking up thewater and providing a procedural blank.

Fig. 1. (a) Locations of the CTD stations (white squares with numbers 1, 5 and 7) and the two Tow-Yo lines (CTD4 and CTD6) relative to theLoihi Seamount. (b) Enlarged location map of the summit indicates the locations of the mat samples.

5528 S.A. Bennett et al. / Geochimica et Cosmochimica Acta 75 (2011) 5526–5539

2.1.2. Microbial mats

Mat samples were collected using a canister constructedfrom PVC tubing that is opened and closed with a ballvalve. Sampling was carried out by ROV Jason-II by push-ing the open canister through the mat, and closing after col-lection. On board the ship, the samples were allowed tosettle and the overlying seawater was poured into acidcleaned glass bottles and processed for DOC and POC.Prior to deployment, the canisters were triple rinsed with ul-tra pure deionised water, followed by an hour long soak in70% ethanol. This was followed by a further triple rinsewith filtered ultra pure deionised water, leaving the canistersfull of ultra pure deionised water ready for deployment.The main purpose of these canisters was for microbialmat collection and the cleaning procedures reflect this.These fluids can be considered akin to ‘pore-water’ fluidscollected from sediment cores, and we believe that contam-ination from the PVC tubing (also used in push-core appa-ratus) should be minimal relative to the high organiccarbon content of the fluids. No procedural blank was pro-cessed for these canisters.

2.1.3. Hydrothermal plume

A suite of water column samples were collected by CTDrosette from the dispersing hydrothermal plume using 12-Lbottles internally sprung with silicone. The bottles weremounted in a CTD rosette consisting of a SeaBird 9/11+ sys-

tem with a Wetlabs C-Star Transmissometer, SBE43 oxygensensor and SBE 3-02/F temperature sensor. In addition, anEh sensor was attached to the bottom of the CTD frame closeto the temperature sensor. Water samples were collectedbased on real time feedback from the CTD and in particular,the in-situ transmissometer and Eh sensor, both interfacedinto the SeaBird CTD. On recovery of the CTD rosette tothe ship, water samples were collected for total dissolvableiron (TdFe), total dissolvable manganese (TdMn), dissolvediron (dFe), dissolved manganese (dMn), dissolved organiccarbon (DOC) and particulate organic carbon (POC).

Out of the six CTD hydrocast operations reported inthis paper, two consisted of a series of casts while the shipwas moving (Tow-Yo) with a sample taken from the centerof the bisected plume. One of these Tow-Yo operationspassed lateral to the Loihi summit and the other proceededdirectly towards the summit from a distance of 4 km(Fig. 1a). The three stationary CTD casts were directed intoPele’s Pit (CTD1), The Pit of Death (CTD5) and 1.6 kmSW of Pele’s Pit (CTD7). For these casts, samples were col-lected throughout the plume.

2.2. Sample processing

2.2.1. DOC and POC

The separation of DOC from POC is operationally de-fined as that which passes through a filter with a nominal


pore size of 0.7 lm. For our samples, seawater was filteredthrough pre-combusted (450 �C, >4 h) GF/F filters (47 mm,Whatman) into 40 ml I-CHEM certified low level total or-ganic carbon (TOC) vials. The seawater was either filtereddirectly from the CTD bottle or sub-sampled into glass bot-tles and then filtered. The filtered seawater was acidifiedwith 40 ll trace metal grade HCl (Fisher Scientific, storedin glass) to pH 2 and stored at 4 �C until analysis back ina land based laboratory. The GF/F filters were folded incombusted aluminum foil and stored frozen. The volumeof water filtered was recorded for future quantitativeanalysis.

2.2.2. Total and dissolved Fe and Mn

On recovery of the CTD rosette, water samples were col-lected directly from the CTD bottle into acid cleaned low-density polyethylene (LDPE) 500 ml bottles (with 3-foldrinsing) for TdFe and TdMn analysis back in a land basedlaboratory. The remaining sample was filtered under pres-sure (N2 gas) through an acid cleaned 0.2 lm membrane fil-ter (Whatman polycarbonate, 47 mm) and a portion of thefiltrate was collected in an acid-cleaned LDPE 500 ml bot-tle. These samples (both total dissolvable and dissolved)were later acidified in a land-based laboratory to pH 1.6using HCl (Fisher, OPTIMA grade).

2.3. Sample analysis

2.3.1. DOC analysis

Analysis of DOC concentration was carried out at theUniversity of California, Santa Barbara (UCSB) on a Shi-madzu TOC-V series TOC analyzer as described in Carlsonet al. (2010). All samples were systematically referencedagainst low carbon water every 6–8 analyses, with deep Sar-gasso Sea reference waters (2600 m) and surface SargassoSea water (Hansell and Carlson, 1998; Carlson et al.,2004). Daily reference waters were calibrated with DOCConsensus Reference Waters. The analytical precision was±0.9 lM (1r, n = 3) for deep reference water and±0.7 lM (1r, n = 3) for surface reference water. Three tofive replicates were carried out on each sample to determinean additional analytical precision.

2.3.2. POC and d13C analysis

Samples for POC and d13C analysis were prepared at theUniversity of Southern California and analyzed at the Cal-ifornia Institute of Technology on a Costech InstrumentsElemental Analyzer (EA) interfaced to a Finnigan-MATDelta-S isotope ratio mass spectrometer (IRMS) via a Con-flo III interface. The GF/F filters were unfolded, removedfrom the foil, placed in small polystyrene containers anddried in an oven at 60 �C. The filters were then treated withconcentrated HCl under vacuum for 24 h to remove theinorganic carbonates. The filters were again dried, quar-tered, and packaged in pre-combusted tin cups ready foranalysis. For the microbial mat samples, enough materialwas present on the filters to be scraped off and analyzedwithout the filter (20–58 lg). These samples also had a sec-ondary acid treatment due to the thickness of the material,adding 2 M HCl directly to the samples as per Kennedy

et al. (2005). Isotopic compositions were calibrated relativeto CO2 reference gas, and acetanilide was used as an exter-nal calibration standard. Values are reported in the conven-tional d13C notation relative to the VPDB standard.

Clean filters were processed throughout to provide theblank correction due to the filter and an estimate of analyt-ical precision. The average carbon blank from the tin cupsalone was 8.7 ± 0.5 lg (n = 5) and for clean filters plus tincups the average was 13.1 ± 0.7 lg (n = 4). The tin cupshad an average d13C value of �27.1 ± 0.8& (n = 5) andthe filters plus tin cups had an average value of�29.6 ± 1.0& (n = 4). The variation in the standard devia-tion between the tin cups and the tin cups + blank filters issmall and therefore we suggest an analytical precision of0.7 lg (1r) for the mass, and 1.0& (1r) for d13C. The uncer-tainty in the size of the blank is large relative to the reportedvalues; however the actual measured value on each filterwas 11.5 times more concentrated as a result of the volumeof sample passed through each filter. The accuracy of d13Cvalues from EA analyses, based on repeat analyses of anacetanilide standard, was within 0.5& of the accepted valuefor this standard (�29.1 ± 0.4& (n = 17)). Due to the lowconcentration of carbon collected on each filter, only oneset of measurements was made on each plume samplewhereas 2–3 analyses were made on mat samples. Valuesof d13C for samples were corrected for blank carbon bymass balance, and uncertainties in blank-corrected d13Cvalues were calculated by propagating uncertainties in bothd13C value and size of the analytical blank (Hayes, 1983).Samples collected for POC analysis typically use in-situ fil-tration devices that can process thousands of liters of water.However, in our study this type of equipment was not avail-able and the amount of seawater available for POC analysiswas on the order of 11.5 L. Hence, blank carbon was oftena large fraction of the carbon within our samples (8–104 lg), and the propagated uncertainties for d13C valuesof POC in our plume samples are much larger than is typ-ical, up to 10&.

2.3.3. Fe and Mn analysis

Fe and Mn analysis was carried out at the University ofSouthern California using inductively coupled plasma massspectrometry (ICP-MS) after the addition of an isotopespike and Mg(OH)2 co-precipitation as described previ-ously (Wu and Boyle, 1998; Saito and Schneider, 2006).Briefly, 13 ml aliquots of acidified seawater were pouredinto acid cleaned centrifuge tubes and weighed. Each sam-ple was spiked with 200 ll of a 1.3 lM 57Fe solution and al-lowed to equilibrate overnight. Ammonium hydroxidesolution was added to each sample to precipitate Mg(OH)2

and the co-precipitation of metals. After ammoniumhydroxide addition, the samples were first left for 90s with-out disturbance and then mixed and left for an additional180s. The samples were centrifuged for 3 min at 3000 rpmusing an Eppendorf Centrifuge, decanted, and then centri-fuged a second time and decanted. The remaining pelletwas re-dissolved in 1 ml of 5% nitric acid (Fisher, OPTIMAgrade) containing 5 ppb Indium. All samples were run intriplicate, and a certified reference material (NASS-5) wasprocessed with the samples providing an analytical preci-


sion of 0.3 nM (1r, n = 11) for Fe and 0.7 nM (1r, n = 11)for Mn. Stock 1 mg l�1 standards of Fe and Mn were di-luted in preparation for instrument calibration and the con-centration of the 57Fe spike was confirmed by running thestock solution with a 1:5 dilution. The 5% nitric acid solu-tion was run as a blank. ICP-MS measurements were madeusing medium resolution mode on a Thermo-Finnigan Ele-ment2 attached to a Peltier Cooled Cyclonic Spray Cham-ber. The scavenging efficiency of Mn was assumed to bethe same as Fe, as reported by Saito and Schneider(2006), and instrument sensitivity differences between Feand Mn were corrected for by carrying out a standard addi-tion of Mn to one of the samples. The limit of detection wascalculated as 1.7 nM for Fe and 0.15 nM for Mn (n = 5)from analysis of a 5% HNO3 blank.

3. RESULTS

3.1. Hydrothermal fluids

Organic carbon analysis was carried out on hydrother-mal fluids collected from four different sites within Pele’sPit (Fig. 1b and Table 1). The samples were collected fromPele’s Pit at the Hiolo and Spillway area. The temperatureof the fluids ranged between 42.7 and 50.7 �C and the DOCconcentrations ranged between 54.0 and 92.2 lM. The POCconcentrations were below detection limit. Process blanksfor the titanium sampler had a DOC concentration of15.9 lM.

Table 1Temperature, DOC, POC and d13C isotope composition of POC in samplLoihi Seamount.

Sample ID Location Temperature(�C)

Hydrothermal fluids within Pele’s Pit

J2-482-Red M39 steamer hole/Hiolo area 42.7J2-483-Black Between M34 and M38/Spillway area 47.4J2-483-Blue Red smoker/Spillway area 47.4J2-483-White M34/M38 chimlet/Spillway area 50.7

Microbial mats within Pele’s Pit

J2-479-SC2 M38/Spillway area 43.0–52.2J2-479-SC1 M34/M38 chimlet/Spillway 29.7J2-481-SC4 M34/Spillway area –J2-479-SC5 M36/Hiolo area 48.0J2-482-SC2 M39/Hiolo area 47.2J2-476-SC5 M48/Tower area 20.3J2-482-SC4 Upper M31/Tower area

Peak of the Loihi mound

J2-481-SC2 M2/Lohiau 3.9

Slope of the Loihi mound

J2-478-SC7 M/Naha 6.4J2-478-SC3 M17a 2.4

Within the Pit of Death

J2-476-SC1 M56/Pit of Death 7.3

bd = Below detection limit, dashes indicate that samples were not measua Standard deviation from 3 to 5 repeat analyses of the same sample.b Propagated uncertainties from blank correction (see Section 2.3.2 for

3.2. Microbial mats

A much larger range of microbial mat samples were col-lected for organic carbon analysis and included samplesfrom Pele’s Pit (Spillway, Hiolo and Tower area), The Pitof Death, the summit of the Loihi mound (Lohiau), andalong the southern slope of the mound (Naha and M17)(Fig. 1 and Table 1). Prior to sampling, the temperaturewithin the mat was measured with a temperature probe.The mat samples collected within Pele’s Pit ranged in tem-perature from 20.3 to 52.2 �C whereas the mat samples col-lected outside of Pele’s Pit ranged from 2.4 to 7.3 �C. TheDOC concentrations within the ‘pore’ fluids of the matsamples within Pele’s Pit ranged from 68.4 to 78.2 lM,and outside of the Pit from 105.5 to 179.2 lM. Microbialmats were 0.28–0.63% organic carbon (dry weight) withd13C values ranging from �23& to �28&.

3.3. Hydrothermal plume

A total of three vertical CTD casts and two Tow-Yooperations were carried out to sample the Loihi hydrother-mal plume for organic carbon. Detailed sampling was car-ried out in three vertical casts and profiles of TdFe, dFe,DOC, transmission and Eh are shown in Fig. 2 and datain Table 2. The three CTD casts took place at varying dis-tance from Pele’s Pit, with one cast deployed directly intoPele’s Pit (CTD 1), one cast 1.24 km north of Pele’s Pit intoThe Pit of Death (CTD 5) and a final cast south-west of the

es collected from the hydrothermal fluids and microbial mats at the

Depth(m)

DOC(lM)

r(lM)a

POC(%)

d13C POC(&)

r(&)b

1300 92.2 1.4 bd bd1273 60.3 0.8 bd bd1253 69.6 1.4 bd bd1273 54.0 0.7 bd bd

Pore-water DOC(lM)

TOC(%)

1273 73.9 1.3 0.51 �28 11273 74.9 0.4 0.28 �23 11272 – – 0.58 �24 11300 78.2 0.7 –1300 – – 0.51 �26 11301 – – 0.55 �25 11300 68.4 0.9 –

1175 – – 0.33 �25 1

3034 105.5 1.8 0.28 �25 11716 179.2 2.2 0.63 �26 1

1197 – – 0.46 �23 1

red for the particular component.

further details).


mound, 1.60 km from Pele’s Pit (CTD7). For all CTD castsa transmissometer and Eh sensor were used to determine thepresence of the plume in real time; decreases in transmissionindicate elevated particles within the water column and de-creases in Eh indicate the presence of chemically reducedspecies. Back in the laboratory, chemical analysis demon-

Pele's Pit (CTD1)

Transmission (%)

97.4 97.5 97.6 97.7

Dep

th (m

)

800

900

1000

1100

1200

1300

Eh (mV)

150 155 160 165 170 175 180

Fe concentration (nM)0 100 200 300 400 500

DOC concentration (μM)40 45 50 55 60 65

Pit of Death

Transmiss

97.2 97.3 97.

Eh (m

130 135 140 14

Fe concentr0 100 200

DOC concent40 45 50

TransmissionEhTdFedFe DOC

Fig. 2. Depth profiles of TdFe, dFe, DOC, transmission and Eh for the Cvalue is the direct reading of Pt electrode voltage against Ag–AgCl referenvoltage value against the standard hydrogen electrode (SHE).

Table 2TdFe, dFe, TdMn, DOC, POC and d13C isotope composition of POC inLoihi seamount.

Location (CTD) Depth(m)

TdFe(nM)

dFe(nM)

TdMn(nM)

dMn(nM)

Pele’s Pit CTD1 1300 310.0 235.9 17.9 18.11240 465.1 347.4 22.9 23.01050 105.3 61.3 5.0 5.2800d 3.6 1.8 0.4 0.3

Pit of Death CTD5(1.24 km N of Pele’s Pit)

1285 188.3 102.8 6.3 5.91265 211.4 117.6 6.7 6.51175 229.3 132.6 6.8 6.81075 138.2 91.9 5.0 5.0

1.6 km SW of Pele’s PitCTD7

1250 – – –1229 233.7 183.0 9.0 9.11140 – – –1090 93.6 79.3 5.0 4.9982 – – –

South of Loihi SummitCTD3

1003d 3.8 1.5 2.2 2.11072d

bd = Below detection limit, dashes indicate that samples were not measua Fe/Mn ratio (both in the Td fraction).b Calculated as (1-(dFe/TdFe)) � 100.c POC has been blank corrected (see Section 2.3.2).d Background seawater samples.e Standard deviation from 3 to 5 repeat analyses of the same sample.f Propagated uncertainties from blank correction (see Section 2.3.2 forg Samples not considered in the discussion – see Sections 3.3 and 4.3.2

strated elevated Fe and Mn concentrations (Table 2), whichconfirmed that hydrothermally influenced water had beensampled. Dissolved Mn concentrations slightly higher thanTdMn (max 0.2 nM) were measured, and are interpretedas the result of slight contamination during filtering. Thedissolved organic carbon concentrations within the plume

(CTD5)

ion (%)

4 97.5 97.6

V)

5 150 155 160

ation (nM)300 400 500

ration (μM)55 60 65

1.5 km SW of Pele's Pit (CTD7)

Transmission (%)

97.2 97.3 97.4 97.5 97.6

Eh (mV)

110 115 120 125 130 135 140

Fe concentration (nM)0 100 200 300 400 500

DOC concentration (μM)40 45 50 55 60 65

TD stations that intercepted the hydrothermal plume at Loihi. Ehce electrode in saturated KCl solution and is not converted to the Pt

samples collected from the dispersing hydrothermal plume at the

Fe/Mna

%pFeb DOC(lM)

re

(lM)POC c

(lg/L)d13CPOCc (&)

rf

(&)

17.3 23.9 48.5 0.5 1.8 �22 620.3 25.3 51.1 0.5 1.9 �25 521.1 41.8 – bd bd9.0 50.0 40.8 0.1 0.7 �20 10

29.9 45.4 41.8 0.6 2.4 �28 431.6 44.4 55.3 1.0 1.6 �24 433.7 42.2 58.8 0.6 bd bd27.6 33.5 50.0 0.4 bd bd

56.9 0.2 2.1 �27 326.0 21.7 59.0 0.3 1.9g �16g

60.3 1.0 9.0g �28g

18.7 15.3 54.3 0.8 bd bd45.7 0.1 bd bd

1.7 60.5 40.4 0.8 bd bd39.2 0.3 1.2 – –

red for the particular component.

further details)..


ranged between 41.8 and 60.3 lM, as compared to the back-ground values of 40.1 ± 0.8 lM (1r calculated as a result ofnatural variation). Background samples were measuredsouth of the Loihi Summit at 1003 and 1072 m (CTD 3)and above the plume at 800 m (CTD 1) (Table 1). Particu-late organic carbon concentrations ranged between 1.2and 9.0 lg L�1 and had d13C values ranging from �16&

to �28& (Table 2). This compares to background POC val-

Fig. 3. Tow-Yo profiles of the dispersing hydrothermal plume at Loihi aswater column increased i.e. within the plume, the transmission signal decron the map in Fig. 1. (a) Tow-Yo heading towards the summit with 0 dsummit with 0 distance at the most southern point of the Tow-Yo. The lindicated with black diamonds and the location of CTD 7 is indicated wiTow-Yo operations crossed.

Table 3TdFe, dFe, TdMn, DOC, POC and d13C isotope composition of POC inLoihi seamount.

Location (CTD) ID Depth(m)

Distance fromPPa (km)

TdFe(nM)

dFe(nM

Tow-Yo-Lateral to theSummit (CTD4)

1 1127 2.27 163.9 77.02 1200 2.37 186.8 78.13 1168 2.71 107.6 49.9

Tow-Yo-into thesummit (CTD6)

4 1153 1.26 226.3 185.45 1245 3.97 58.3 22.8

a PP = Pele’s Pit. The distance was calculated using the latitude and lob POC concentrations have been blank corrected (see Section 2.3.2).c Standard deviation from 3 to 5 repeat analyses of the same sample.d Propagated uncertainties from blank correction (see Section 2.3.2 for

ues of 0.7–1.2 lg L�1 and a d13C value of �20&. Thesample with an isotope composition enriched in 13C(d13C = �16&) relative to the background seawater, islikely a result of carbonate contamination and therefore nei-ther the mass nor the isotope composition are reliable. Thissample will not be considered in any further discussion.

Samples located even further from the Loihi summit werecollected during two Tow-Yo operations, and the plume

detected with a transmissometer sensor. As the particles within theeased. The distances indicate the length of the Tow-Yo as indicatedistance closest to Pele’s Pit and (b) Tow-Yo heading lateral to theocation of the samples collected during the Tow-Yo operations areth black squares. The asterisk indicates where the paths of the two

samples collected from the dispersing hydrothermal plume at the

)TdMn(nM)

dMn(nM)

DOC(lM)

rc

(lM)POCb

(lg/L)d13CPOCb (&)

rd

(&)

7.4 7.4 47.8 0.8 0.8 �22 96.0 6.2 44.7 0.6 1.5 �22 54.3 4.2 44.5 0.9

9.3 9.4 46.2 0.7 0.9 �21 92.3 2.3 44.1 0.9

ngitude of the CTD cast relative to the center of Pele’s Pit.

further details).


profiles are shown in Fig. 3 and data in Table 3. The Tow-Yothat proceeded directly towards the Loihi summit is shown inFig. 3a, with the X-axis indicating the distance along theTow-Yo line as indicated on the map in Fig. 1. The Tow-Yo, which passed lateral to the Loihi summit, is shown inFig. 3b, with distance covered shown South (0 km) to North(�4 km). The transmission signal i.e. particle anomalies, de-tected in the water column are plotted relative to depth as anisosurface plot. The paths of each of these Tow-Yo opera-tions crossed, and the point at which this occurred is indi-cated with an asterisk on each figure. The location of thesamples collected during the Tow-Yo operations are shownwith black diamonds, and the location of CTD 7 is indicatedon the plot by black squares. The Tow-Yo operation at thesummit took place on a different day to CTD 7 and thereforethe plume signals on the Tow-Yo plot do not represent theconditions during the CTD 7 cast. The location is indicatedfor reference, demonstrating the dynamic nature of plumedispersal and changes on a day-to-day basis.

During Tow-Yo operations, an individual sample wascollected during a chosen ‘upcast’ from within the core ofthe plume as determined by the transmissometer (or Eh sen-sor closer to the source). Limited sampling resulted from thelength of each Tow-Yo operation (>12 h) and the number ofbottles on the rosettes that were available for this study.

4. DISCUSSION

4.1. Hydrothermal fluids

Iron and manganese data for the hydrothermal fluids col-lected during this research cruise will be reported elsewhere,however, the temperature of the fluids did fall within thesame range as those collected in 2006 and 2008 (Glazer andRouxel, 2009) and suggest that venting at Loihi remains inits ‘steady state’ (Wheat et al., 2000; Malahoff et al., 2006).Glazer and Rouxel (2009) reported Fe concentrations withinend-member fluids collected from Loihi between 2006 and2008 to range between 234 and 727 lM, and Mn concentra-tions to range between 8.1 and 25.8 lM (Fe/Mn ratio be-tween 20 and 40). In previous years, venting of hydrothermal fluids was also present at the Lohiau and the Pohakusites (shown on Fig. 1). Glazer and Rouxel (2009) reportedthe fluids to be cooler (up to 25 �C) with lower Fe concentra-tions at Lohiau (resulting in lower Fe/Mn ratios of 7–20) buthigher Fe concentration at Pohaku (Fe/Mn ratio of 40–45).Venting at Lohiau was mostly inactive between 2007 and2008 and therefore not sampled in 2009. Fluids within Pele’sPit had small amounts of free sulfide and Fe sulfide com-plexes whereas fluids collected outside of the Pit did not.

Particulate organic carbon within hydrothermal fluidsfrom Pele’s Pit were less than 3 lg/L, the detection limit asa result of the limited sample availability (500 mL). In com-parison, elevated concentrations of DOC (54.0–92.2 lM)were detected. DOC has been found to be elevated in end-member fluids from sediment hosted or ultramafic systemswhereas bare-rock basalt hosted hydrothermal systems aredepleted in DOC (�15 lM, (Pearson et al., 2005; Cruseand Seewald, 2006; Lang et al., 2006). The hydrothermal flu-ids at Loihi seep out of cracks within the seafloor, are much

cooler (50 �C) and are surrounded by thick microbial mats.Any DOC released from the surrounding microbial commu-nities could easily be entrained into these fluids only a fewcentimeters below the surface and would remain stable inthe cool temperatures of these fluids. Additionally, anymicrobial communities present in the deep-biosphere mayprovide a source of labile organic carbon to these fluids(D’Hondt et al., 2004; Edwards et al., 2004; McCarthyet al., 2011).

From an organic point of view the fluids at Loihi shouldbe considered more as a diffuse hydrothermal setting, butthe high Fe concentrations sets Loihi apart from MOR dif-fuse fluids. Diffuse fluids from typical MOR settings haveelevated DOC concentrations (39–69 lM) but their Fe con-centrations are low (0.3–277 lM (Von Damm and Lilley,2004; Lang et al., 2006).

4.2. Microbial mats

Total organic carbon (TOC) makes up less than 1% ofthe total dry weight of the microbial mats at the Loihi ventsites (Table 2). This is high compared to typical organic car-bon contents in deep-sea sediments (e.g. average 0.15% or-ganic carbon in sediments from the Western EquatorialPacific (DSDP 7, (Gealy, 1971)) and reflects the elevatedmicrobial biomass in these mats. Within this TOC fractionare the microbial cells that carry out chemoautotrophy,acquiring their energy from reduced species within thehydrothermal fluids. Both autotrophic and heterotrophicmicroorganisms are expected in our mat samples.

Autotrophic microbial communities fix CO2 using differ-ent carbon fixation pathways, potentially resulting in adistinct C isotope signature in the mats compared to thatin the deep ocean. For example, some Fe-oxidizing micro-organisms have been reported to utilize the Calvin cycleto fix CO2 (Davis et al., 2010; Singer et al., 2010), whereassulfur oxidizers of the e-proteobacteria utilize the reductivetricarboxylic acid cycle (TCA cycle) (Hugler et al., 2005).The isotopic fractionation resulting from these two carbonfixation pathways differs, with a greater fractionation ob-served during the Calvin cycle (10–22&) compared to theTCA cycle (4–16&) (Conway et al., 1994; Hayes, 2001;House et al., 2003).

The isotopic compositions of fixed carbon will also de-pend on the composition of source CO2. Within Loihihydrothermal fluids, Sedwick et al. (1994) reported thed13C values of DIC extracted from gas-tight samples tobe �5.5 to �1.7&, compared to a typical background sea-water value of �0&. Moreover, the isotopic composition ofdissolved CO2 (the substrate for autotrophic carbon fixa-tion) will depend on DIC speciation, which in turn dependson the temperature and pH of the fluids and are largely var-iable in the hydrothermal system. Thus our expectation wasthat carbon fixed by microorganisms within the hydrother-mal system (both vents and plume) should be quite variablein isotopic composition.

In this study, the d13C values of TOC from the mats ran-ged between �23 and �28&, with a mean of �25 ± 1.5&

(1r, n = 9). This compares to the deep ocean, where d13Cvalues of POC range between �20 and �22& (Druffel


et al., 1992; Bauer, 2002), reflecting primarily photosyn-thetic carbon fixation by algae (Pardue et al., 1976; Michen-er and Schell, 1994). Within marine sediments, the isotopiccomposition of carbon is dependent on the amount of mar-ine and terrestrial derived organic matter, as terrestrial or-ganic matter is substantially depleted in 13C (estimatedmean value of �27&) (Bickert, 2000). However, in ourmicrobial mat samples it is unlikely that terrestrial inputsare present because the organic carbon concentrations andisotopic compositions in the water column either side ofhydrothermal plume waters sampled during this study aretypical of marine derived organic carbon (DOC = 40.1 ±0.8 lM, PO13C = �20&). Thus it can be assumed that theorganic carbon within the microbial mats is a result of theassociated microbial communities and algal fallout fromthe water column. The percent of organic carbon in the matsis greater than typical deep ocean sediments from this region(Gealy, 1971), suggesting that at least 50% of the organiccarbon in the mats is from associated microbial communi-ties. The isotope composition of our mat samples suggeststhat the associated microbial communities have an isotopiccomposition depleted in 13C relative to the POC in the deepocean.

The ‘pore-water’ fluids for the microbial mat sampleswere rich in DOC, ranging from 68.4 to 179.2 lM com-pared to 40.1 ± 0.8 lM in the surrounding deep waters.This is not unusual, as pore-water DOC in many marinesediments is often elevated by an order of magnitude overbottom water values (Burdige, 2006). In our microbialmat samples, the DOC fraction is likely the result of an ac-tive microbial community, with the DOC fraction resultingfrom extracellular release from bacteria, grazer mediatedrelease and excretion, release via cell lysis (from virusesand bacteria), solubilization of particles, and bacterialtransformation and release (Carlson, 2002). The additionalDOC in these samples, relative to the background DOC,provides a 28–139 lM increase in organic carbon. This rep-resents a source of labile organic carbon that will be en-trained into hydrothermal fluids that flow through themats. These fluids will then rise up into the water column,providing a source of dissolved organic carbon to thehydrothermal plume.

At the Loihi summit, a hydrothermal plume is dispersedby the deep ocean currents and has been detected up to2000 km west of the Loihi Seamount (Lupton, 1996). Thehydrothermal plume has the potential to transport a chem-ically and biologically rich package of seawater out into thedeep ocean. Therefore the distinct isotopic composition ofthe microbial mats (�25&) provides us with a baseline va-lue for a hydrothermal contribution to the plume that canhelp us interpret the isotope composition of carbon founddownstream from the vents.

4.3. Hydrothermal plume

The three vertical CTD casts and two Tow-Yo opera-tions conducted during this study intercepted hydrother-mally-influenced seawater. Due to limited time, we choseto follow the plume to the west of the summit with Tow-Yo operations, as well as carry out three individual casts,

two of which were located at the summit of the Loihimound and one to the west. During the two Tow-Yo oper-ations the plume was detected up to 4 km SW of the summitand 2.7 km NW of Pele’s Pit.

For both CTD casts deployed within Pele’s Pit or The Pitof Death, the transmission and Eh signal from the in-situsensors remained anomalous even at the bottom of the cast.Hydrothermal plume dispersal at Loihi is also very differentfrom a typical mid-ocean ridge system. Within Pele’s Pit, thehydrothermal fluids seep out of cracks within the crater andthe Pit fills up with hydrothermally influenced seawater.This fluid spills out over the edges of the crater as a meansof dispersal. The Fe/Mn ratios of the seawater within Pele’sPit (17.3–21.1) are on the low end compared to those mea-sured in the hydrothermal end-member fluids within Pele’sPit (20–40), and may be a result of Fe oxidation and there-fore fallout of Fe from the plume. As Mn can be treated as aconservative tracer in the near field plume, supported by Mnconcentrations within the dissolved and total dissolvablefraction having similar value, any decrease in the Fe/Mn islikely a result of Fe precipitation. In addition to Fe loss,there did appear to be a decreasing trend in the Fe/Mn ratiofrom 2006 to 2008 and therefore the end-member fluids in2009 may have had a slightly lower Fe/Mn ratio (Glazerand Rouxel, 2009).

In comparison, there is no known hydrothermal ventingat the Pit of Death and therefore it is more difficult to ex-plain the anomalous transmission signal at the bottom ofthe Pit. One would expect hydrothermal plume signals fromPele’s Pit to spill over into the Pit of Death but not to thebase of the Pit. As expected the signals from the transmis-someter and the Eh sensor are not as large in the Pit ofDeath compared to Pele’s Pit, and the Fe and Mn concen-trations are lower. However, the Fe/Mn ratios in the seawa-ter within the Pit of Death (27.6–33.7) are higher than thosemeasured in Pele’s Pit (17.3–21.1), as well as the percent ofFe in the particulate phase (45% compared to 25% in Pele’sPit, Table 2). This could be a result of particle re-suspensionduring ROV operations, because as soon as the ROVtouches the seafloor, the flocculent mat is disturbed andrises up into the water column. This would therefore in-crease the Fe/Mn ratio and increase the fraction of pFein the water column. CTD 5 (Pit of Death) followed directlyon from an ROV operation into Pele’s Pit. Therefore Fe re-suspended from Pele’s Pit and then detected in the Pit ofDeath will need to have dispersed 1.24 km. If currents atthis time were flowing in the right direction, this would takeapproximately 3.5 h (using a typical deep-ocean currentspeed of 10 cm s�1) and therefore the detection of re-sus-pended material from Pele’s Pit would begin to drop off�4 h after the ROV left the seafloor. The CTD cast tookplace within 2 h of ROV recovery and therefore plume sam-ples likely included re-suspended material from Pele’s Pit.

The transmissometer signal in the CTD cast 1.5 km SW ofthe summit (CTD 7) displays more typical plume character-istics, with two distinct particle anomalies and backgroundseawater signals either side of the plume. The two plume sig-nals can be identified in the CTD cast at Pele’s Pit (CTD 1),with peaks in the Eh and transmission signal detected aroundsimilar depths to those detected at CTD 7. The Fe/Mn ratios


within the plume are 18.7 and 26, similar to those measured inthe Pele’s Pit plume as well as having a similar percent of pFe(22%). This suggests that entrainment of re-suspended parti-cles was not an issue in this cast. The continued presence ofanomalies in the Eh signal suggests the presence of reducedspecies such as Fe(II).

4.3.1. DOC within the hydrothermal plume

The DOC concentrations within Pele’s Pit are elevatedover background DOC concentrations (48.5 and51.1 lM), most likely due to the entrainment of pore-waterfluids from the microbial mats and the hydrothermal pointsources. Away from Pele’s Pit, DOC within the water col-umn would be expected to decrease as a result of dilutionwith the surrounding seawater. However, the DOC concen-trations within Pit of Death and 1.5 km SW of Pele’s Pit areeven greater in value than those at Pele’s Pit.

It is unlikely that this is solely the result of re-suspensionduring ROV dives. The ROV dive to The Pit of Death oc-curred 5 days prior to CTD 5 (Pit of Death) and at the bot-tom of cast 5 the DOC concentration was the same asbackground seawater (40.1 ± 0.8 lM), whereas only 20 mshallower the DOC concentration was 55.3 lM. This back-ground equivalent concentration of DOC at the bottom ofPit of Death provides evidence that re-suspension of matmaterial from the bottom of the Pit, 5 days earlier did notaffect the DOC concentrations within our plume.

Although the ROV dive five days earlier in The Pit ofDeath probably did not affect the chemical compositionof the plume, the ROV dive into Pele’s Pit, which tookplace immediately prior to CTD 5, may have affected sam-ples collected during that cast. Re-suspension during thedive followed by dispersal may have added Fe, Mn, POCand DOC to the plume at The Pit of Death sampled duringCTD 5. We have already suggested above that the elevatedFe/Mn ratios within the plume at the Pit of Death are likelya result of re-suspended particulate Fe occurring during thisROV dive. Re-suspended DOC would have a maximumconcentration of �73.8 lM (average measured concentra-tion within the mats at Pele’s Pit – Table 1), but duringdispersal this organic carbon would dilute with the sur-rounding seawater. By using Mn as a conservative tracerof plume dilution, we can see that from Pele’s Pit (max[Mn] measured prior to re-suspension = 22.9 nM), to Pitof Death (max [Mn] = 6.8 nM), a minimum 70% dilutionhas occurred with the surrounding seawater. Dilutionmay have been even larger if re-suspension resulted in an in-crease in Mn within Pele’s Pit as well. Therefore, at a max-imum, if we assume an average re-suspension of 73.8 lM ofDOC into Pele’s Pit, followed by a 70% dilution with sea-water (40.1 ± 0.8 lM), we would expect a maximumDOC concentration within the Pit of Death plume to be50.2 lM, 5–8 lM less than that measured.

The concentrations of DOC within the plume 1.5 kmSW of Pele’s Pit were higher still, with a maximum of60.3 lM. Within this plume, a 60% dilution has occurredwith the surrounding seawater (Max [Mn] = 22.9 nM).Therefore re-suspended material from the Loihi summitcould only account for 53.6 lM of DOC within this plume,resulting in an addition of �7 lM. Also Fe entrainment is

not indicated by the Fe/Mn ratios in these samples (seeabove) and therefore is an unlikely explanation for the ex-cess DOC. These elevated DOC concentrations are in starkcontrast to those measured in plumes from a more typicalhydrothermal system. On the East Pacific Rise the DOCconcentrations within the hydrothermal plumes, even20 m up in the buoyant plume, were within the range ofbackground seawater (Bennett et al., 2011).

Hydrothermal plume samples were also collected duringTow-Yo operations and represent more distal points in theplume (Table 3 and Fig. 3). The DOC concentrations atthese more distal stations (44.1–47.8 lM) are slightly ele-vated compared to background seawater (40.1 ± 0.8 lM)and we can compare these data with the three verticalCTD casts by plotting DOC data vs Mn concentration(Fig. 4a). Such a plot can be used to determine whetherDOC is sourced solely from the end-member fluids andmats, or if there are any additional DOC inputs to theplume. With the plume within Pele’s Pit representing thehigh DOC:Mn end-member and background seawater rep-resenting the dilution fluid, we calculated a linear dilutionrange from 22.9 nM Mn to 0.5 nM Mn, with DOC decreas-ing from 51.1 ± 0.5 lM to 40.1 ± 0.8 lM (Fig. 4a). All thesamples collected in Pele’s Pit (CTD 1), fall within thisrange. However, the rest of the samples, bar one samplefrom the Pit of Death, lie above this range, implying thatthere must be an additional source of DOC to thesesamples.

Even though some entrainment of re-suspended materialis likely in these samples, it cannot fully explain the excessDOC in our plume samples. Unlike Fe and Mn, DOCcan be produced and consumed via biological activity andtherefore these plume samples provide evidence that amicrobial community exists in the plume and that produc-tion of organic carbon is greater than mixo- or heterotro-phic consumption. Direct detection of microbial biomasscan be achieved by analyzing the particulate fraction(>0.7 lm) because this is where the majority of cells will ex-ist. The next section will carry out a more detailed look atthe POC fraction within the plume to evaluate cellularproductivity.

4.3.2. POC within the hydrothermal plume

The concentrations of POC within the hydrothermalplume at Loihi are low ranging from 0.7 to 9.0 lg/L (Table2). However, the value at 9.0 lg/L is much higher than thenext highest value (2.4 lg/L) and suggests an additionalsource of carbon to this sample that is not representativeof the plume. There is no clear explanation for this anom-alous value and therefore we will not consider this sampleany further. We have plotted POC vs Mn to determineany dilution characteristics and we have calculated a dilu-tion range using the maximum POC and Mn concentra-tions measured in the plume at Pele’s Pit (POC = 1.9 ±0.4 lg/L) diluting with background seawater (POC = 0.7–1.2 lg/L) (Fig. 4b). The calculated dilution range demon-strates scatter within our data, with samples from Pele’sPit lying within the dilution range. Two samples lie just be-low the lower boundary and three samples lie above theupper boundary. Particle fallout from the plume may ex-

TdMn (nM)

0 5 10 15 20 25

DO

C (μ

M)

40

45

50

55

60Pele's Pit (CTD 1)Pit of Death (CTD 5)1.6 km SW of PP (CTD 7)TowYo (CTD 4)TowYo (CTD 6)

Dilution range

TdMn (nM)0 5 10 15 20 25

POC

(μg/

L)

0.0

0.5

1.0

1.5

2.0

2.5

3.0(a) (b)

Fig. 4. (a) Relationship between DOC and TdMn in the hydrothermal plume. The dilution range has been calculated by assumingconservative mixing between plume water at Pele’s Pit ([Mn] = 22.9 nM, [DOC] = 51.1 ± 0.5 lM) and background seawater ([Mn] = 0.5 nM,[DOC] = 40.1 ± 0.8 lM). Only two samples are shown for CTD 7 as the DOC values had no complementary Mn data. (b) Relationshipbetween POC and TdMn in the hydrothermal plume. The dilution range has been calculated by assuming conservative mixing between plumewater at Pele’s Pit ([Mn] = 22.9 nM, [POC] = 1.9 ± 0.4 lg/L) and background seawater ([Mn] = 0.5 nM, [POC] = 0.7–1.2 lg/L).

0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6-35

-30

-25

-20

-15

-10

-5

r = 0.78y intercept = -40 ± 11‰

2

1/[C] (μg/L)-1

POC

C

(

δ13‰

)

Pele's Pit (CTD 1)Pit of Death (CTD 5)1.6 km SW of PP (CTD 7)TowYo (CTD 4)TowYo (CTD 6)

Fig. 5. The isotopic composition of POC in the hydrothermalplume relative to the reciprocal of carbon concentration. Error barsdemonstrate the propagated uncertainties from blank correction(see Section 2.3.2 for further details). The errors are large becauseof the carbon contribution from the GF/F filters and lowconcentration of carbon collected from the hydrothermal plume(See methods). The y-intercept of a linear fit (r2 = 0.78) is�40 ± 11& (error propagated using the Monte Carlo method).More POC data appears in Fig. 5 compared to Fig. 4b because all[POC] data had complimentary isotope data.


plain the lower than expected POC concentrations detectedin samples below the lower dilution boundary, whereassamples lying above the upper dilution boundary can be ex-plained by an addition of carbon to the plume.

In a study carried out at the East Pacific Rise vents at9�500N, POC concentrations in the dispersing plume rangedfrom 0.1 to 5.5 lg/L with dilution characteristics suggestingan additional source of POC to the plume during dispersal(Bennett et al., 2011). These elevated POC values wereattributed to adsorption of DOC onto Fe oxyhydroxideparticles and/or in-situ microbial carbon production. Bothsuch processes may be occurring in the Loihi plume alongwith entrainment of material re-suspended during ROVoperations. It is of note, those sites with elevated DOCconcentrations in the plume at the Pit of Death and1.6 km SW of the Loihi summit, also have elevated POCconcentrations.

The d13C values of POC ranged between �20 and�28&. In the far field plume samples (CTD4 and 6) andin one sample taken higher up in the water column at800 m (CTD1), the concentrations were low and had d13Cvalues between �20& and �22&, typical of the deepocean. The most distal plume sample collected and success-fully measured for its carbon isotopic composition was2.4 km away from Pele’s Pit (CTD 4, sample 2) and had ad13C value of �22&. This sample could therefore not bedistinguished from background seawater. This suggests that13C isotopes are useful in hydrothermal plumes close to thesource, but farther from the source detection is limited. Allthe rest of the plume samples had isotope compositions de-pleted in 13C compared to the deep-ocean.

In Fig. 5 the d13C values are plotted relative to the con-centration of organic carbon (as 1/[C]). The deep-oceanlimits of POC concentration and isotopic composition areindicated with dashed lines. The samples with ‘background’POC concentrations lie within these ranges whereas the restof the samples lie within the lower left hand quadrant.Uncertainties in the data are too large to say with confi-dence whether the relationship between d13C and 1/[C] is

linear, as would be expected from 2-component mixturecomprising background seawater POC and a 13C-depletedhydrothermal component. Nevertheless, if a 2-componentmixture is assumed, the y-intercept of a linear fit(r2 = 0.78) predicts the 13C value of the hydrothermal com-ponent as �40 ± 11& (error propagated using the MonteCarlo method). In comparison, the microbial mats sampledat Loihi had an isotope composition ranging from �23& to�28&, more enriched in 13C than the predicted end-mem-ber for the plume.


POC within these plume samples may have been en-trained from mats, adsorbed from DOC onto Fe-oxyhy-droxide particles and/or produced in-situ throughmicrobial processes. Carbon entrained from the mats wouldresult in more 13C enriched POC than that measured in theplume and adsorption of DOC onto the particulate phases isunlikely to result in isotope fractionation (Slater et al.,2000). Therefore in-situ microbial productivity may be thebest explanation for the depletion of 13C within these sam-ples. Due to the dynamic nature of the hydrothermal plumeenvironment, one might predict the plume to be dominatedby autotrophic rather than heterotrophic bacteria, as seenduring colonization experiments (Rassa et al. 2009). Addi-tionally, we would not expect e-proteobacteria, which utilizethe TCA cycle to fix carbon, to be present due to the absenceof sulfur within the plume. This compares to the microbialmats, where heterotrophs and e-proteobacteria have beendetected. Therefore, differences in d13C values between theplume and mats are presumed to arise because of differentcommunities of microbes in each setting, which fractionatethe isotopic composition of carbon to varying extents.

4.4. In-situ carbon production in hydrothermal plumes

We suggest that the combination of both DOC and POCincreasing in the dispersing plume that cannot be solely theresult of entrainment and DOC adsorption, is best ex-plained by in-situ carbon fixation. In a more typical hydro-thermal system, DOC concentrations within the plumes arenot similarly elevated (9�500 EPR, (Bennett et al., 2011)).This suggests that variations in the chemistry of the plumeare important in controlling the microbial metabolic activ-ities. At basalt hosted, mid-ocean ridge environments, sul-fide and hydrogen oxidation is considered to be importantin the buoyant or early non-buoyant plume, whereas met-hanotrophy and ammonia oxidation are more likely to beimportant in non-buoyant plumes (Lam et al., 2008). AtLoihi, the lack of elevated H2 in the end-member fluidsand H2S in the plume suggests that microbial communitiesrequiring these metabolic pathways are unlikely. In com-parison, the persistence of a large concentration of reducedFe within the plume suggests that microbial Fe oxidationcould play an important role. However, it is unlikely thatFe oxidation accounts for all of the primary productivity.Even in the absence of significant S and H2 oxidation, otherautotrophic pathways are possible. For example, thepresence of elevated concentrations of ammonium in theplume suggests the potential for ammonium oxidizers tobe present (Lam et al., 2004; Sylvan et al., 2010).

It has recently been recognized that the shallow subsur-face ocean crust is likely to be home to significant numbersof Fe-oxidizing microorganisms (Bach and Edwards, 2003;Edwards et al., 2004; Orcutt et al., 2011). Residing in thepermeable, basaltic ridge flanks, Fe oxidizers can harnessthe energy released during chemical reactions betweenseawater and rocks. However, the inaccessibility of thisenvironment makes it difficult to assess the potential bio-geochemical effects of the deep biosphere on open oceanchemistry (Deming and Baross, 1993). The Loihi hydro-thermal plume system presents us with a relatively simple

and accessible environment in which to study Fe-oxidizingmicroorganisms and hydrothermal processes in detail, andour results suggest that during microbial oxidation, a largeamount of dissolved organic carbon is released. Approxi-mately 100 times greater concentration of carbon is releasedas DOC relative to microbial biomass within the Loihiplume, based on the measurements made in this study.Therefore DOC released as a result of Fe-oxidizing micro-organisms within the deep biosphere could either support aheterotrophic community within this environment, or be re-leased to the open ocean and provide a previously unrecog-nized, labile DOC source to the deep ocean (McCarthyet al., 2011).

5. SUMMARY

In this study, we have investigated the microbial produc-tion of organic carbon within the Loihi hydrothermalplume that may occur as a result of the activities of auto-trophic microorganisms, including Fe-oxidizing bacteria.Within the plume, the isotopic composition of POC demon-strates mixing between background seawater POC and a13C-depleted hydrothermal component. This data com-bined with elevated DOC and POC concentrations, suggestin-situ microbial production and the release of dissolved or-ganic carbon to the water column. The large release ofDOC from the Loihi seamount demonstrates a previouslyunrecognized source of organic carbon to the water columnthat may have wider implications, depending on the fre-quency of magmatically driven hydrothermal circulationand the consequential chemical environment.

ACKNOWLEDGEMENTS

We would like to thank the Associate editor, Tom McCollomand three anonymous reviewers, whose comments significantly im-proved this manuscript. We thank the captain, crew and technicalsupport on board the Kilo Moana, KM09-23 and the ROV pilotsof Jason-II, who contributed to the success of this work. We thankOlivier Rouxel, Jason Sylvan and Deb Jaisi who assisted in CTDoperations and Olivier Rouxel and Rick Davis for on board pro-cessing of vent fluid and mat samples. We thank Craig Carlsonfor DOC measurements and Jim Moffett for access to his cleanroom facilities at USC. This work was supported by the NSFMicrobial Observatories Program (MCB 0653265), the MooreFoundation and NSF-OCE 0648287. This is the Center for DarkEnergy Biosphere Investigations contribution #106.

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