Time of exposure and light intensity modify the photosynthetic and

calcification response of coralline algae to ocean acidification (OA)

Laura Sordo*, Rui Santos, Isabel Barrote, João Silva CCMar – Centre of Marine Sciences, University of Algarve, Gambelas, 8005-139 Faro, Portugal

Figure 1. Study area in SE

Portugal (a), Armação de

Pêra (b). Natural mäerl beds

at 20-25 m depth (c).

Phymatolithon sp. (d) Source: http://t4vistamar.blogspot.pt/2010/03/mapa-de-armacao-de-pera.html

a)

b)

c)

d)

Background and objectives

It is unclear how ocean acidification (OA) will affect

mäerl/ rhodolith beds in a near future. Some

inconsistencies among OA experiments are related to

differences in methodology, time of exposure and the

synergetic effects of other stressors. The objectives of

this study were; 1) to measure the long-term effects of

OA on the respiration, photosynthesis and calcification

of Phymatolithon sp. and 2) to study the simultaneous

effect of light and OA on the photosynthesis and

calcification of the algae.

Methods

Acidification effects (air-CO2 mix bubbling) were

evaluated over a 20 months period. Three pCO2

levels were tested: control (~390 ppm),

intermediate (550ppm) and high (750 ppm) (Figure

2). Algae were gradually acclimatized to experimental

conditions.

Photosynthetic and calcification rates were measured

using chamber incubations (dissolved oxygen

evolution and alkalinity anomaly technique) under

different photosynthetically active radiation (PAR)

levels (figure 3a). Calcification was also measured

using the buoyant weight technique (BW) (figure

3b).

Figure 3. Incubation

chambers (a) and

BW measurements

(b).

Figure 2. OA

experimental

system (a) and

key components

of CO2 control

system (b).

b)

a) b)

Results and discussion

Respiration, photosynthesis and calcification vs. PAR

The first 11 months, respiration decreased with

CO2 and photosynthesis increased with CO2 and

light (figure 4). Calcification also increased with

CO2 and light, saturating at around 45 µmol m-2s-1

for acidified algae at 750 ppm, a much lower level

than for photosynthesis (~200 PAR) (figure 5).

Photosynthesis and calcification vs. time

Photosynthetic rates of acidified algae increased

during the first 11 months but decreased with CO2

after 20 months of exposure (figure 6).

Calcification/growth rate also decreased with time

in acidified algae and proportionally to CO2 (figure

7). Control algae kept continuous rates and after 20

months of acidification presented the highest

cumulative growth.

PAR( mol photons.m-2

. s-1

)

0 8 45 195 450

NP

P(

mo

lO2.g

FW

-1.

h-1

)

-1

0

1

2

3

4

control

550ppm

750ppm

Figure 4. Photosynthetic rates

(NPP) at different pCO2 as a

function of PAR. Means ± SE.

Figure 5. Calcification rates (CG)

at different pCO2 as a function of

PAR after 11 months of treatment.

Means ± SE.

PAR ( mol. photons. m-2

. s-1

)

0 8 45 195 450

CG

(m

olC

aC

O3gF

W-1

. h

-1)

-0.4

-0.2

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

control

550ppm

750ppm

time (months)

0 11 20

NP

P(

mol O

2.g

FW

-1.

h-1

)

0

1

2

3

4

control

550ppm

750ppm

Figure 6. Photosynthetic rates

(NPP) at different pCO2 as a

function of time. Means ± SE.

Figure 7. Calcification/growth

rates at different pCO2 as a

function of time. Means ± SE.

Conclusions

We found that both the time of exposure and the light

intensity affect the predictions of how Phymatolithon

sp. will respond to OA. While in the short-term algae

increased their rates to compensate the corrosive

conditions of low pH, in the long-term both

photosynthetic and calcification rates decreased.

Long term effects of OA, particularly at higher

irradiances will be detrimental to these algae.

Acknowledgements: This study is a contribution to the FCT project

PTDC/MAR/115789/2009 “Mäerl calcification, photosynthesis and metabolism

in an acidified ocean” (jmsilva@ualg.pt). The first author* (lsnieves@ualg.pt) is

supported by the FCT doctoral grant SFRH/BD/76762/2011. The SOLAS

project awarded LS with a travel grant to attend this conference.

Source: http://geology.com/world/europe-physical-map.shtml

time (months)

7 12 16 20

Gro

wth

ra

te

mm

olC

aC

O3/(

g.d

ay)

0.000

0.001

0.002

0.003

0.004

a)