PHYTOPLANKTON ABSORPTION IN RELATION TO PIGMENT

COMPOSITION

Importance

• Primary production

• Light penetration in the ocean

• Remote sensing of phytoplankton biomass and primary production

• Mixed-layer dynamics

Influence of absorption on the attenuation of light in the ocean

a: total absorption coefficient (m-1) bb: backscattering coefficient (m-1)

: average cosine of light field

b

d

baK

R(): reflectance at wavelength bb(): backscattering coefficient at

a(): absorption coefficient at

)(

)()(

a

bfR b

Influence of phytoplankton absorption on reflectance

Ocean Colour

Components of absorption in the ocean

)()()()()( *** ydphw YaDaCaaa

:)(wa absorption coefficient of pure seawater (m-1):)(a total absorption coefficient of seawater (m-1)

:C concentration of chlorophyll-a (mg m-3)

:)(* ya specific absorption coefficient of yellow substances (dimensionless)

:Y concentration of yellow substances (expressed in absorption m-1)

:D concentration of detritus (mg m-3)

:)(* pha specific absorption coefficient of phytoplankton [m-1 (mg m-3)-1]

:)(* da specific absorption coefficient of detritus [m-1 (mg m-3)-1]

Phytoplankton pigments

• Chlorophyll-a (or its substitutes bacteriochlorophyll-a or divinyl-chlorophyll-a) is located in the RCs of all photosynthetic organisms.

• Three main types of accessory pigments: chlorophylls, carotenoids and biliproteins are located in the subantennae and LHCs of different taxonomic groups of algae.

Chlorophylls• Green coloured pigments.• Absorb light energy in the blue

and red regions of the spectrum.• Porphyrin ring – conjugated

double bonds, magnesium ion, nonpolar phytol tail.

• Three main types: a, b, and c (divinyl-chl-a, -b, chl-c1, -c2, -c3).

• Fluoresce (máximum 680 nm).• Photochemistry, and light-

harvesting.

Carotenoids

• Red, orange or yellow pigments.

• Absorb light in the blue-green region.

• Conjugated hydrocarbons.• Two main groups: carotenes

(e.g., -carotene) and xanthophylls (e.g., fucoxanthin).

• Do not fluoresce per se.• Some light-harvesting, some

photoprotective.

Phycobilins

• Brightly coloured pigments (red, orange, pink).

• Absorb light in the green-yellow region.

• Linear tetrapyrroles (water soluble).

• Four major types: phycocyanin, phycoerythrin, allophycocyanin, phyoerythrocyanin.

• Fluoresce (máximum 570 nm).

• Light-harvesting.

Pigment composition in phytoplankton taxaAlgal Division/Class Common Name GeneraGolden-brown algae (chl-a and c)Bacillariophyta diatoms 210Dinophyta dinoflagellates 550Chrysophyta Golden-brown flagellates

Chrysophyceae chrysophytes,silicoflagellates 120 Raphydophyceae chloromonads 4Haptophyta Golden-brown flagellates

Prymnesiophyceace coccolithophorids 50Xanthophyta Yellow-green algae 600Cryptophyta* cryptomonads 8Eustigmatophyta Yellow-green algae 6Green algae (chl-a and –b)

Chlorophyta

Chlorophyceae green algae 350 Prasinophyeceae green flagellates 13Euglenophyta euglenoids 43Rhodophyta (chl-a and biliproteins)Rhodophyta red algae 3Blue-green algae (chl-a and biliproteins)Cyanophyta cyanobacteria ??

prochlorophytes 3

Pigment composition in phytoplankton taxa

Absorption spectra of pigment-protein-complexes

(from Barrett and Anderson, 1980)

Absorption spectra of different algae

(from Kirk, 1994)

Absorption spectra of different algae

• Why in the visible range?• First determination of the action spectrum of photosynthesis

Response to the light field

• Different algae have pigment composition suitable for growth under their typical natural light environments.

• Intracellular pigment concentration is also variable with the intensity of the light field.

• Both pigment composition and intracellular pigment concentration influence the absorption characteristics of the phytoplankton.

Photoadaptation & Photoacclimation

• The pigment characteristics of a species reflects adaptation at evolutionary time scales to their environment (Photoadaptation).

• The response of phytoplankton to the light field may also be temporary (Photoacclimation).

Photoacclimation

• Short-term changes.

• Long-term changes:

–Changes in the number of PSUs.

–Changes in the size of the PSUs.

–Changes in the proportion of photosynthetic (PS) and photoprotective (PP)pigments.

Short-term photoacclimationXanthophyll cycle

Long-term photoacclimation:Changes in the size of the PSU

(from Falkowski, 1983)

Variations in the absorption characteristics with photoacclimation

(from Lutz et al., 2001)

Packaging effect

Radiation absorption in a discrete medium containing an absorbing substance as suspended particles is different from absorption in a continuous medium containing an equal amount of the same substance, supposedly dissolved.Because of this effect of discreteness, absorption by a suspension of particles does not follow Beer’s Law.

Packaging effect

If the concentration of particles is small or the flux absorbed by individual particles is small, then the absorption coefficient of the particles can be approximated as:

)()( NsQaa p

Absorption efficiency of one particle at

)),0(

),(),0(()(

I

rIIQa

Where N is the number of particles per unit volume of the medium, and s is the cross section of one particle

Packaging effect

’ is a function of the cell size and the intracellular concentration of pigments

(from Platt and Sathyendranath, 2002)

Variations in the absorption characteristics with cell-size and photoacclimation

(from Lutz et al., 2001)

Spectrophotometric determination of phytoplankton in vivo absorption

• Suspension: opal glass technique

• Quantitative filter technique– Correction for detritus– Correction for the pathlength amplification

factor ( factor)

Components of absorption in the ocean

)()()()()( *** ydphw YaDaCaaa

:)(wa absorption coefficient of pure seawater (m-1):)(a total absorption coefficient of seawater (m-1)

:C concentration of chlorophyll-a (mg m-3)

:)(* ya specific absorption coefficient of yellow substances (dimensionless)

:Y concentration of yellow substances (expressed in absorption m-1)

:D concentration of detritus (mg m-3)

:)(* pha specific absorption coefficient of phytoplankton [m-1 (mg m-3)-1]

:)(* da specific absorption coefficient of detritus [m-1 (mg m-3)-1]

Scheme of a bio-optical model: optical components for MERIS

Gelbstoffyellow substance

Water sample

gelbstoffabsorption spectrum

spectral exponent

gelbstoffabsorption spectrum

spectral exponent

particletotal absorption

Absorption ofbleachedfraction

= spm absorption

Absorption ofbleachedfraction

= spm absorption

Absorption ofTotal - bleached

fraction= phytoplankton

absorption

Absorption ofTotal - bleached

fraction= phytoplankton

absorption

In situAC-9BB-4

particlescattering

backscattering

particlescattering

backscattering

Absorption ofbleachedFraction +gelbstoff

= total gelbstoff

Absorption ofbleachedFraction +gelbstoff

= total gelbstoff

TSM

Gelb

Chlor

Courtesy R. Doerffer

Detritus determination

• Extraction method (Kishino et al., 1985).• Statistical estimation of relationship between total

absorption and absorption at wavelength dominated by detritus (Morrow et al., 1989; Bricaud and Stramski, 1990).

• Microphotometry (Iturriaga and Siegel, 1989).• Spectral reconstruction (Bidigare, 1989)• Fitting an exponential shape (Hoepffner and

Sathyendranath, 1993).

Detritus correction

400 500 600 7000.00

0.02

0.04

0.06

TotalDetritusPhyto.

EPEA-24-0 m

Wavelength (nm)

abso

rpti

on (

m-1

)

Detritus correction: Fitting an exponential curve (Hoepffner and Sathyendranath, 1993)

• ap(): observed total absorption from each sample, (m-1)

• anph(): average absorption spectrum of phytoplankton,

normalised at 440 nm (Hoepffner and Sathyendranath, 1993)

• aph(440): phytoplankton absorption coefficient at 440 nm, (m-1)

• ad(440): detritus absorption coefficient at 440 nm, (m-1)

• q: exponential coefficient

))440(exp()440()()440()( qaaaa dnphphp

Pathlength amplification factor,

• apf: absorption of particles on the filter, (m-1)

• ap: absorption of particles in suspension, (m-1)

p

pf

a

a

Pathlength amplification factor,

Quadratic equation - Hoepffner and Sathyendranath 1992

Accounting for Prochlorophytes – Kyewalyanga et al. 1998

2)]([)]([)( pfpfps OBOAO

222

211 )()(1)()()( pfpfdvpfpfdvps OBOAFOBOAFO

Fdv: ratio div-chla/total-chla

Modelling of phytoplankton absorption:“Single pigment” method

• aph(): absorption coefficient of phytoplankton at wavelength , (m-1)

• a*ph(): specific absorption coefficient of phytoplankton at , (m-1(mg chl m-3)-1)

• C: concentration of chlorophyll-a, (mg m-3)

Caa phph )()( *

Modelling of phytoplankton absorption:“Multi pigment” method

• aph(): absorption coefficient of phytoplankton at wavelength , (m-1)

• a*i(): specific absorption coefficient of the i-th pigment at , (m-1(mg pigment m-3)-1)

• Ci: concentration of the i-th pigment, (mg m-3)

i

n

i iph Caa )()( *

Decomposition of the in vivo absorption spectrum of phytoplankton: pigments

and chromoproteins

(from Johnsen and Sakshaug, 1996)

Decomposition of the in vivo absorption spectrum of phytoplankton: Gaussian curves

(from Hoepffner and Sathyendranath, 1991)