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)  b b : backscattering coefficient (m-1)   : average cosine of light field

R( ): reflectance at wavelength b b ( ): backscattering coefficient at a( ): absorption coefficient at Influence of phytoplankton absorption on reflectance

Ocean Colour

Components of absorption in the ocean absorption coefficient of pure seawater (m -1 ) total absorption coefficient of seawater (m -1 ) concentration of chlorophyll-a (mg m -3 ) specific absorption coefficient of yellow substances (dimensionless) concentration of yellow substances (expressed in absorption m -1 ) concentration of detritus (mg m -3 ) specific absorption coefficient of phytoplankton [m -1 (mg m -3 ) -1 ] 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-c 1, -c 2, - c 3 ). 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 taxa Algal Division/ClassCommon NameGenera Golden-brown algae (chl-a and c) Bacillariophytadiatoms210 Dinophytadinoflagellates550 ChrysophytaGolden-brown flagellates Chrysophyceaechrysophytes,silicoflagellates120 Raphydophyceaechloromonads4 HaptophytaGolden-brown flagellates Prymnesiophyceacecoccolithophorids50 XanthophytaYellow-green algae600 Cryptophyta*cryptomonads8 EustigmatophytaYellow-green algae6 Green algae (chl-a and –b) Chlorophyta Chlorophyceaegreen algae350 Prasinophyeceaegreen flagellates13 Euglenophytaeuglenoids43 Rhodophyta (chl-a and biliproteins) Rhodophytared algae3 Blue-green algae (chl-a and biliproteins) Cyanophytacyanobacteria?? prochlorophytes3

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 photoacclimation Xanthophyll 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: Absorption efficiency of one particle at 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 absorption coefficient of pure seawater (m -1 ) total absorption coefficient of seawater (m -1 ) concentration of chlorophyll-a (mg m -3 ) specific absorption coefficient of yellow substances (dimensionless) concentration of yellow substances (expressed in absorption m -1 ) concentration of detritus (mg m -3 ) specific absorption coefficient of phytoplankton [m -1 (mg m -3 ) -1 ] specific absorption coefficient of detritus [m -1 (mg m -3 ) -1 ]

Scheme of a bio-optical model: optical components for MERIS Gelbstoff yellow substance Water sample gelbstoff absorption spectrum spectral exponent gelbstoff absorption spectrum spectral exponent particle total absorption Absorption of bleached fraction = spm absorption Absorption of bleached fraction = spm absorption Absorption of Total - bleached fraction = phytoplankton absorption Absorption of Total - bleached fraction = phytoplankton absorption In situ AC-9 BB-4 particle scattering backscattering particle scattering backscattering Absorption of bleached Fraction + gelbstoff = total gelbstoff Absorption of bleached Fraction + 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

Detritus correction: Fitting an exponential curve (Hoepffner and Sathyendranath, 1993) a p ( ): observed total absorption from each sample, (m -1 ) a n ph ( ): average absorption spectrum of phytoplankton, normalised at 440 nm (Hoepffner and Sathyendranath, 1993) a ph (440): phytoplankton absorption coefficient at 440 nm, (m -1 ) a d (440): detritus absorption coefficient at 440 nm, (m -1 ) q: exponential coefficient

Pathlength amplification factor,  a pf : absorption of particles on the filter, (m -1 ) a p : absorption of particles in suspension, (m -1 )

Pathlength amplification factor,  Quadratic equation - Hoepffner and Sathyendranath 1992 Accounting for Prochlorophytes – Kyewalyanga et al F dv : ratio div-chla/total-chla

Modelling of phytoplankton absorption: “Single pigment” method a ph ( ): 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 )

Modelling of phytoplankton absorption: “Multi pigment” method a ph ( ): 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 ) C i : concentration of the i-th pigment, (mg m -3 )

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)