FROM ELECTRON TO BIOMASS: A MECHANISTIC MODEL TO DESCRIBE PHYTOPLANKTON PHOTOSYNTHESIS AND STEADY‐STATE GROWTH RATES¹

Abstract

A system of differential equations was presented which describes the rate of linear and cyclic electron flow through photosystems II and I. The system describes the rate of photochemistry in terms of electrons generated that are available for cellular metabolism, and results in a realistic description of photosynthesis as a function of irradiance without implicit assumptions for the relationship. The system allows a concise and detailed simulation of fluorescence kinetics. The derivation of the general degree of reduction (c x ) and its application to translate the rates of photochemistry (or measured fluorescence yields) to steady-state rates of carbon fixation and growth was shown. The efficiency of light-limited photosynthesis (a) was shown to depend on the cellular ratio of carbon to nitrogen. For any given antenna size, a increases with nitrate as N-source, and decreases with ammonium as N-source, if the cellular carbon to nitrogen ratio of the phytoplankton increases. Cyclic electron transport around photosystem I increases the ratio of ATP generated relative to linear electron (e --) flow. The increase of ATP/e --is larger under extreme light-limiting conditions. The long-known fact that protein synthesis saturates at lower light intensities than carbonate synthesis was explained in terms of the decrease of ATP/e --with increasing irradiance and the higher ATP demand of protein synthesis.Key index words: carbon fixation; degree of reduction; electron transport model; fluorescence; photosynthesis; phytoplankton growth Abbreviations: Fd, ferredoxin; FNiR, Fd-nitrite oxidoreductase; FNR, Fd-NADP þ oxidoreductase; FRR, fast repetition rate; FTR, Fd-thioredoxin oxidoreductase; GOGAT, glutamine-oxoglutarate aminotransferase; ODEs, ordinary differential equations; PC, plastocyanin; PFD, photon flux density; PQ, plastoquinoneThe importance of phototrophic microorganisms and especially oceanic phytoplankton for life on earth was never challenged. Most research on photosynthesis, however, was driven by the need to investigate photosynthetic growth, in order to optimize yields and production rates of agricultural crops. While basic research successfully unravelled structural and functional features of the photosynthetic machinery, applied measurements focussed on the quantification of the most prominent signals of photosynthesis (oxygen evolution or carbon fixation) relative to irradiance. Many theories and models were developed to describe these so-called photosynthesis-irradiance (P/E) curves (Bannister 1979, Gallegos andPlatt 1981) and these models were helpful to derive ecologically relevant photosynthetic production rates. Fewer models were developed that describe algal activity based on the biochemistry of macromolecular compounds

Fluorescence as a Tool to Understand Changes in Photosynthetic Electron Flow Regulation

“…However, the ratio of oxygen released to carbon incorporated is highly variable, for several reasons. Firstly, the reduction in biomass strongly depends on the species and the environmental conditions (Kroon and Thoms 2006). For example, under N or P limitation, the relative proportion of carbon incorporated into carbohydrates is strongly increased and therefore the reduction in biomass is relatively low.…”

Gross and net primary production: closing the gap between concepts and measurements

“…Again, differences in nutrient supply might cause different energy demand patterns, leading to deviations in the stoichiometry between electrons transported and carbon fixed. Kroon & Thoms (2006) describe an integrative model that accounts for nutrient status and acclimation of the photosynthetic energy output. Franklin & Badger (2001) showed reasonable correlation between fluorescence determined ETR and gross O 2 evolution based on 18 O studies, except when algae were exposed to very high light or starved of inorganic carbon (as a sink for electron flow).…”

On the approaches applied in formulation of a kinetic model of photosystem II: Different approaches lead to different simulations of the chlorophyll a fluorescence transients

“…Baake and Schlöder, 1992;Stirbet et al, 1998;Vredenberg et al, 2001;Lazár, 2003;Zhu et al, 2005;Belyaeva et al, 2006), but also complex models were considered describing, in addition to PSII, also subsequent electron transport reactions (e.g. Lebedeva et al, 2002;Kroon and Thoms, 2006;Laisk et al, 2006).…”