In search of a physiological basis for covariations in light-limited and light-saturated photosynthesis

The photosynthesis-irradiance (PE) relationship links indices of phytoplankton biomass (e.g. chl) to rates of primary production. The PE curve can be characterized by two variables: the light-limited slope (alpha(b)) and the light-saturated rate (P-max(b)) of photosynthesis. Variability in PE curves can be separated into two categories: that associated with changes in the light saturation index, E-k (=P-max(b)/alpha(b)) and that associated with parallel changes in alpha(b)and P-max(b) (i.e. no change in E-k). The former group we refer to as ``E-k-dependent'' variability, and it results predominantly from photoacclimation (i.e. physiological adjustments in response to changing light). The latter group we refer to as ``E-k-independent'' variability, and its physiological basis is unknown. Here, we provide the first review of the sporadic field and laboratory reports of E-k-independent variability, and then from a stepwise analysis of potential mechanisms we propose that this important yet largely neglected phenomenon results from growth rate-dependent variability in the metabolic processing of photosynthetically generated reductants (and generally not from changes in the oxygen-evolving PSII complexes). Specifically, we suggest that as growth rates decrease (e.g. due to nutrient stress), reductants are increasingly used for simple ATP generation through a fast (<1s) respiratory pathway that skips the carbon reduction cycle altogether and is undetected by standard PE methodologies. The proposed mechanism is consistent with the field and laboratory data and involves a simple new ``twist'' on established metabolic pathways. Our conclusions emphasize that simple reductants, not reduced carbon compounds, are the central currency of photoautotrophs

Diel variations in the photosynthetic parameters of Prochlorococcus strain PCC 9511: Combined effects of light and cell cycle

International audienceWe examined the mechanisms related to the diel variations in the parameters of the relationship between the rate of carbon fixation of phytoplankton and irradiance (P vs. E curve). Our goal was to understand what determines the phase of these variations relative to that of the light cycle. We grew the marine prokaryote Prochlorococcus in an axenic cyclostat culture system under a light-dark cycle that mimicked natural conditions at sea surface and followed changes in cell physiology with a 2-h resolution. Individual cells divide mostly in phase with each other, once a day at the beginning of the dark period. The quantum yields of chlorophyll fluorescence, the maximum quantum yield of carbon fixation and the maximum rate of carbon fixation (P-max(B)) exhibited diel variations over about factors of 2, 4, and 4, respectively, with maxima at the beginning of the light period. The morning drop in phi(Cmax) and the quantum yield of fluorescence, which was accompanied by only a small decrease (< 15%) of photochemial efficiency of PSII (F-v/F-m), suggests regulation by light and preceded the drop in P-max(B) by 4 h. The decrease in P-max(B) during the day matched a decrease in the transcription level of Rubisco. The quantum yield of fluorescence, phi(Cmax), and P-max(B) increased again during the dark period, but this recovery was slowed at the time of cell division. Our results suggest that the pattern of diel variations in the photosynthetic parameters is determined both by photoacclimation and the cell-division cycle

� 2005, by the American Society of Limnology and Oceanography, Inc. Diel variations in the photosynthetic parameters of Prochlorococcus strain PCC 9511: Combined effects of light and cell cycle

European Commission research project PROMOLEC (contract MAS3-CT97-0128). We thank Ms. Florence Le Gall and Sandrine Boulben for preparing culture medium. M. Dominique Marie and Drs Daniel Vaulot, Stéphan Jacquet, and Jean Blanchot also greatly contributed to the success of this experiment b

Fluorescence as a tool to understand changes in photosynthetic electron flow regulation

International audienceThe physiological state of a chloroplast is stronglyinfluenced by both biotic and abiotic conditions.Unfavourable growth conditions lead to photosyntheticstress. Chlorophyll a fluorescence is a widelyused probe of photosynthetic activity (specificallyPSII), and therefore stress which specifically targetsthe electron transport pathway and associated alternativeelectron cycling pathways. By manipulating theprocesses that control photosynthesis, affecting thechlorophyll a fluorescence, yields detailed insight intothe biochemicalpathways. Light that is captured by achlorophyll molecule can be utilised in three competingprocesses; electron transport, energy dissipation(via heat) and chlorophyll a fluorescence emission.Electrons produced by water-splitting are not alwaysused in carbon fixation; if the incident irradiancegeneratesmore electrons than the dark reactionscan use in carbon fixation, damage will occur to the photosynthetic apparatus. If carbon fixation is inhibitedby temperature or reduced inorganic carbon (Ci), ATPor NADPH availability, then the photosystem dynamicallyadjusts and uses alternate sinks for electrons, suchas molecular oxygen (water-water cycle or Mehler ascorbateperoxidase reaction). The process of stress acclimationleads to a number of photoprotective pathwaysand we describe how inhibitors can be used to identifythese particular processes. In this chapter, we describethe processes controlling electron transport as influencedby light-induced stress