Abstract
AbstractPicophytoplankton dominate the phytoplankton community in wide ocean areas and are considered efficient in the acquisition of light compared to other phytoplankton groups. To quantify their photophysiological parameters we use three strains of picoprokaryotes and four strains of picoeukaryotes. We measure the acclimated response of the exponential growth rates and chlorophyll a (Chl a) to carbon ratios, as well as the instantaneous response of photosynthesis rates at 5–7 light intensities. We then use a dynamic photosynthesis model (Geider et al. ) and extend it with a photoinhibition term. We derive five photophysiological parameters: the maximum rate of photosynthesis (), the affinity to light (αchl), the photoinhibition term (βchl), the respiration rate (resp), and the maximum Chl a to carbon ratio (θmax). We show that is significantly lower for picoprokaryotes than for picoeukaryotes and increases significantly with increasing cell size. In turn, αchl decreases significantly with increasing maximum growth rate (μmax). The latter finding is contrary to a previously reported relationship for phytoplankton, but agrees with theoretical assumptions based on size. The higher efficiency in light acquisition gives picoprokaryotes an advantage in light limited environments at the expense of their maximum growth rate. In addition, our results indicate that the accumulation of long‐term damage through photoinhibition during acclimation is not well represented by the dynamic photosynthesis model. Hence, we would recommend to distinguish between the effects of irreversible damage (on a time scale of days) on growth rates and of reversible damage (on a time scale of minutes) on photosynthesis rates.
Highlights
Picophytoplankton dominate the phytoplankton community in wide ocean areas and are considered efficient in the acquisition of light compared to other phytoplankton groups
Experimental procedures and analyses To investigate the effect of light on the exponential growth rates, photosynthesis rates and chlorophyll a (Chl a) to carbon ratios of picophytoplankton, seven strains from diverse phytoplankton classes were obtained from the Roscoff culture collection (RCC, Vaulot et al 2004)
To model the response of exponential growth rates, photosynthesis rates and Chl a to carbon ratios to light we use the dynamic photosynthesis model of Geider et al (1997, their Eqs. 2–4). We extended their Eq 1 with a photoinhibition term, which we obtained by reformulating the steady state light inhibition model (Platt et al 1980) to match the dependence on a variable Chl a to carbon ratio in the dynamic photosynthesis model (Eqs. 1, 2)
Summary
Picophytoplankton dominate the phytoplankton community in wide ocean areas and are considered efficient in the acquisition of light compared to other phytoplankton groups To quantify their photophysiological parameters we use three strains of picoprokaryotes and four strains of picoeukaryotes. Picophytoplankton include cells with a diameter 3 lm (e.g., Vaulot et al 2008) and consist of two distinct groups: picoprokaryotes represented by Prochlorococcus and Synechococcus, and picoeukaryotes with representatives from diverse phytoplankton classes Both groups contribute substantially to phytoplankton biomass (Buitenhuis et al 2013), primary production (Grossman et al 2010), and to the recycling of organic matter within the microbial loop in the surface ocean (Azam et al 1983; Fenchel 2008). The acclimation to high light intensities leads to a decline in Chl a, but to an increase of energy storage components (Geider 1987), which in turn affects the growth and photosynthesis rates
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