Seasonal photosynthetic activity of autotrophic picoplankton in Lake Kinneret, Israel

Abstract

Autotrophic picoplankton populations in Lake Kinneret are composed of picocyanobacteria and picoeukaryotes. Overall, the rates of photosynthetic carbon fixed by autotrophic picoplankton during this study were low (0.01-1.5 mg C m~ h). The highest chlorophyll photosynthetic activity of the <3 (i.m cell-size fraction was found in spring, when picoeukaryotes predominated and in addition small nanoplankton passed through the filters. The maximum cell-specific photosynthetic rate of carbon fixation by picocyanobacteria and picoeukaryotes was 2.5 and 63 fg C cell h, respectively. The highest specific carbon fixation rate of autotrophic picoplankton was 11 p.g C (ig~' Chi h~'. The proportional contribution of autotrophic picoplankton to total photosynthesis usually increased with depth. Picocyanobacteria collected from the dark, anaerobic hypolimnion were viable and capable of active photosynthesis when incubated at water depths within the euphotic zone. Maximum rates of photosynthesis (/ma,) for picocyanobacteria ranged from 5.4 to 31.4 fg C cell h~' with the highest values in hypolimnetic samples exposed to irradiance. Photosynthetic efficiency (a) was -4fold higher in picocyanobacteria sampled from 40 m than in cells from near-surface waters. Light saturation (/fc) was lower in picocyanobacteria from 40 m, suggesting that these cells were acclimated to lower light intensities. The relative contribution of autotrophic picoplankton to total phytoplankton photosynthesis in Lake Kinneret was low, but occasionally, at seasons and depths where picocyanobacteria or picoeukaryotes were abundant, could account for most of photosynthetic activity.