Light availability is one of the major factors guiding production patterns of photoautotrophs such as cyanobacteria. In this study, photosynthesis-irradiance (PI) curves were used to determine photosynthetic efficiency (α), photosynthetic maxima (Pmax) and onset of light-saturated photosynthesis (Ik). Those parameters are used to characterize photoacclimation to provide information on productivity under varying light levels. Radioactively labeled carbon was incorporated into organic matter in water samples from Lake Superior and Synechococcus sp. isolates from Lake Erie and Lake Superior growing under low- and high-light conditions (15 vs. 70 μmol photons m-2s-1). Measurements of in vivo chlorophyll a fluorescence were used to determine growth rates of the picocyanobacterial strains.
Photosynthetic performance of the pico-size fraction in comparison to total phytoplankton in Lake Superior displayed an increase in photosynthetic contribution by the size fraction with water depth. Differences of photosynthetic performances between Lake Erie and Lake Superior isolates could not be found. Most isolates from Lake Superior did not show significant differences in photosynthetic efficiencies for low-light vs. high-light adapted cultures. Lake Erie isolate KD3a showed higher efficiencies under high light conditions. Lake Erie strain Arc11 showed higher photosynthetic maxima for the low-light adapted culture. These surprising patterns might explain the dominance of these strains within Lake Erie, having the ability to respond to changing light fields with the lake.