Turnover rates of photosynthesis determined by fluorescence kinetic analysis.

Maximum turnover rates of photosynthesis impose an upper limit on global primary production, yet traditional estimation methods remain tedious and prone to significant uncertainty. This research presents a new method and operational instrumentation to deduce the maximum turnover rates of photosynthesis from instantaneous kinetic analysis of chlorophyll fluorescence under saturating background light. Measurements of maximum turnover rates in model species of green algae and diatoms over a range of environmental conditions revealed that the rates vary from 150 to 600 s- 1 per PSII. These rates correspond to the minimum turnover times of 1.6 to 7 ms. Decreases in turnover rates are driven by low-light photoacclimation and nutrient limitation. In natural populations of near-surface phytoplankton in tropical and subtropical regions, the maximum turnover rates may reach approximately 800 s- 1, reflecting photoacclimation to high solar irradiance. In contrast, in cold polar waters, especially under severe nitrogen limitation, the turnover rates decline to 80 to 140 s- 1, highlighting a strong temperature and nutrient dependence. Importantly, variations in these turnover rates with nutrient availability show a strong linear correlation with cell growth rates. This discovery provides an efficient, operational tool to quantify the effect of nutrient limitation on phytoplankton growth rates and productivity in the global ocean.