Using chlorophyll fluorescence kinetics to determine photosynthesis in aquatic ecosystems

Variable fluorescence techniques are increasingly used to assess phytoplankton photosynthesis. All fluorescence techniques and models for photosynthetic electron transport rates (ETRs) are amplitude-based and are subject to errors, especially when phytoplankton growth is nutrient-limited. Here we develop a new, kinetic-based approach to measure, directly and in absolute units, ETRs and to estimate growth rates in phytoplankton. We applied this approach to investigate the effects of nitrogen limitation on phytoplankton photophysiology and growth rates. Nutrient stress leads to a decrease in the quantum yield of photochemistry in Photosystem II (F_v/F_m); however, the relationship between F_v/F_m and growth rates is highly nonlinear, which makes it impossible to quantify the reduction in phytoplankton growth rates from F_v/F_m alone. In contrast, the decline in growth rates under nitrogen stress was proportional to the decrease in kinetic-based photosynthetic rates. Our analysis suggests the kinetic fluorescence measurements markedly improve the accuracy of ETR measurements, as compared to classical amplitude-based measurements. Fluorescence-based methods for primary production rely on measurements of ETRs and then conversion to carbon fixation rates by using the electron yields of carbon fixation. The electron yields exhibit 10-fold variability in natural phytoplankton communities and are strongly affected by nutrient limitation. Our results reveal that a decrease in the growth rates and the electron yields of carbon fixation is driven by, and can be quantified from, a decrease in photosynthetic turnover rates. We propose an algorithm to deduce the electron yields of carbon fixation, which greatly improve fluorescence-based measurements of primary production and growth rates.