New Kinetic Fluorescence Technique for Measurements of Photosynthetic Rates and Instantaneous Growth Rates of Phytoplankton, in Relation to Nutrient Stress in the Ocean

Phytoplankton photosynthetic rates are commonly assessed using variable chlorophyll fluorescence techniques that rely on recording a change in fluorescence amplitude induced by saturating light. All modern fluorescence techniques (such as PAM, FRR, and FIRe) and models for electron transport rates (ETR) are amplitude-based and are subject to uncertainties and errors, especially when phytoplankton growth is nutrient-limited.

Here we propose and have implemented a conceptually new kinetic-based approach to measure, directly and in absolute units, the rates of electron transport and to assess growth rates in phytoplankton. This approach relies on the kinetic analysis of photosynthetic electron flow. This kinetic approach has been incorporated into new operation miniaturized Fluorescence Induction and Relaxation instruments (mini-FIRe fluorometers).

We applied this novel approach to quantify the effects of nitrogen limitation on phytoplankton photophysiology, electron transport rates and growth rates. Nutrient stress leads to a reduction in the quantum yield of photochemistry in PSII (Fv/Fm). However, the relationship between Fv/Fm and phytoplankton growth rates is highly non-linear, which makes it impossible to quantify the reduction in phytoplankton growth rates from Fv/Fm along. In contrast, the reduction in growth rates under nutrient stress was found to be directly proportional to the reduction in kinetic-based photosynthetic rates. Our analysis revealed several-fold better correlation between the kinetic-based measurements of photosynthetic rates and growth rates, as compared to amplitude-based ETRs. Our laboratory and field experiments showed that these kinetic fluorescence measurements provide a sensitive quantitative proxy of nutrient stress in the ocean. Finally, we discuss applications and limitations of kinetic fluorescence measurements to estimate gross and net primary production in the ocean, in relation to nutrient stress.