Linking FRRF Derived Photophysiology with Carbon-based Primary Productivity: Insights from Concepts of Cellular Energy Allocation

Active chlorophyll a fluorescence approaches, including fast repetition rate fluorometry (FRRF), have the potential to provide estimates of phytoplankton primary productivity at unprecedented spatial and temporal resolution. FRRF-derived productivity rates are based on estimates of charge separation in photosystem II (ETR_RCII), which must be converted into ecologically relevant units of carbon fixation. Understanding sources of variability in the coupling of ETR_RCII and carbon fixation provides important physiological insight into phytoplankton photosynthesis, and is critical for the application of FRRF as a primary productivity measurement tool.

We present data from a series of experiments during which we simultaneously measured phytoplankton carbon fixation and ETR_RCII in the iron-limited NE subarctic Pacific. Our results show significant variability of the derived conversion factor (Ф_e:C/n_PSII), with highest values observed under conditions of excess excitation pressure at the level of photosystem II, caused by high light and/or low iron. Our results will be discussed in the context of metabolic plasticity, which evolved in phytoplankton to simultaneously maximize growth and provide photoprotection under fluctuating light and limiting nutrient availabilities. Because the derived conversion factor is associated with conditions of excess light, it correlates with the expression of non-photochemical quenching (NPQ) in the pigment antenna, also derived from FRRF measurements. Our results demonstrate a significant correlation between NPQ and the conversion factor Ф_e:C/n_PSII, and the potential of this relationship to improve FRRF-based estimates of phytoplankton carbon fixation rates is discussed.