Exploring Variability of Photophysiology Using Multi-Excitation Wavelength Fast Repetition Rate Fluorometry.

Fast repetition rate fluorometry (FRRf) allows for rapid non-destructive assessment of phytoplankton photophysiology yet has rarely been applied to This gap reflects long-standing concerns that (and other cyanobacteria) contain pigments that are less effective at absorbing blue light which is often used as the sole excitation source in FRR fluorometers-potentially leading to underestimation of key fluorescence parameters. In this study, we use a multi-excitation FRR fluorometer (equipped with blue, green, and orange LEDs) to investigate photophysiological variability in assemblages from two sites. Using a multi-LED measurement protocol (447+519+634 nm combined), we assessed maximum photochemical efficiency ( / ), functional absorption cross section of PSII (σ ), and electron transport rates (ETRs) for assemblages in both the Northwest Pacific (NWP) and North Indian Ocean in the vicinity of Sri Lanka (NIO-SL). Evaluating fluorometer performance, we showed that use of a multi-LED measuring protocol yields a significant increase of / for compared to blue-only excitation. We found distinct photophysiological differences for at both locations with higher average / as well as lower σ and non-photochemical quenching ( ) observed in the NWP compared to the NIO-SL (Kruskal-Wallis -test = 1, < 0.05). Fluorescence light response curves (FLCs) further revealed differences in ETR response with a lower initial slope (α ) and higher maximum electron turnover rate ( ) observed for in the NWP compared to the NIO-SL, translating to a higher averaged light saturation (= /α ) for cells at this location. Spatial variations in physiological parameters were both observed between and within regions, likely linked to nutrient supply and physiological stress. Finally, we applied an algorithm to estimate primary productivity of using FRRf-derived fluorescence parameters, yielding an estimated carbon-fixation rate ranging from 7.8 to 21.1 mgC mg Chl-a h across this dataset. Overall, our findings demonstrate that capacity of multi-excitation FRRf to advance the application of Chl-a fluorescence techniques in phytoplankton assemblages dominated by cyanobacteria and reveals novel insight into environmental regulation of photoacclimation in natural populations.