Preliminary assessment of carbonic acid dissociation constants: Insights from observations in China's east coastal oceans.

Based on observations in China's east coastal oceans, we conducted a preliminary assessment of 16 sets of carbonic acid dissociation constants (K1* and K2*) by comparing spectrophotometrically measured pH values at 25 °C with those calculated from total alkalinity and dissolved inorganic carbon. We obtained that K1* and K2* often performed differently within different salinity ranges, and that the constants of Millero et al. (2002) (M02) demonstrated the best performance for the salinity range of 24-35. In contrast, the often recommended constants of Mehrbach et al. (1973) refit by Dickson and Millero (1987) (DM87-M) and Lucker et al. (2000) (L00) would underestimate pH at salinities of 24-30. This was mainly associated with the higher product of K1* and K2* by DM87-M and L00 than by M02 at this salinity range. Also, we found almost no differences between pH values calculated with DM87-M and L00.

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

Fast repetition rate fluorometry (FRRf) allows for rapid non-destructive assessment of phytoplankton photophysiology in situ yet has rarely been applied to Trichodesmium. This gap reflects long-standing concerns that Trichodesmium (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 Trichodesmium assemblages from two sites. Using a multi-LED measurement protocol (447+519+634 nm combined), we assessed maximum photochemical efficiency (F v /F m ), functional absorption cross section of PSII (σ PSII ), and electron transport rates (ETRs) for Trichodesmium 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 F v /F m for Trichodesmium compared to blue-only excitation. We found distinct photophysiological differences for Trichodesmium at both locations with higher average F v /F m as well as lower σ PSII and non-photochemical quenching (NPQ NSV ) observed in the NWP compared to the NIO-SL (Kruskal-Wallis t-test df = 1, p < 0.05). Fluorescence light response curves (FLCs) further revealed differences in ETR response with a lower initial slope (α ETR ) and higher maximum electron turnover rate ( E T R P S I I m a x ) observed for Trichodesmium in the NWP compared to the NIO-SL, translating to a higher averaged light saturation E K (= E T R P S I I m a x /α ETR ) 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 Trichodesmium using FRRf-derived fluorescence parameters, yielding an estimated carbon-fixation rate ranging from 7.8 to 21.1 mgC mg Chl-a-1 h-1 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 Trichodesmium populations.