Physiological and Ecological Responses of Photosynthetic Processes to Oceanic Properties and Phytoplankton Communities in the Oligotrophic Western Pacific Ocean

Understanding the dynamics of primary productivity in a rapidly changing marine environment requires mechanistic insight into the photosynthetic processes (light absorption characteristics and electron transport) in response to the variability of environmental conditions and algal species. Here, we examined the photosynthetic performance and related physiological and ecological responses to oceanic properties [temperature, salinity, light, size-fractionated chlorophyll a (Chl a) and nutrients] and phytoplankton communities in the oligotrophic Western Pacific Ocean (WPO). Our results revealed high variability in the maximum (F(v)/F(m); 0.08–0.26) and effective (F(q)′/F(m)′; 0.02–0.22) photochemical efficiency, the efficiency of charge separation (F(q)′/F(v)′; 0.19–1.06), the photosynthetic electron transfer rates (ETR(RCII); 0.02–5.89 mol e(–) mol RCII(–1) s(–1)) and the maximum of primary production [PP(max); 0.04–8.59 mg C (mg chl a)(–1) h(–1)]. All these photosynthetic characteristics showed a depth-specific dependency based on respective nonlinear regression models. On physiological scales, variability in light absorption parameters F(v)/F(m) and F(q)′/F(m)′ notably correlated with light availability and size-fractionated Chl a, while both ETR(RCII) and PP(max) were correlated to temperature, light, and ambient nutrient concentration. Since the presence of nonphotochemical quenching (NPQ(NSV); 2.33–12.31) and increasing reductant are used for functions other than carbon fixation, we observed nonparallel changes in the ETR(RCII) and F(v)/F(m), F(q)′/F(m)′, F(q)′/F(v)′. In addition, we found that the important biotic variables influencing F(v)/F(m) were diatoms (cells > 2 μm), picosized Prochlorococcus, and eukaryotes, but the PP(max) was closely related to large cyanobacteria (cells > 2 μm), dinoflagellates, and picosized Synechococcus. The implication is that, on ecological scales, an interaction among temperature, light, and nutrient availability may be key in driving the dynamics of primary productivity in the WPO, while large cyanobacteria, dinoflagellates, and picosized Synechococcus may have a high contribution to the primary production. Overall, the photosynthetic processes are interactively affected by complex abiotic and biotic variables in marine ecosystems, rather than by a single variable.