Contrasting Photophysiological Characteristics of Phytoplankton Assemblages in the Northern South China Sea

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.

Fast Repetition Rate Fluorometry (FRRF) Derived Phytoplankton Primary Productivity in the Bay of Bengal

The approach of fast repetition rate fluorometry (FRRF) requires a conversion factor (Φ_{e : C}/n _PSII) to derive ecologically-relevant carbon uptake rates (PP _z,t). However, the required Φ_{e : C}/n _PSII is commonly measured by ¹⁴C assimilation and varies greatly across phytoplankton taxonomy and environmental conditions. Consequently, the use of FRRF to estimate gross primary productivity (GP _z,t), alone or in combination with other approaches, has been restricted by both inherent conversion and procedural inconsistencies. Within this study, based on a hypothesis that the non-photochemical quenching (NPQ_NSV) can be used as a proxy for the variability and magnitude of Φ_{e : C}/n _PSII, we thus proposed an independent field model coupling with the NPQ_NSV-based Φ_{e : C}/n _PSII for FRRF-derived carbon, without the need for additional Φ_{e : C}/n _PSII in the Bay of Bengal (BOB). Therewith, this robust algorithm was verified by the parallel measures of electron transport rates and ¹⁴C-uptake PP _z,t. NPQ_NSV is theoretically caused by the effects of excess irradiance pressure, however, it showed a light and depth-independent response on large spatial scales of the BOB. Trends observed for the maximum quantum efficiency (F_v/F_m), the quantum efficiency of energy conversion ( F q ' / F m ' ) and the efficiency of charge separation ( F q ' / F v ' ) were similar and representative, which displayed a relative maximum at the subsurface and were collectively limited by excess irradiance. In particular, most observed values of F_v/F_m in the BOB were only about half of the values expected for nutrient replete phytoplankton. FRRF-based estimates of electron transport at PSII (ETR_RCII) varied significantly, from 0.01 to 8.01 mol e^- mol RCII^-1 s^-1, and showed profound responses to depth and irradiance across the BOB, but fitting with the logistic model. N, P, and irradiance are key environmental drivers in explaining the broad-scale variability of photosynthetic parameters. Furthermore, taxonomic shifts and physiological changes may be better predictors of photosynthetic parameters, and facilitate the selection of better adapted species to optimize photosynthetic efficiency under any particular set of ambient light condition.