Abstract
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 (Fv/Fm; 0.08–0.26) and effective (Fq′/Fm′; 0.02–0.22) photochemical efficiency, the efficiency of charge separation (Fq′/Fv′; 0.19–1.06), the photosynthetic electron transfer rates (ETRRCII; 0.02–5.89 mol e– mol RCII–1 s–1) and the maximum of primary production [PPmax; 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 Fv/Fm and Fq′/Fm′ notably correlated with light availability and size-fractionated Chl a, while both ETRRCII and PPmax were correlated to temperature, light, and ambient nutrient concentration. Since the presence of nonphotochemical quenching (NPQNSV; 2.33–12.31) and increasing reductant are used for functions other than carbon fixation, we observed nonparallel changes in the ETRRCII and Fv/Fm, Fq′/Fm′, Fq′/Fv′. In addition, we found that the important biotic variables influencing Fv/Fm were diatoms (cells > 2 μm), picosized Prochlorococcus, and eukaryotes, but the PPmax 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.
Highlights
Current trends of change in oligotrophic marine ecosystems with ongoing climate change include warming, acidification, oligotrophication, and the increases in water column stratification and light penetration (Gao et al, 2019)
We can infer (1) how the photosynthetic processes respond to specific environmental variable and species composition on physiological scales and (2) what is the key in driving the dynamics of Western Pacific Ocean (WPO) primary productivity on ecological scales
While our dataset is too small to draw general conclusions, our experimental results allow us to gain some physiological and ecological insights into how the dominant environmental constraints and algal species regulate the light absorption characteristics and electron transport and what is the key in driving the dynamics of WPO primary productivity
Summary
Current trends of change in oligotrophic marine ecosystems with ongoing climate change include warming, acidification, oligotrophication, and the increases in water column stratification and light penetration (Gao et al, 2019) All of these anticipated changes will inevitably interact to affect the photosynthetic performance of phytoplankton and marine primary productivity (Gao et al, 2012; Hoppe et al, 2015; Schuback et al, 2017; Hughes et al, 2018). In situ measurements of FRRF-derived primary productivity in marine ecosystems can be achieved at the photophysiological level
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