Abstract
The approach of fast repetition rate fluorometry (FRRF) requires a conversion factor (Φe : C/nPSII) to derive ecologically-relevant carbon uptake rates (PPz,t). However, the required Φe : C/nPSII 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 (GPz,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 (NPQNSV) can be used as a proxy for the variability and magnitude of Φe : C/nPSII, we thus proposed an independent field model coupling with the NPQNSV-based Φe : C/nPSII for FRRF-derived carbon, without the need for additional Φe : C/nPSII in the Bay of Bengal (BOB). Therewith, this robust algorithm was verified by the parallel measures of electron transport rates and 14C-uptake PPz,t. NPQNSV 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 (Fv/Fm), the quantum efficiency of energy conversion (/) and the efficiency of charge separation (/) 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 Fv/Fm in the BOB were only about half of the values expected for nutrient replete phytoplankton. FRRF-based estimates of electron transport at PSII (ETRRCII) 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.
Highlights
A convenient starting-point in marine ecosystem cycle is the photosynthesis from phytoplankton, with a possible very minor contribution of a few species of truly photosynthetic bacteria (McDermott et al, 1995)
Active chlorophyll a fluorescence (ChlF) approaches which refer to measures of the quantum yield of linear electron transport through photosystem II (PSII), including fast repetition rate fluorometry (FRRF), can afford instantaneous estimates of gross primary productivity (GPz,t) at unprecedented high spatial and temporal resolution, avoiding the artifacts related to bottle containment (Kolber and Falkowski, 1993; Kolber et al, 1998; Smyth et al, 2004)
We presented an independent field model based on the NPQNSV-proxy hypothesis, without the need for additional e : C/nPSII in natural phytoplankton assemblages
Summary
A convenient starting-point in marine ecosystem cycle is the photosynthesis from phytoplankton, with a possible very minor contribution of a few species of truly photosynthetic bacteria (McDermott et al, 1995). The rates of phytoplankton primary productivity have been measured tracing the evolution of O2 or the assimilation of CO2 (Tortell, 2000), as well as using the 14C-method in conjunction with a simulated in situ incubator (Gall et al, 1999). These techniques have a number of well-known limitations, e.g., high labor intensity and cost associated with routine sample processing, low spatial and temporal resolution, and bottle artifacts due to exclusion of contamination. Active chlorophyll a fluorescence (ChlF) approaches which refer to measures of the quantum yield of linear electron transport through photosystem II (PSII), including fast repetition rate fluorometry (FRRF), can afford instantaneous estimates of gross primary productivity (GPz,t) at unprecedented high spatial and temporal resolution, avoiding the artifacts related to bottle containment (Kolber and Falkowski, 1993; Kolber et al, 1998; Smyth et al, 2004)
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