Diurnal variation in the coupling of photosynthetic electron transport and carbon fixation in iron-limited phytoplankton in the NE subarctic Pacific

Abstract

Abstract. Active chlorophyll a fluorescence approaches, including fast repetition rate fluorometry (FRRF), have the potential to provide estimates of phytoplankton primary productivity at unprecedented spatial and temporal resolution. FRRF-derived productivity rates are based on estimates of charge separation at PSII (ETRRCII), which must be converted into ecologically relevant units of carbon fixation. Understanding sources of variability in the coupling of ETRRCII and carbon fixation provides physiological insight into phytoplankton photosynthesis, and is critical for the application of FRRF as a primary productivity measurement tool. In the present study, we simultaneously measured phytoplankton carbon fixation and ETRRCII in the iron-limited NE subarctic Pacific, over the course of a diurnal cycle. We show that rates of ETRRCII are closely tied to the diurnal cycle in light availability, whereas rates of carbon fixation appear to be influenced by endogenous changes in metabolic energy allocation under iron-limited conditions. Unsynchronized diurnal oscillations of the two rates led to 3.5 fold changes in the conversion factor coupling ETRRCII and carbon fixation (Φe:C / nPSII). Consequently, diurnal variability in phytoplankton carbon fixation cannot be adequately captured with FRRF approaches if a constant conversion factor is applied. Utilizing several auxiliary photophysiological measurements, we observed that a high conversion factor is associated with conditions of excess light, and correlates with the expression of non-photochemical quenching (NPQ) in the pigment antenna, as derived from FRRF measurements. The observed correlation between NPQ and the conversion factor Φe:C / nPSII has the potential to improve estimates of phytoplankton carbon fixation rates from FRRF measurements alone.

Primary productivity and the coupling of photosynthetic electron transport and carbon fixation in the Arctic Ocean

“…, the previously observed correlation from the Subarctic Pacific holds well for all samples taken from within the mixed layer. Both the Arctic phytoplankton assemblages we sampled here and the iron‐limited phytoplankton assemblages examined by Schuback et al () showed a very similar slope in the highly significant regression between the conversion factor and NPQ (Κ C /n PSII = 486 NPQ NSV + 1854; r 2 = 0.75, p < 0.01, df = 140). Moreover, the y‐intercept of this regression, 1854, is strikingly close to the theoretical minimum value of Κ C /n PSII (1600 e − C −1 Chl a RCII −1 , assuming 4 mol e − mol C −1 and 400 Chl a mol RCII −1 ).…”

“…While our dataset is too small to draw general conclusions, our experimental approach allows us to gain some insight into how Arctic phytoplankton assemblages achieve optimal balance of the different components of the photosynthetic process. We also use our new observations to expand on recent work (Schuback et al ) exploring the feasibility of predicting the conversion factor between ETR RCII and 14 C‐uptake rates from FRRF derived estimates of non‐photochemical quenching (NPQ NSV ) in the pigment antenna.…”

“…Extending on previous work in the Subarctic Pacific (Schuback et al ), we sought to assess the correlation between the derived conversion factor Κ C /n PSII and the expression of NPQ NSV . As shown in Fig.…”

“…The conversion factor between rates of ETR RCII and 14 C‐uptake (Κ C /n PSII ; mol e − mol C −1 mol Chl a mol RCII −1 ) was derived as described in detail in Schuback et al ().…”

Diversity in Xanthophyll Cycle Pigments Content and Related Nonphotochemical Quenching (NPQ) Among Microalgae: Implications for Growth Strategy and Ecology

“…, Schuback et al. ). Under relatively steady light exposure, high light acclimated cells/organisms have much larger xanthophyll pools (Dd+Dt or Z+A+V), often leading to a greater capacity for XC‐mediated NPQ in both plants (Demmig‐Adams et al.…”