Seasonal regulation of the coupling between photosynthetic electron transport and carbon fixation in the Southern Ocean

Abstract

Active fluorescence measurements can provide rapid, non‐intrusive estimates of phytoplankton primary production at high spatial and temporal resolution, but there is uncertainty in converting from electrons to ecologically relevant rates of CO2 assimilation. In this study, we examine the light‐dependent rates of photosynthetic electron transport and 13C‐uptake in the Atlantic sector of the Southern Ocean to derive a conversion factor for both winter (July 2015–August 2015) and summer (December 2015–February 2016). The results revealed significant seasonal differences in the light‐saturated chlorophyll specific rate of 13C‐uptake, ( PmaxnormalB), with mean summer values 2.3 times higher than mean winter values, and the light limited chlorophyll specific efficiency, (αB), with mean values 2.7 times higher in summer than in winter. Similar patterns were observed in the light‐saturated photosynthetic electron transport rates ( ETRmaxRCII, 1.5 times higher in summer) and light limited photosynthetic electron transport efficiency (αRCII, 1.3 times higher in summer). The conversion factor between carbon and electrons (Φe:C (mol e− mol C−1)) was derived utilizing in situ measurements of the chlorophyll‐normalized number of reaction centers (nRCII), resulting in a mean summer Φe:C which was ∼ 3 times lower than the mean winter Φe:C. Empirical relationships were established between Φe:C, light and NPQ, however they were not consistent across locations or seasons. The seasonal decoupling of Φe:C is the result of differences in Ek‐dependent and Ek‐independent variability, which require new modelling approaches and improvements to bio‐optical techniques to account for these inter‐seasonal differences in both taxonomy and environmental mean conditions.

Primary Productivity Dynamics in the Summer Arctic Ocean Confirms Broad Regulation of the Electron Requirement for Carbon Fixation by Light-Phytoplankton Community Interaction

“…Spearman correlation and nMDS analyses revealed that PAR indeed had the highest correlation coefficient with K C ( Supplementary Table 2; Supplementary Figure 6, r = 0.66, p < 0.001), which is consistent with our previous results (Zhu et al, 2016(Zhu et al, , 2017. Changes in light availability drive an increase in the need to dissipate absorbed excitation energy through nonphotochemical dissipation (McKew et al, 2013) and hence a positive association between K C and NPQ may also be expected, as broadly observed previously (Schuback et al, 2015(Schuback et al, , 2016(Schuback et al, , 2017Hughes et al, 2018b;Ryan-Keogh et al, 2018). Since the K C of this study was derived from ETR and primary productivity in daily scales, similar to the method applied for daily ETR calculation, we also integrated the NPQ NSV values according to their light dependent functions (Supplementary Figure 7A).…”

“…However, no positive relationship between NPQ NSV and K C was evident for this data set ( Supplementary Figure 7B). Lack of a relationship (or negative relationship) between NPQ or PAR with K C was also reported before (Schuback et al, 2017;Ryan-Keogh et al, 2018). Whilst the reason is not well-known yet, it may reflect a decoupling of the NPQ NSV (or PAR)-K C relationship under relatively low light (Schuback et al, 2017) and further investigation is needed.…”

“…Early efforts to reconcile conventional CO 2 uptake rates with FRRf-derived CO 2 uptake attributed FRRf "overestimates" to use of a constant K C value (i.e., 4 or 5 mol e − (mol C) −1 ) (Kromkamp et al, 2008;Mino et al, 2014), in particular under excess irradiance (Ralph et al, 2010). Subsequent studies demonstrated that miss-matches between CO 2 uptake rates and FRRf were largely due to K C variability, which in turn could be explained (predicted) from co-variability with key environmental factors known to regulate PP, i.e., light, temperature and/or inorganic nutrient availability (Lawrenz et al, 2013;Hughes et al, 2018b;Ryan-Keogh et al, 2018). Schuback et al (2015Schuback et al ( , 2016Schuback et al ( , 2017 and (Schuback and Tortell, 2019) further demonstrated that such K C variability correlated with the extent of nonphotochemical quenching (NPQ), interpreted as an indication of excess light energy, which leads to a decoupling of ETR PSII and carbon uptake (see also Hughes et al, 2018b).…”

Photosynthetic adaptation to low iron, light, and temperature in Southern Ocean phytoplankton

“…Region Iron level Growth rate, d −1 A B C D α B under iron limitation has been observed previously with O 2based measurements of photosynthesis (20), but not with the short-term (hours) 14 C-based method (21)(22)(23). The difference in the results of the two methods may be linked to the fact that the shortterm 14 C method measures neither gross nor net C-fixation rates, while the O 2 method measures rates of photosynthetic O 2 production minus O 2 consumption from respiration and photorespiration, which may be lessened under iron and light limitation (24,25).…”

Seasonal Depletion of the Dissolved Iron Reservoirs in the Sub‐Antarctic Zone of the Southern Atlantic Ocean

“…Depth‐integrated PP rates (PP wc , mol C·m −2 ·day −1 ) were calculated from quenching corrected glider‐derived chlorophyll (Thomalla, Moutier, et al, ) and PAR according to Platt et al (), Platt and Sathyendranath (), and Thomalla et al (). PP parameters were determined from a linear relationship with chlorophyll using experimental values from both cruises (supporting information Figure S2; Ryan‐Ryan‐Keogh, Thomalla, Little, et al, ). Seasonal variation in nutrient concentrations within the euphotic and winter mixed layer isopycnals required three different methods to derive depth‐integrated nutrient inventories.…”