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

Abstract

We examined the light absorption properties and light-dependent rates of photosynthetic electron transport (ETR RCII ) and 14 C-uptake in phytoplankton of the Canadian Subarctic and Arctic Ocean. Our results reveal high variability in the light-saturated, chlorophyll a-specific rate of 14 C-uptake (P Chla max ; 0.7 to 8.0 mg C mg chl a 21 h 21 ) , and the light-dependant efficiency of 14 C-uptake (a Chla ; 0.01 to 0.09 mg C mg Chl a 21 h 21 [lmol quanta m 22 s 21 ] 21 ). Variability in P Chla max correlated with light availability in the stratified watercolumn, while both P Chla max and a Chla were correlated to the degree of nitrate depletion in the mixed layer. For all samples, we observed that 14 C-uptake rates reached light saturation at lower irradiances than ETR RCII , leading to significant, light-dependent de-coupling of carbon fixation and photosynthetic electron transport. For samples taken from within the mixed layer, we found that the conversion factor from ETR RCII to carbon fixation was strongly correlated to the magnitude of non-photochemical quenching, as derived from fast repetition rate fluorometry (FRRF) measurements. This correlation supports recent findings from the Subarctic Pacific Ocean, and has the potential to improve FRRF-based estimates of phytoplankton carbon fixation. Our observations reveal distinct environmental regulation of phytoplankton light absorption, electron transport and carbon fixation in phytoplankton assemblages within and below the shallow summer mixed layer, and will help to inform more robust predictions of future primary production in a rapidly changing Arctic Ocean.

Resistance of Arctic phytoplankton to ocean acidification and enhanced irradiance

“…Furthermore, the assemblage was dominated by diatoms, which are known to possess highly efficient photo-protective machinery (Lavaud et al 2004). Our FRRF measurements indeed revealed a high capacity for energy dissipation via NPQ (Table 4), even when compared to relatively high values previously measured in diatom-dominated Arctic phytoplankton assemblages as well as individual diatom and haptophyte strains (McKew et al 2013;Hoppe et al 2015;Schuback et al 2017). Moreover, relatively high temperatures occurring in the incubators may have helped to prevent high-light stress by increasing the turnover rate of carbon fixation relative to electron transport, thereby allowing the Calvin cycle to be a more efficient sink for light energy (Mock and Hoch 2005;Goldman et al 2015).…”

“…Samples were kept at low light (\10 lmol photons m -2 s -1 ) and at incubator temperatures for at least 30 min before measurements, in order to achieve a dark-regulated photo-physiological state (i.e., all photochemical and non-photochemical quenching processes relaxed). All FRRF measurements were conducted on a benchtop FRRF instrument (Soliense Instuments) as described in Schuback et al (2017). For each sample, a single turnover protocol (70 flashlets with 0.7 ls length and 2.5 ls interval, 87,800 lmol photons m -2 s -1 peak power intensity, resulting in a excitation sequence of 225 ls, providing *7-12 photons per RCII) was applied to derive basal and maximal Chl a fluorescence (Chl F) yields F o and F m , respectively.…”

“…On dilution and final sampling days, we conducted additional measurements of steady-state light curves, as described in Schuback et al (2017). Rates of initial charge separation in reaction center II (ETR RCII , mol emol RCII s -1 ) were calculated from Chl F yields measured at nine incrementally increasing background PAR levels.…”

“…in Schuback et al (2017). Spectrally corrected rates of 2-h 14 C-uptake and ETR RCII were used to derive the conversion factor J/n PSII (mol e -mol C -1 (mol Chl a) mol RCII -1 ) at light limitation (a RCII /a Chl a ) and light saturation (P RCII max / P Chl a max ), as described in more detail in Schuback et al (2015Schuback et al ( , 2016.…”

“…Large changes in irradiance, however, also occur during upwelling events, which play an important role in triggering autumn blooms in ice-free Arctic areas (Ardyna et al 2014). As discussed below, Arctic phytoplankton assemblages can therefore be expected to possess mechanisms that allow them to thrive under sudden high-light conditions (Platt et al 1982;Gosselin et al 1990;Campbell et al 2015;Schuback et al 2017).…”