Photophysiological responses of Southern Ocean phytoplankton to changes in CO2 concentrations: Short-term versus acclimation effects

Abstract

The present study examines how different pCO 2 acclimations affect the CO 2 -and light-dependence of photophysiological processes and O 2 fluxes in four Southern Ocean (SO) key phytoplankton species. We grew Chaetoceros debilis (Cleve), Pseudo-nitzschia subcurvata (Hasle), Fragilariopsis kerguelensis (O'Meara) and Phaeocystis antarctica (Karsten) under low (160 μatm) and high (1000 μatm) pCO 2 . The CO 2 -and lightdependence of fluorescence parameters of photosystem II (PSII) were determined by means of a fluorescence induction relaxation system (FIRe). In all tested species, nonphotochemical quenching (NPQ) is the primary photoprotection strategy in response to short-term exposure to high light or low CO 2 concentrations. In C. debilis and P. subcurvata, PSII connectivity (p) and functional absorption cross-sections of PSII in ambient light (σ PSII ′ ) also contributed to photoprotection while changes in re-oxidation times of Q a acceptor (τ Qa ) were more significant in F. kerguelensis. The latter was also the only species being responsive to high acclimation pCO 2 , as these cells had enhanced relative electron transport rates (rETRs) and σ PSII ′ while τ Qa and p were reduced under short-term exposure to high irradiance. Low CO 2 -acclimated cells of F. kerguelensis and all pCO 2 acclimations of C. debilis and P. subcurvata showed dynamic photoinhibition with increasing irradiance. To test for the role and presence of the Mehler reaction in C. debilis and P. subcurvata, the light-dependence of O 2 fluxes was estimated using membrane inlet mass spectrometry (MIMS). Our results show that the Mehler reaction is absent in both species under the tested conditions. We also observed that dark respiration was strongly reduced under high pCO 2 in C. debilis while it remained unaltered in P. subcurvata. Our study revealed species-specific differences in the photophysiological responses to pCO 2 , both on the acclimation as well as the short-term level.Abbreviations: a* PSII , optical cross section for photosystem II; α, maximum light-use efficiency; ATP, adenosine triphosphate; CCM, carbon concentrating mechanism; cf, conversion factor; Chl a, chlorophyll a; C i , inorganic carbon; CO 2 , carbon dioxide; DBS, dextran-bound sulfonamide (inhibitor for eCA); DIC, dissolved inorganic carbon; eCA, extracellular carbonic anhydrase; ETR, electron transport rate; FIRe, fluorescence induction relaxation system; F 0 , minimum fluorescence; F 0 ′ , light-adapted minimum fluorescence; F m , maximum fluorescence; F m ′ , light-adapted maximum fluorescence; F v /F m , maximum quantum yield of photochemistry in photosystem II according to Genty et al. (1989); F q ′ /F m ′ , effective quantum yield of photochemistry in photosystem II; HCO 3 − , bicarbonate; HEPES, 2-[4-(2-Hydroxyethyl)-1-piperazinyl]ethanesulfonic acid; I, irradiance; I k , light acclimation index; J, connectivity parameter according to Lavergne and Trissl (1995); MIMS, membrane inlet mass spectrometry; MTF, multiple turnover flash; NADH, nicotinamide adenine dinucleo...

Ocean acidification of a coastal Antarctic marine microbial community reveals a critical threshold for CO<sub>2</sub> tolerance in phytoplankton productivity

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