ALBERT

Description: Previous field studies in the Southern Ocean (SO) indicated an increased occurrence and dominance of cryptophytes over diatoms due to climate change. To gain a better mechanistic understanding of how the two ecologically important SO phytoplankton groups cope with ocean acidification (OA) and iron (Fe) availability, we chose two common representatives of Antarctic waters, the cryptophyte Geminigera cryophila and the diatom Pseudo-nitzschia subcurvata. Both species were grown at 2°C under different pCO2 (400 vs. 900 μatm) and Fe (0.6 vs. 1.2 nM) conditions. For P. subcurvata, an additional high pCO2 level was applied (1400 μatm). At ambient pCO2 under low Fe supply, growth of G. cryophila almost stopped while it remained unaffected in P. subcurvata. Under high Fe conditions, OA was not beneficial for P. subcurvata, but stimulated growth and carbon production of G. cryophila. Under low Fe supply, P. subcurvata coped much better with OA than the cryptophyte, but invested more energy into photoacclimation. Our study reveals that Fe limitation was detrimental for the growth of G. cryophila and suppressed the positive OA effect. The diatom was efficient in coping with low Fe, but was stressed by OA while both factors together strongly impacted its growth. The distinct physiological response of both species to OA and Fe limitation explains their occurrence in the field. Based on our results, Fe availability is an important modulator of OA effects on SO phytoplankton, with different implications on the occurrence of cryptophytes and diatoms in the future.

Description: Climatic changes in the Southern Ocean have strong implications for the global marine carbon cycle, for example through changes in phytoplankton community composition. These shifts, in turn, can affect the strength and efficiency of the biological carbon pump, i.e. the process by which carbon is exported from the surface ocean to the deep sea via the aggregation and sinking of phytoplankton and other organic matter. At depth, carbon can be sequestered over long periods of time, effectively “buffering” increasing atmospheric CO2 concentrations. For the Southern Ocean, specifically the Weddell Sea, we only have limited data on carbon export due to the difficulties of accessing these remote and often ice-covered regions. Based on various phytoplankton bottle incubation experiments which simulated future climatic changes a possible shift in phytoplankton community composition from large diatoms to small flagellates such as Phaeocystis sp. is indicated, with unknown consequences for nutrient cycling and carbon export. To address these unknowns, we conducted in situ measurements and roller tank experiments with contrasting diatom-to-Phaeocystis ratios during a Polarstern cruise to the Southern Weddell Sea in spring 2021 to characterize aggregate formation degradation and sinking of marine snow. The same set of parameters were also assessed in controlled laboratory experiments with well-defined diatom-to-Phaeocystis ratios. Based on our results from field and laboratory, we hypothesised that a climate-mediated shift towards Phaeocystis in the future would reduce the efficiency of the biological carbon pump due to decreased silica-ballasting and increased concentrations of positively buoyant exopolymeric substances associated with Phaeocystis colonies. To our surprise, preliminary results reveal that higher Phaeocystis cell numbers relative to diatoms do not lead to a statistically significant reduction in aggregate mass density and size-specific sinking velocity. At the same time, there was a trend towards larger particles when Phaeocystis was abundant. Our results from field and laboratory observation together reveal that Phaeocystis-dominated communities do not impede carbon export in the Weddell Sea, but may actually enhance it.