ALBERT

Description: The acquisition of dissolved inorganic carbon by aquatic primary producers became increasingly challenging with higher structural complexity of algae, and with simultaneously declining atmospheric CO2 partial pressure. The seemingly easy diffusive supply of CO2 to RubisCO turned into a bottleneck for photosynthesis, which consequently required alternative inorganic carbon acquisition processes and pathways to evolve. In order to ensure sufficient CO2 supply to RubisCO, aquatic photosynthesizing organisms started to employ facilitated CO2 uptake, active HCO3- trafficking across multiple membranes as well as carbonic anhydrases, located at the outer cell membrane and in several cellular compartments. The modes of these so-called CO2-concentrating mechanisms (CCMs) are very diverse, non-canonical even within phylogenetic groups, and possess differently efficient CO2 accumulation capacities, depending on the requirements of RubisCO, the physico-chemical conditions in the boundary layer, membrane properties and cellular architecture. However, different independently evolved CCMs also exhibit a high degree of functional similarity, owing to the functional similarity of the photosynthetic process. To introduce the topic to the reader, this chapter starts with a brief outline of RubisCO´s properties and the reasons why CCMs are required (4.2). Then, the principle chemical nature of dissolved inorganic carbon in water is described (4.3): Its speciation and kinetic behavior and relevant co-determinants of carbonate chemistry. We furthermore touch upon the physico-chemical basis of carbon availability in aquatic environments (4.4.), and subsequently elaborate on the known transport modes of different inorganic carbon species. Subsequently, the current state of knowledge on existing strategies in main algal groups is presented (4.5-4.9). Finally, we consider the operation of CCMs in the context of co-occurring cellular processes (4.10), such as calcification and N2 fixation, which rely on the provision of ample inorganic carbon and/or energy and, in the case of calcification, can have important consequences for compartmental pH homeostasis.

Description: To investigate the balance between net photo- and heterotrophy throughout the Arctic autumn-winter-spring transition, we assessed the abundances of O2 and Ar in surface waters by means of membrane-inlet mass spectrometry . We derived biologically mediated O2 super-/undersaturation (ΔO2/Ar), reflecting the difference between gross primary production and the community’s combined autotrophic and heterotrophic respiration (i.e., ‘net community production’, NCP). We present first results on the magnitude of NCP over the autumn-winter-spring transition and extrapolate biological carbon drawdown and release. Further correlation with biological and chemical parameters assessed during MOSAiC is used to identify the controls on net community production and to better understand the ecological mechanisms that drive biogeochemical fluxes in the rapidly changing Arctic.

Description: We assessed the responses of solitary cells of Arctic Phaeocystis pouchetii grown under a matrix of temperature (2°C vs. 6°C), light intensity (55 vs. 160 μmol photons m-2 s-1) and pCO2 (400 vs. 1000 μatm). Next to acclimation parameters (growth rates, particulate and dissolved organic C and N, chlorophyll a content), we measured physiological processes in-vivo (electron transport rates and net photosynthesis) using fast-repetition rate fluorometry and membrane-inlet mass spectrometry. Within the applied driver ranges, elevated temperature had the most pronounced impacts, significantly increasing growth, elemental quotas and photosynthetic performance. Light stimulations manifested prominently under high temperature, underlining its role as a 'master variable'. pCO2 was the least effective driver, exerting mostly insignificant effects. The obtained data were used in a simplified ecosystem model to simulate P. pouchetii's bloom dynamics in the Fram Strait with increasing temperatures over the 21st century. Model results suggest that global warming will accelerate bloom dynamics, with earlier onsets of blooms and higher peak biomasses. Despite remaining uncertainties about the magnitude of these effects, data strongly suggest that increasing temperatures over the coming century will affect the phenology of Phaeocystis and other Arctic phytoplankton with likely important implications for higher trophic levels.