Description: Sea ice retreat, changing stratification and ocean acidification are fundamentally changing the light availability and physico-chemical conditions for primary producers in the Arctic ocean. However, detailed studies on ecophysiological strategies and performance of key species in the pelagic and ice-associated habitat remain scarce. We therefore investigated the acclimated responses of the diatoms Thalassiosira hyalina and Melosira arctica towards elevated irradiance and CO2 partial pressures. Next to growth, elemental composition and biomass production, we assessed detailed photophysiological responses through fluorometry and gas-flux measurements, including respiration and carbon acquisition. In the pelagic T. hyalina, growth rates remained high in all treatments and biomass production increased strongly with light. Even under low irradiances cells maintained a high-light acclimated state, allowing them to opportunistically utilize high irradiances by means of a highly plastic photosynthetic machinery and carbon uptake. The ice-associated M. arctica proved to be less plastic and more specialized on low-light. Its acclimation to high irradiances was characterized by minimizing photon harvest and photosynthetic efficiency, which led to lowered growth. Comparably low growth rates and strong silification advocate a strategy of persistence rather than of fast proliferation, which is also in line with the observed formation of resting stages under low-light conditions. In both species, responses to elevated pCO2 were comparably minor. Although both diatom species persisted under the applied conditions, their competitive abilities and strategies differ strongly. With the anticipated extension of Arctic pelagic habitats, flexible high-light specialists like T. hyalina seem to face a brighter future.

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.

Description: Atmospheric and oceanic CO2 concentrations are rising at an unprecedented rate. Laboratory studies indicate a positive effect of rising CO2 on phytoplankton growth until an optimum is reached, after which the negative impact of accompanying acidification dominates. Here, we implemented carbonate system sensitivities of phytoplankton growth into our global biogeochemical model FESOM-REcoM and accounted explicitly for coccolithophores as the group most sensitive to CO2. In idealized simulations in which solely the atmospheric CO2 mixing ratio was modified, changes in competitive fitness and biomass are not only caused by the direct effects of CO2, but also by indirect effects via nutrient and light limitation as well as grazing. These cascading effects can both amplify or dampen phytoplankton responses to changing ocean pCO2 levels. For example, coccolithophore growth is negatively affected both directly by future pCO2 and indirectly by changes in light limitation, but these effects are compensated by a weakened nutrient limitation resulting from the decrease in small-phytoplankton biomass. In the Southern Ocean, future pCO2 decreases small-phytoplankton biomass and hereby the preferred prey of zooplankton, which reduces the grazing pressure on diatoms and allows them to proliferate more strongly. In simulations that encompass CO2-driven warming and acidification, our model reveals that recent observed changes in North Atlantic coccolithophore biomass are driven primarily by warming and not by CO2. Our results highlight that CO2 can change the effects of other environmental drivers on phytoplankton growth, and that cascading effects may play an important role in projections of future net primary production.

Description: Biogeochemical models are used to project future plankton community composition and biogeochemical fluxes. Phytoplankton growth therein is usually parametrized by sensitivities to the bottom-up factors temperature, light, and nutrient availability. However, many laboratory studies identify the carbonate system as an additional and essential growth-determining factor, especially in light of ongoing ocean acidification. Besides, growth-responses towards one factor are often altered by the level of another factor, and these so-called interactive effects are barely considered in models. In the presented work, model functions for carbonate system dependencies of growth and calcification were developed based on published results of laboratory data and implemented into a biogeochemical model. Using the results of an earlier meta-analysis on dual driver interactions, this new model setup was then substituted by interactive growth effects between the carbonate system, temperature, and light. End-of-century phytoplankton biomass and community composition in a high-emission scenario was projected by using one model version with and one model version without driver interactions. The results reveal that interactive growth effects considerably alter the future community composition compared to the model version without interactions. These alterations are largest in the Southern Ocean. Globally, the model with interactions projects a future phytoplankton community consisting out of more small phytoplankton and fewer diatoms and coccolithophores. Hence, considering interactive growth effects between bottom-up factors can essentially modify the projections of future phytoplankton community composition and related biogeochemical fluxes, and should be considered more closely in biogeochemical models.