Focus – The Arctic is warming faster than any other oceanic region of the planet and its sea-ice cover is declining faster than models predict. Due to changes in sea-ice cover and stratification, the Arctic Ocean may transition from predominantly light-controlled to more nutrient-controlled productivity. Also the phenomenon of ocean acidification is most pronounced in the Arctic due to the higher solubility of CO2 at low temperatures as well as the lower alkalinities. Rapid changes in the Arctic marine environment have already led to changes in productivity and species distribution patterns, and more dramatic shifts are expected for the future. The Arctic Ocean therefore appears to be the ‘miner’s canary’ of global change. In order to elucidate synergistic or antagonistic interactions between ocean acidification, warming and secondary effects imposed by melting of sea ice, we apply so-called multi-factorial matrices. In such experiments, two or more environmental parameters (e.g. temperature, CO2, light, nutrients) are independently varied so that the combined as well as individual effects can be investigated. Despite the complexity in these responses, they can principally be divided into physiological and ecological aspects, e.g. changes in the rates of processes and shifts in the dominance of species that drive the community responses. To address this systematically, we assess the effects of multiple stressors on the level of physiology in isolates of key species and the ecological consequences by testing the sensitivities of natural assemblages. 

Process Understanding – The determination of an empirical relationship between stressors and the respective responses in growth and elemental composition of a phytoplankton species is the necessary first step to estimate the impact on organisms, ecosystems and the cycling of elements. Nevertheless, this commonly used approach cannot explain the observed responses, as it does not provide any information about the underlying processes. Moreover, the sensitivity of a phytoplankton species towards stressors may differ depending on the investigated ‘cellular level’. Only by extending the observations to the level of subcellular processes, we are, therefore, able to make sound statements on what is actually happening in the cells and more importantly why. To achieve this goal, we apply several approaches that target different levels of physiology by using a combination of mass-spectrometric (membrane-inlet mass spectrometry, MIMS), fluorometric (fast repetition rate fluorometry, FRRF) and isotope tracer techniques (14C-/18O-labelled substrates). These in vivo measurements are supported by ‘functional transcriptomics’ to identify the underlying molecular machinery and regulation patterns therein. The deduced patterns are confirmed with basic ‘targeted metabolomics’ approaches, in which quotas of key metabolites are assessed. Measurements on these different levels of cell biology then allow us to explain changes in cellular fluxes of elements (e.g. C, N, P) and energy (e.g. ATP, [e-]) under different future scenarios, and to identify bottlenecks and trade-offs in metabolic pathways that explain different sensitivities of species and groups.  

Species Shifts – Studies that focus on single species and strains alone can obviously not address the complexity of natural communities, as they lack critical aspects of ecological interactions, e.g. competition or grazing. Predictions being purely based on physiological data were in fact found to underestimate the responses in natural environments. For instance, the CO2-dependent decrease in calcification of coccolithophores is far more pronounced in natural assemblages than in single species incubations. This stronger sensitivity was attributed to the fact that rising CO2 not only causes reduced calcification rates per cell, but over time provokes floristic shifts from more heavily to less calcified species and morphotypes. We, therefore, interlink approaches on physiology and ecology by testing the responses of natural assemblages as well as the identified ‘winners and losers’ under the individual scenarios. In this context it is important to address the diversity in response patterns by measuring the reaction norms in several strains, by focusing on those traits that have been shown to affect the competitive abilities of cells and those that may be indicated in our experiments with natural assemblages. This enables us to assess the consequences of altered physiology in an ecological context, identifying the traits that affect the fitness of species, which is a prerequisite to judge whether cellular responses are ‘amplified’ or ‘buffered’ by ecology. The implementation of ecological concepts increases our abilities to predict floristic and functional shifts in future climate scenarios.

FRAM - Our Work group is also involved in the context of the Arctic long-term observatory FRAM (Frontiers of Arctic Monitoring). Based on innovative developments as well as approved technologies, this project will enable the collection of chemical and biological data in high spatial and temporal resolution in the future. The foundation of the FRAM observatory is the preexisting long-term observatory 'Hausgarten'  in the Fram Strait, which connects the Arctic and the Atlantic Ocean. Different from the shallow Aleutian connection to the Pacific, this connection is 5569 meters deep, making it the dominant pathway for the exchange of water masses between the Arctic and the world oceans. The existent, locally fixed measuring units, so-called moorings, are getting equipped with many mobile components like deep-sea robots, ice buoys, gliders and autonomously navigating underwater vehicles. These will enable us to look beyond the 'Mooring Chain' and the 'Hausgarten' into the European northern seas and the Arctic. Phytochange operates the involved in-situ sensors for CO2 and inorganic nutrients that are the primary controlling factors for phytoplankton productivity.