Iron manganese co-limitation – a potential driver of Southern Ocean phytoplankton ecology

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The Southern Ocean (SO) exerts a disproportional control on the global carbon cycle, thus affecting climate at a global scale. Southern Ocean phytoplankton are major drivers of global carbon cycling accounting for 20% of the global annual primary production. In the SO, the biological uptake of carbon dioxide is mainly controlled by the availability of the trace metal iron (Fe) and light, both being essential for photosynthesis. Whereas most studies so far focused on disentangling how Fe and light influence SO phytoplankton growth and productivity, almost nothing is known whether other trace metals such as manganese (Mn) also act as limiting or co-limiting nutrient in the SO. Mn is required for various cellular processes in phytoplankton cells and has been suggested to be (co-)limiting with Fe in some parts of the SO such as the Drake Passage. Currently, we lack information on growth, photosynthesis, oxidative stress and Mn and Fe requirements under Fe-Mn limitation for SO phytoplankton species. This research project aims to improve our understanding about the potential effects of Fe-Mn co-limitation on SO phytoplankton physiology and their implications for the ecology and biogeochemistry in the present and future SO. To this end, the responses of SO phytoplankton will be studied in response to altered Fe and Mn availabilities under different light conditions, simulating different climate change scenarios. A multidisciplinary approach will be followed that integrates biology and marine chemistry and combines laboratory and field work. Laboratory experiments with ecologically and biogeochemically important phytoplankton species of the SO will provide a mechanistic understanding on physiological processes such as photosynthesis and trace metal requirements. Shipboard manipulation experiments with natural SO phytoplankton assemblages of different regions of the SO will further reveal the occurrence of Fe-Mn co-limitation and will help to identify phytoplankton species in the field, which are particularly sensitive, but also tolerant towards altered trace metal and light availabilities. All together this research project will help to assess an ecophysiological explanation for the spatial distribution of SO key phytoplankton species in the present and the future SO. Improved knowledge on the functioning and the sensitivity of the SO ecosystem is pivotal to improve our existing predictive tools (e.g. modelling) and increase our understanding on the mechanisms, by which the SO affects climatic processes at global scale. Therefore, this research project will significantly contribute to the key research topic 2.2.4 Response to Environmental Change within the DFG Schwerpunktprogramm 1158 Antarktis-Forschung.