Combined effects of temperature and resource availability on organic matter degradation by Antarctic bacterioplankton

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Global warming poses new threats to marine ecosystems since rising seawater temperature potentially induces cascading effects in biogeochemical cycles and food webs. The Scientific Committee on Antarctic Research (SCAR) identified a better understanding of potential effects of climate change on the physical and biological uptake of CO2 by the Southern Ocean as one of the most pressing tasks in Antarctic research. Heterotrophic bacteria are the main producers of CO2 in the ocean, thereby counteracting the biological drawdown of CO2 by primary production. In Antarctic marine systems, low seawater temperature, and the low availability of labile organic matter are major environmental constraints on bacterial growth and degradation activity. However, temperature and the availability of resources for heterotrophic bacteria undergo considerable change induced by climate warming combined with subsequent ice melt and changes in primary productivity. Previous laboratory experiments conducted with batch cultures revealed a high potential of temperature effects on bacterial growth near the lower temperature limit to interact with the concentration of organic substrates. Due to this interaction temperature effects on bacterial growth were disproportionately strong when substrates became available. However, the relevance of this synergistic effect for bacterial productivity and subsequent CO2 release by natural communities of the polar oceans is still unclear. This project aims to test single and combined effects of temperature and organic matter availability on Antarctic marine bacterioplankton. For this purpose, activation energies of extracellular enzymatic reactions, substrate uptake rates and biomass production will be examined in bacterial communities of the Weddell Sea. Rate measurements in the Weddell Sea will be combined with the analysis of organic matter to estimate temperature effects on fluxes of labile and semi-labile organic carbon. Results will be used to explore the natural range of temperature sensitivity and its modulation by abiotic and biotic factors. On-board experiments will investigate combined effects of warming and substrate addition on bacterial community composition, patterns in gene expression and organic matter turnover to link changes in community structure to changes in community functioning. Furthermore, chemostat experiments with bacterial isolates from the Southern Ocean will be conducted to quantify temperature effects on growth efficiencies and the chemical composition of bacterial biomass at substrate-limited growth on carbohydrates of different molecular complexity. A better determination of bacterial remineralization and its dependence on temperature and substrate concentration will help to improve the parameterization of biogeochemical models that aim to project the marine carbon cycle and the ocean-atmosphere CO2 exchange in a changing climate.