There is no doubt, that global emissions of carbon dioxide have dramatically increased, due to human activities like burning of fossil fuels, deforestation or agricultural activities. Next to the atmosphere, the ocean is the second largest sink for anthropogenic CO2 (1). When entering the oceans surface, CO2 reacts with seawater to form bicarbonate and protons, thereby consuming carbonate ions. The net result of this process - which is termed ocean acidification - is an increase in CO2 and bicarbonate concentrations and a decrease in seawater pH and carbon ion concentration (2,3). The resulting change in seawater chemistry is a predictable consequence and does not suffer from uncertainties associated with climate forecasts (4). If CO2 emissions continue to rise at current rates marine organisms will be exposed to conditions, which they have not experienced during their recent evolutionary history and which may pose a threat to the competitive fitness of pH/CO2 sensitive species and groups (5). Dramatic effects of this ongoing process can already be observed globally. Still, very little is known about the consequences for marine microorganisms, which play a central role in marine nutrient cycling. Therefore the main objective of my research effort is to determine effects of the increased CO2 uptake of the ocean on microbial communities, while studying shifts in their diversity, composition and abundance. Long-term effects are studied on natural systems, mimicking future CO2 levels (e.g. sediments at CO2 vents), while short-term effects are investigated by using mesocosm experiments. The experimental set-up consists of a flow-through system with interconnected, temperature regulated bioreactors, which are filled with heterotrophic sandy sediments. For the bioreactors different gradients of pH, temperature, nutrient supply and CO2 in the porewater can be established. A CO2 gassing system is used to manipulate the carbonate system of the applied seawater at different pCO2 levels. Abundance, composition and activity of the communities in each bioreactor are determined at different time intervals, with a toolkit of diverse molecular techniques (e.g. T-RFLP, ARISA, FISH, qPCR, 16S rRNA and functional gene sequencing, metagenomics). Furthermore parameters like O2-consumption, pH, sulfate reduction, ammonia concentration and accumulation of DIC are monitored. The goal is to assess structural and functional changes as well as resistance, and resilience of the studied communities. My PhD is part of the major national BMBF-research project "Biological Impacts of Ocean ACIDification" (BIOACID), with 14 participating Institutes and Universities. Together with other projects BIOACID is pioneering the large scale research effort on ocean acidification and is closely coordinated with the "European Project on OCean Acidification" (EPOCA, 7th EU Framework).

1. Sabine, C. L., Feely, R. A., Gruber, N., Key, R. M., et al. The oceanic sink for anthropogenic CO2. Science 305, 367 (2004).
2. Zeebe, R. E. & Wolf-Gladrow, D. CO2 in seawater: equilibrium, kinetics, isotopes, 65. Elsevier Oceanography Series (2001).
3. Caldeira, K. & Wickett, M. E. Anthropogenic carbon and ocean pH. Nature 425, 365-365 (2003).
4. Doney, S. C., Fabry, V. J., Feely, R. A. & Kleypas, J. A. Ocean acidification: the other CO2 problem. Annual Review of Marine Science 1, 169-192 (2009).
5. BIOACID - A proposed national ‘Verbundprojekt’ of the Federal Ministry of Education and Research (2009)