Anthropogenic CO2 emissions are causing an acidification of the world’s oceans. The consequences for marine organisms and especially heterotrophic bacteria remain under debate, and almost nothing is known concerning marine fungi. Both microbial groups are important players in organic matter decomposition and nutrient cycling, and their pH tolerance is known to be broad in relation to the predicted acidification. So far, ocean acidification effects on marine bacterial communities have mainly been investigated in large-scale mesocosm studies. In these systems, indirect effects mediated through complex food web interactions come into play. Until now, these experiments were not carried out in sufficient replication. In this thesis, we chose an alternative approach and investigated bacterial and fungal communities in highly replicated microcosm experiments (1-1.6 L). The duration of the experiments was four weeks. We incubated the natural microbial community from Helgoland Roads (North Sea) at in situ seawater pH, pH 7.82 and pH 7.67. These pH levels represent the present-day situation and acidification at atmospheric CO2 of 700 or 1000 ppm, projected for the southern North Sea for the year 2100. For the bacterial community, different dilution approaches were used to select for different ecological groups. Seasonality was accounted for by repeating the experiment four times (spring, summer, autumn, winter). In a second experiment repeated in two consecutive years, we investigated direct pH effects on marine fungal communities. We additionally isolated marine yeasts and identified them by Matrix-assisted laser desorption/ionization-time-of-flight mass spectrometry (MALDI-TOF MS) and partial sequencing of the large subunit (LSU) rRNA gene. To reveal changes in community structure, we applied the culture-independent fingerprint method automated ribosomal intergenic spacer analysis (ARISA) for both bacteria and fungi. Bacterial communities were furthermore analyzed by 16S ribosomal amplicon pyrosequencing. Abundances were determined by flow cytometry (bacteria) and colony forming unit counts (fungi). To be able to interpret results comprehensively, we determined the natural variability of the carbonate system at Helgoland Roads over a yearly cycle. We found that from September 2010 to September 2011, pH at Helgoland Roads ranged from 8.06 to 8.43, corresponding to partial pressures of carbon dioxide (pCO2) of 215-526 µatm. The acidification predicted for the year 2100 consequently represents a strong perturbation of the system. Bacterial communities developing in the microcosms were primarily influenced by season and dilution, demonstrating that diverse communities had been generated. We predominantly found pH-dependent shifts in bacterial community structure already at pH 7.82. Groups involved in these shifts were different members of Gammaproteobacteria, Flavobacteriaceae, Rhodobacteraceae, Campylobacteraceae and further less abundant groups. While Rhodobacteraceae were consistently less characteristic for reduced pH, Campylobacteraceae profited from pH reduction. For most other bacterial groups however, pH effects were context-dependent, i.e. dependent on season, dilution or an interaction of effects. Regarding bacterial abundance, no pH effect was found. Fungal community structure was significantly different between both years of the experiment, hinting at inter-annual variability. Shifts in response to pH occurred predominantly only at pH 7.67. In contrast, a strong pH effect was observed on fungal abundance. In comparison to in situ pH, fungal numbers were on average 9 times higher at pH 7.82 and 34 times higher at pH 7.67. Concerning marine yeasts, Leucosporidium scottii, Rhodotorula mucilaginosa and related species, as well as Cryptococcus sp. and Debaromyces hansenii reacted positively to low pH. Our findings demonstrate that already small reductions in pH have direct effects on both bacterial and fungal communities. A tipping point for community shifts appears to be reached earlier for bacteria than for fungi. Regarding bacteria and yeasts, both naturally abundant groups and rare species were affected by pH reductions. The strong increase in fungal numbers at reduced pH suggests that with ocean acidification, marine fungi may reach higher importance in marine biogeochemical cycles and as infectious agents. Using a microcosm approach, a robust analysis of direct ocean acidification effects on marine bacterial and fungal communities was accomplished. Results yield valuable hypotheses to test in future large-scale and long-term studies.