| Summary: | Microbe-mediated processes are crucial for biogeochemical cycles and the functioning of marine ecosystems (Azam and Malfatti 2007 ). If these processes are affected by ocean acidification, major consequences can be expected for the functioning of the global ocean and the systems that it influences, such as the atmosphere. In contrast to phytoplankton, which have been relatively well studied (see Chapter 6), there is comparatively little information on the effect of ocean acidification on heterotrophic microorganisms. Two reviews on the potential effects of ocean acidification on microbial plankton have recently been published (Liu et al. 2010 Joint et al. 2011) . In a recent perspective paper, Joint et al. (2011) concluded that marine microbes possess the flexibility to accommodate pH change and that major changes in marine biogeochemical processes that are driven by microorganisms are unlikely. Narrative reviews, which look at some of the relevant literature, are potentially biased and could lead to misleading conclusions (Gates 2002). Metaanalysis was developed to overcome most biases of narrative reviews. It statistically combines the results (effect size) of several studies that address a shared research hypothesis. Liu et al. (2010) used a metaanalytic approach to comprehensively review the current understanding of the effect of ocean acidification on microbes (including phytoplankton) and microbial processes, and to highlight the gaps that need to be addressed in future research. In the following, a brief digest on oceanic microbes and their role is provided for readers unfamiliar with this topic. Then the research that has been performed to assess the effects of ocean acidification on the diversity and activity of heterotrophic marine microorganisms is reviewed. Finally, scenarios are developed and potential implications are discussed. Microorganisms are defined as organisms that are microscopic, i.e. too small to be seen by the naked human eye, and mostly comprise single-celled organisms. Viruses are ...
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