Table 1. Current state of knowledge on microbially mediated process that may be susceptible to ocean acidification.

Process	Significance	State of knowledge
Primary productivity	Major influence on global carbon cycle, as ocean productivity is equivalent to terrestrial primary production.	Laboratory and field incubations showing both little change or enhanced productivity with elevated CO₂. No reliable estimates of how global ocean productivity will change in relation to higher pCO₂/lower pH.
Dominant phytoplankton species	Size of phytoplankton is important, influencing food availability (different grazing efficiencies) and potentially changing population structure of herbivores and carbon export.	Little information available; Kim et al. (2006) found only small changes in diatoms populations, whereas Tortell et al. (2008) report increase in chain-forming diatoms.
Phytoplankton biochemical composition	Food quality may change (for example, protein, lipid and carbohydrate content).	Changes in C/N ratio, with higher C content reported in some studies (Riebesell et al., 2007) but not others (Hutchins et al., 2009); experimental conditions (light and nutrients) are important (Leonardos and Geider, 2005).
Calcification and carbonate dissolution	Calcifying phytoplankton and zooplankton produce particulate inorganic carbon that can alter particle sinking and export flux (ballast hypothesis) and influence bio-optical properties; balance between calcification and carbonate dissolution influences large-scale inorganic carbon cycle and geochemical processes.	Conflicting data on effect of high pCO₂ on coccolithophore calcification with reports of both reduced (Riebesell et al., 2000, 2007) and enhanced (Iglesias-Rodriguez et al., 2008) rates. Ridgwell et al. (2009) emphasize importance of strain differences in culture experiments. There is clearer evidence of reduced calcification for foramaniferia and pteropods (Fabry et al., 2008).
Particle sinking and export of organic carbon	The biological carbon pump controls carbon sequestration in the deep ocean (linked to species composition, as small cells do not sink) and “ballast”—the organic carbon associated with sinking inorganic particles.	Delille et al. (2005) suggested vertical flux will increase in higher pCO₂.
Bacterial respiration and remineralization	The vast majority of organic carbon in the surface oceans is utilized by heterotrophic bacteria; changes in bacterial activity and growth efficiency could profoundly affect the oceanic carbon balance.	Grossart et al. (2006) found changes in bacterial metabolism in high pCO₂, but it was difficult to distinguish direct effects on bacteria from changes in phytoplankton assemblages at high pCO₂.
Nitrogen cycle	Microbes control the cycling of nutrients, particularly, biologically available nitrogen which limits primary productivity in many oceanic provinces. Key processes are nitrogen fixation, denitrification and nitrification.	Hutchins et al. (2007, 2009) and Levitan et al. (2007) found increased rates of nitrogen fixation by Trichodesmium at high pCO₂. Huesemann et al. (2002) and Beman et al. (2009) report reductions in nitrification under elevated CO₂. The impact of realistic CO₂ variations on denitrification has not been examined (Hutchins et al., 2009).
Trace gas production	Microbes drive the production and consumption of many other potent climate-active gases (for example, methane, nitrous oxide) and alter atmospheric chemistry (dimethylsulphide and organohalides).	Hopkins et al. (2010) found decreased production of dimethylsulphide and volatile iodocarbon compounds under high pCO₂ conditions.
Trace metal availability	Altered pH may change trace metal availability, for through changes in pH- and CO₂-dependent chemical speciation and dust dissolution.	Shi et al. (2010) report a reduction in iron bioavailability and phytoplankton uptake under elevated CO₂.