CO2 concentrating mechanisms and environmental change

• Great diversity of aquatic organism CCMs, sometimes operating in parallel or series in a given cell.
• CCMs of aquatic organisms occur in lower PARs and lower temperatures than does terrestrial C4.
• Aquatic organism CCMs, but not terrestrial C4, typically acclimate to environmental changes.
• Aquatic CCMs show complex responses to components of global environmental change.

The diversity of CCMs among aquatic oxygenic photolithotrophs causes difficulties in generalising about their responses to environmental change. A frequent response of organisms with CCMs to increasing CO2 is increased organic carbon content, even if (as is often the case) the growth rate is not increased. In the few studies performed, the internal carbon dioxide concentration relative to the growth carbon dioxide concentration is decreased for organisms grown at elevated CO2, but not sufficiently so to alter the degree of Rubisco saturation with CO2. The more commonly measured affinity for external CO2 generally also decreases with increased CO2 concentration for growth. The present global distribution of cyanobacteria and algae with CCMs over a wide temperature range contrasts with terrestrial C4 plants which mainly grow in warmer habitats. This suggests that algae and cyanobacteria will benefit less from increasing global temperatures in terms of geographical range than will terrestrial C4 plants. The general correlate of increased temperature and a shallowing (shoaling) of the thermocline means that phytoplankton will experience a higher mean PAR and UV-B flux, and a decreased availability of combined nitrogen, phosphorus and iron. The general conclusion is that this combination of environmental changes will in part offset the decreased CCM activity in response to increased CO2. The few data available on responses to environmental change of the minority of aquatic oxygenic photolithotrophs lacking CCMs, suggest some similarities of responses of these organisms and those with CCMs. Organic carbon burial (“blue carbon”) by undisturbed seagrass beds will probably increase under environmental change.