CO₂ concentrating mechanisms and 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.