Cellular and ecological effects   

Ultraviolet radiation can kill algae and other marine microorganisms, and moderate levels can cause changes in cell size and morphology (Karentz et al. 1991, Beardall and Raven 2004 and references therein). It has also been demonstrated to influence marine bacteria community composition (Arrieta et al. 2000). Phytoplankton species that produce UV screening compounds are likely to show greater fitness than other taxa, and this would promote change in algal assemblages. The same mechanism may force microevolution toward more effective UVBR protection. Cell size, habitat and growth habit also influence species’ level of exposure to UVBR (Jeffery et al. 1999, Shelly et al. 2002, Beardall and Raven 2004), and UVBR exposure has been correlated with larger average cell size (Bothwell et al. 1993). This also suggests a succession toward larger species as dominant. However, because of their inefficient harvesting of ambient CO₂, many marine phytoplankton will be unable to increase their size beyond certain limits. Any increase in cell size also implies a reduced ratio of surface area to volume. Because of the cells’ reliance on diffusion both for gas exchange and to obtain dissolved inorganic nutrients, immutable physical barriers will limit size increase in algal cells. Any putative increase in primary production as a result of increasing temperature would also be subject to such restrictions. Nevertheless, changes to algal cell ultrastructure and physiology, and to species composition of algal communities, have been observed in response to both artificially enhanced CO₂ concentrations and UVBR flux (Beardall and Raven 2004 and references therein). It is also possible that some extant taxa are capable of benefiting from changed environmental conditions as described. There is scope to investigate effect of cell size and its interactions with the environmental stresses discussed. This could possibly be done by culturing a number of marine algae species, selected to represent a range of cell sizes, in separate bioreactors and measuring their performance under experimentally varied conditions, after the methods of Shelly et al. (2002, 2005) and Roberts et al. (2008). 

Behrenfeld et al. (2006) found that global net photosynthetic production by oceanic algae had decreased, but localised eutrophication is linked with increasing algal biomass (Suikkanen et al. 2006). This points to the variable and sometimes-localised nature of fluctuations marine phytoplankton abundance and activity. Localised blooms in inshore waters can be toxic to other marine life, and increased stratification is also associated with changes in microorganism assemblages (Suikkanen et al. 2006). Selection pressure will cause widespread changes to phytoplankton species assemblages in a warming world. Changing equilibria between photosynthesis and respiration indicate the potential for significant alteration of the functioning of trophic systems (Wohlers et al. 2009). The combined effects of rising ocean CO₂ concentrations, temperature, UVBR flux and nutrient limitation due to increased stratification, are of considerable importance in pelagic ecosystems. Turnover of the entire global planktonic biomass takes only a few days (Behrenfeld et al. 2006). This confirms the importance of marine microorganisms as primary producers at the base of oceanic food chains and points to the potentially catastrophic impact of the disruption of these systems. Through its contribution to positive feedback loops associated with global warming, damage to marine microalgae and alteration of algal assemblages have broad impacts that extend far above the highest tide.