Introduction   

Oceans cover 70% of the Earth’s surface and water is a substrate or reactant in nearly all chemical reactions in the biosphere, so changes in the properties of environmental water are of particular importance to life on the planet. The pelagic ocean is the largest of the Earth’s ecosystems, extending over this huge proportion of the planet’s surface and extending to great depth. The euphotic zone of the ocean, defined as that area where light penetrates at sufficient intensity to allow photosynthesis, may occupy 200-300 metres depth from the surface. Photosynthetic, unicellular marine organisms are ubiquitous in this zone and are at the base of oceanic food chains, sustaining both ecosystems and global fisheries, and also play an important role in the global carbon cycle. 

Oceanic phytoplankton perform up to half of global primary production, and have removed from the atmosphere around 30% of carbon emitted as a result of the combustion of fossil fuels since the industrial revolution (Beardall and Raven 2004, Behrenfeld et al. 2006). Damage to the ozone layer has increased the amount of ultraviolet-B radiation (UVBR) reaching the biosphere. The wavelength of UVBR is short, and the energy per photon is great, and this light is known to have deleterious effects on organisms’ DNA and metabolic processes.  The ocean has absorbed more than 80% of the Earth’s heat increase, associated with elevated CO₂ and the greenhouse effect, over recent decades, and this trajectory is likely to continue (Barnett et al. 2005, Levitus et al. 2005). Sea surface temperatures are predicted to rise by an average of 2-3°C over the next 100 years, and there is the potential for the Earth’s climate to transition to a permanently warmer state (Beardall and Raven 2004, Wara et al. 2005). Higher temperatures are associated with increased biological activity so may have a positive influence on some species of algae and in some latitudes, but more importantly by enhancing the difference in density between surface and underlying waters, the warming of ocean surface waters increases stratification. This has the potential to limit upwelling and hence reduce the availability of nutrients, especially phosphorus and nitrogen, to photosynthetic plankton (Fig. 1). 

An interesting nexus exists between the effects of environmental changes and the selection pressure they bring to bear on phytoplankton, and this informs the scope of the current review. Will the size of individual algal cells influence the viability of their species under changing UVBR flux and nutrient availability regimes, and hence influence species assemblages? Algal cells are reliant on diffusion for gas exchange and nutrition, so cell size has a strong impact on their metabolic efficiency. Larger cells are also less susceptible to damage by UVBR (Bothwell et al. 1993), and many species are capable of producing UV-screening substances (Jeffrey et al. 1999). 

The long-term effects of environmental change on algae have rarely been studied (Shepherd et al. 2009). Marine phytoplankton species are likely to be differentially resistant to a combination of changes in nutrient availability and UVBR flux, and this may drive significant change in algal assemblages.  Any disruption to algal physiology or assemblages has the potential to seriously perturb marine biodiversity and may also contribute to atmospheric CO₂ through positive feedback loops (Ware and Thomson 2005, Shepherd et al. 2009, Wohlers et al. 2009). These downstream effects will not be addressed in this review. Nor will this paper consider changes to pH, an important co-effect of an increase in dissolved CO₂ in the world’s oceans, or the role of phytoplankton in global carbon cycling.