The effects of UVBR on algae  

UVBR is known to damage marine algae, inhibiting photosynthesis and nutrient uptake, and degrading DNA; this high-energy light penetrates as far as 70m into ocean waters (Karentz et al. 1991, Jeffery et al. 1999), though far less in turbid coastal waters. Ozone depletion, which allows more UVBR to penetrate to the Earth’s surface, has been directly correlated with reduced autotrophic activity in Antarctic waters (Worrest and Häder 1997). Many phytoplankton species produce compounds that screen UV light, and algal cell size and growth habit also influence UVBR exposure levels (Karentz et al. 1991, Jeffery et al. 1999, Beardall and Raven 2004, Shelly et al. 2002)  

Karentz et al. (1991) tested twelve Antarctic marine diatom species for their sensitivity to UVBR as indicated by induction of DNA damage, the rate of repair of this damage, and cell survival. Cultures of each species were irradiated under UVB light of six intensity levels, then treated and assayed. Considerable variation for each response was recorded, and some species were killed by even low levels of irradiation. Higher levels of irradiation caused greater mortality, and moderate levels of irradiation were associated with changes in cell size and morphology. While there was great variation in levels of photodamage suffered, smaller cells sustained significantly more damage per unit DNA than larger diatoms. The ratio of cell surface area to volume was also considered and the linkage between this factor and susceptibility to damage statistically assessed (Karentz et al. 1991); the authors attributed UVBR exposure as one of several determinants of cell size. 

Prior to Bothwell et al. (1993), most studies of the effect of UV radiation on algae had been of very short duration, and longer-term studies are still few. These researchers investigated the effects of photosynthetically active radiation (PAR), and UVBR on freshwater algal growth rates and community composition, and hence cell sizes. UVBR exposure was observed to have a strong positive correlation with larger average cell size, possibly indicating a succession toward larger dominant species (Bothwell et al. 1993). The explanation, in Karentz et al. (1991) that larger cells are less susceptible to UVBR-induced damage, is cited, and the authors postulate that exposure to both UVA and UVB (as a component of PAR) elicited the production of UV-screening compounds. (Karentz et al. 1991, Bothwell et al. 1993).  

Thus increased UVBR flux has a more negative effect on small phytoplankton than on larger species, and can induce shifts in the mean size of organisms in algal assemblages. Further, exposure to high rates of UVBR over a sustained period confers some resistance to subsequent acute exposures (Heraud et al. 2005). It is reasonable to expect that larger cells will have more inherent protection against radiation damage, because light has further to travel through their cytoplasm before reaching the nucleus. Where such cells produce UV-screening compounds they will be further advantaged, and differential capacity to repair photodamage may also confer greater fitness on some species. Interplay of UVBR flux and other environmental factors causes a range of responses in algal cells and communities, and have the potential to alter algal community compositions. (Beardall and Raven 2004). The sea surface radiance data collected by Behrenfeld et al. (2006) could usefully be coupled with measurements of UVBR flux at the ocean surface to gain an appreciation of these phenomena.