Interactions between UVB sensitivity and nutrient availability

Various studies (Shelly et al. 2002, Heraud et al. 2003, Shelly et al. 2005, Roberts et al. 2008) have investigated the relationships between nutrient limitation and exposure to elevated UVBR levels in their effects on rates of photosynthesis in marine phytoplankton. In each of these studies bioreactor cultures of the marine alga Dunaliella tertiolecta were analysed for chlorophyll fluorescence as an indicator of photosystem II (PSII) activity under regimes of replete and limiting nutrient supplies, and with exposure to different levels of both UVBR and PAR. The studies variously measured the effects of nitrogen or phosphorus limitation, and lasted up to 55 days. Shelly et al. (2002) also used a protein synthesis inhibitor that allowed assessment and comparison of rates of repair of UVBR damage. Results demonstrated that exposure to acute doses of UVBR depressed photosynthetic efficiency in all cultures, and that P- or N-limitation both accelerated and exacerbated this response. The exact pathway of these effects is worthy of further investigation. Heraud et al. (2005) propose that P-limitation reduces the availability of P-containing nucleotides, including ATP, and therefore interrupts cellular energy metabolism. Nitrogen limitation may impact cells by depressing their capacity to produce UV screening compounds, and/or by interfering with protein synthesis. 

Dunaliella tertiolecta demonstrated remarkable physiological plasticity by responding to addition of the limiting nutrient within seconds (Heraud et al. 2005, Roberts et al. 2008). Indicators of PSII activity in cultures kept under chronic high levels of UVBR flux suffered less reduction in photosynthesis than other cultures when treated with short-term acute UVBR doses. These cultures also returned to and remained at pre-exposure levels after acute UVBR treatment ceased (Heraud et al. 2005). Some marine algae are apparently able to develop resistance to chronic UVBR exposure, and to recover from both nutrient limitation and acute UVBR dosages. Though UVBR damage was more severe in the most nutrient-limited cultures, these also exhibited much better repair rates after cessation of UVBR exposure (Shelly et al. 2002). Bothwell (1993) likewise found that algal growth was inhibited by UV exposure, but also that this inhibition was unaffected by the degree of phosphorus limitation. This latter finding appears to have been superseded by those of Shelly et al. (2002).