Effects of chronic, low levels of UV radiation on carbon allocation in Cryptomonas erosa and competition between C.erosa and bacteria in continuous cultures

We conducted a long term (4 week) continuous culture study to measure the chronic effects of UV radiation on the alga, Cryptomonas erosa, using three different fluence rates of UV radiation. We measured carbon allocation into carbohydrate, protein and lipid pools, as well as chlorophyll a concentrations and algal and bacterial density. After 21 days, algal density in the control and lowest UV treatment (treatment 1 = 3.4 W m–2 UVR unweighted) was significantly lower than in the two highest UV treatments (treatment 2 = 14.9 W m–2 and treatment 3 = 16.2 W m–2 UVR unweighted), and did not recover in the following week of no UV exposure. Chlorophyll a and carbohydrate content (ng algal cell–1) for the control and treatment 1 were clearly lower than treatments 2 and 3 by day 15, and did not recover by day 28. Percentage total lipid for the control and treatment 1 also decreased compared with treatments 2 and 3 by the end of the exposure period. However, by day 21, protein content for the control and treatment 1 was significantly higher than treatments 2 and 3, and demonstrated a further increase by day 28. The results were largely attributed to competition effects between C.erosa and bacteria in these non-axenic cultures. Bacterial density was significantly (×4) higher in the control and lowest UV treatment compared with the two highest UV treatments. Our findings suggest a competitive advantage of phytoplankton over bacteria under these conditions. If UV radiation, in general, affects bacteria to a greater extent than algae, there are likely to be changes in (i) bacterial utilization of dissolved organic matter produced by phytoplankton, (ii) competition between phytoplankton and bacteria for nutrient minerals and (iii) predation rates on bacteria by micro-flagellates.