Exposure of natural Antarctic marine microbial assemblages to ambient UV radiation: effects on bacterioplankton

Seasonal ozone depletion over Antarctica leads to enhanced UVB (280 to 320 nm) radiation throughout the period of greatest biological production. The effect of UV radiation on bacterioplankton has received little attention, a nd its effects on marine microheterotrophs and viruses, which mediate bacte rial biomass, are poorly understood. This study examined the impact of ambi ent solar UV radiation on bacterioplankton in natural Antarctic microbial c ommunities. Following a lag of 2 d, bacterial concentrations increased all light treatments. Inhibition of bacterial growth increased with increasing UV irradiance and duration of exposure, reaching 27% inhibition in high UV treatments (less than or equal to 2.0 m equivalent depth) compared to contr ols after 7 d exposure. Bacterioplankton growth rates declined in all treat ments during post-UV incubation, particularly at lower UV irradiances (grea ter than or equal to 3.0 m equivalent depth), indicating UV-induced inhibit ion of bacterial mortality during irradiation. Positive bacterial growth co incided with both phytoplankton mortality and increased microheterotroph co ncentrations following exposure to high UV irradiances. Exposure of Antarct ic microbial communities to ambient UV is likely to increase microbial resp iration of carbon in surface waters and reduce vertical carbon flux.