Ultraviolet radiation induced DNA damage in bacterio-plankton in the southern ocean. II. Photochemical & trophic interactions and seasonal patterns of UV response Principal Investigator: Wade H. Jeffrey, PhD Co-principal Investigator: David L. Mitchell, MD Anderson Cancer Center, Smithville, TX Robert V. Miller, PhD. Oklahoma State University, Stillwater, OK Support Agency: National Science Foundation Location of Study: Antarctic Peninsula and Palmer Station, Antarctica

There is now strong evidence that ultraviolet radiation (UVR) is increasing over certain locations including Antarctica and the Southern Ocean where ozone levels have declined as much as 74%. Reduction of ozone concentration selectively limits stratospheric adsorption of UVB radiation, resulting in higher irradiance on the earth's surface. The impact of increased UVB upon phytoplankton and primary production has received extensive interest, while studies of UVR effects on bacterioplankton have been more limited. We examined interactions between bacterioplankton and photochemical processes and interactions with higher trophic groups (e.g. phytoplankton and zooplankton). Direct effects on one trophic group may result in an indirect impact on others. We determined spectral weighting functions for measures of bacterial productivity and compared them to spectral weighting functions determined for phytoplankton from the same water samples. We also determined whether bacterial-phytoplankton coupling modifies bacterial response to UV and the relative proportion of UVR induced DNA damage in different components of the microbial population. Photochemical interactions were examined by determining the impact of chemical photoproducts on bacterial production by adding representative photochemicals (i.e. peroxide, formaldehyde) to seawater samples as well as by examining the effect of irradiated naturally occurring DOM on rates of bacterial growth.
The second goal of this project was to examine microbial community response to changing solar radiation as daylength increases from spring until solstice. These increases in irradiance are greater than those caused by ozone depletion. To monitor seasonal changes in UVR sensitivity, we used a xenon-arc lamp system to provide a constant, reproducible source of full spectrum irradiance. All samples were thus exposed to identical light conditions during measures of bacterial production. Changes in microbial community structure were determined by molecular techniques (DGGE and TRFLPs) and coordinated with changes in sensitivity. Results will provide the most comprehensive trophic analysis of UVR effects in the southern ocean with emphasis on bacterioplankton and indicate whether bacteria in natural communities are adapted to changing solar irradiance conditions. If so, what mechanisms are part of that adaptive process? The larger implication may be in predicting how bacterioplankton in other environments might respond to changing solar UVR and provide a greater understanding of the potential impact that changes in solar UVR (e.g. ozone depletion) may have on marine microbial communities.