Microbial Metabolic Activity and Bioavailability of Dissolved Organic Matter Under the Impact of Intense UV Radiation in Pony Lake, Antarctica

Pony Lake is a saline and hypereutrophic coastal pond located on Cape Royds in the McMurdo Sound area of Antarctica. This shallow lake is ice-covered except in midsummer, when strong winds typically cause thorough mixing of the water column. The source of water appears to be accumulated snow; water is lost by ablation of the ice cover and evaporation of lake water in midsummer. In the west the pond is bordered by an Adelie penguin rookery. Previous studies have shown that Pony Lake can have very high dissolved organic carbon (DOC) concentrations (~ 100 mg C liter -1). Furthermore, Pony Lake is unique because it lacks terrestrial carbon inputs in the watershed, which makes this an excellent example of a system containing autochthonous microbially (algae, cyanobacteria, bacteria and viruses) derived organic matter. From an ecological perspective dissolved organic matter (DOM) acts as a carbon source for microorganisms, absorbs harmful ultraviolet radiation, and can participate in biogeochemical redox reactions, whereas different fractions and chemical characteristics influence the bioavailability and chemical reactivity of DOM in aquatic ecosystems. While the DOM concentration in Pony Lake is high, the percentage of DOC accounted for by fulvics acid is low, as is observed in other lakes with algal derived DOC sources. Algal derived fulvic acids are yellowish in color, and absorb light to a lesser extend compared to terrestrially derived fulvic acids. Fulvic acids from Pony Lake are enriched with nitrogen. Over two field seasons we have investigated the influence of photolytic processes on the microbial utilization of DOM from Pony Lake, Antarctica. We have determined that the intense ultraviolet radiation in Antarctica rapidly photo-bleaches DOM, resulting in the loss of UV absorbing compounds, and rendering fractions of the DOM pool less biologically available to microbes. We monitored microbial community structure, abundance and primary and secondary production over the austral summer as the lake transitioned from fully ice covered to ice free. Bacterial numbers ranged from 1.10 x 106 4.09 x 107 cells ml-1 in ice core samples and from 2.15 x 105 to 3.22 x 107 cell ml-1 in ice free Pony Lake due to seasonal shifts in melting as well as alternations in the DOM pool. Secondary production was higher in the ice free lake compared to the ice core samples. 24 h exposure experiments of natural bacterial assemblages to in situ sunlight demonstrated that tritiated thymidine incorporation was significantly less than compared to dark controls. Our findings indicate that the intense solar radiation in Antarctica effects the microbial community, alters DOM bioavailability and composition and the microorganisms are not only over-wintering in the frozen ice cover of Pony Lake, they are metabolically active.