EFFECTS OF SUNLIGHT ON OCCURRENCE AND BACTERIAL TURNOVER OF SPECIFIC CARBON AND NITROGEN-COMPOUNDS IN LAKE WATER

The effects of solar radiation on concentrations and microbial utiliza tion of various carbon and nitrogen compounds were studied in July in a thermally stratified lake in southern Sweden. Exposure of bacteria-f ree water to natural sunlight in the surface of the lake for 7 h aroun d noon led to higher concentrations of inorganic carbon (39-80%), amin o acids (0-23%) and carbohydrates (0-15%), while lower concentrations of monosaccharides (0-38%), nitrate (0-23%) and urea (0-27%) were meas ured. Ammonium was unchanged. Lake bacteria were inoculated into the i rradiated water and into water that had not been exposed to solar radi ation (dark controls). The bacterial production was 35 to 80% higher d uring exponential growth (20 h after inoculation) in the irradiated sa mples than in the controls. The bacterial utilization of specific carb on and nitrogen compounds in the irradiated samples differed from that in the controls, but the changes in the epilimnion and the hypolimnio n varied. Dominant nutrients to the bacteria were carbohydrates, amino acids, glucose and ammonium. In the controls a release of combined am ino acids (epilimnion) or carbohydrates (hypolimnion) occurred. An app arent non-biological removal of urea in the irradiated hypolimnion sam ples was found, since the microbial urea degradation was only 1% of th e reduction in concentration. Our results suggest that biogeochemical cycling in natural waters is influenced by sunlight, due to changes of microbially available components that were not reported previously, i ncluding amino acids, carbohydrates, nitrate and urea. (C) 1998 Publis hed by Elsevier Science B.V. All rights reserved.