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 utilization of various carbon and nitrogen compounds were studied in July in a thermally stratified lake in southern Sweden. Exposure of bacteria-free water to natural sunlight in the surface of the lake for 7 h around noon led to higher concentrations of inorganic carbon (39–80%), amino acids (0–23%) and carbohydrates (0–15%), while lower concentrations of monosaccharides (0–38%), nitrate (0–23%) and urea (0–27%) were measured. Ammonium was unchanged. Lake bacteria were inoculated into the irradiated water and into water that had not been exposed to solar radiation (dark controls). The bacterial production was 35 to 80% higher during exponential growth (20 h after inoculation) in the irradiated samples than in the controls. The bacterial utilization of specific carbon and nitrogen compounds in the irradiated samples differed from that in the controls, but the changes in the epilimnion and the hypolimnion varied. Dominant nutrients to the bacteria were carbohydrates, amino acids, glucose and ammonium. In the controls a release of combined amino acids (epilimnion) or carbohydrates (hypolimnion) occurred. An apparent non-biological removal of urea in the irradiated hypolimnion samples was found, since the microbial urea degradation was only 1% of the 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, including amino acids, carbohydrates, nitrate and urea.