EFFECTS OF SUNLIGHT ON BACTERIAL-GROWTH IN LAKES OF DIFFERENT HUMIC CONTENT

We investigated the bacterial response to sunlight in 5 oligotrophic l akes of different humic content [dissolved organic carbon (DOC) 3.9 to 19 mg l(-1), water color 0 to 140 mg platinum (Pt) l(-1)] in southern Sweden. Lake water including free-living, heterotrophic bacteria was exposed in situ in quartz tubes al 5 depths (0 m to Secchi depth) from sunrise to sunset during July. In sunlight exposed surface water, H-3 -leucine uptake, measured immediately after retrieval of the tubes at dusk, was inhibited in all lakes by 23 to 85% (most severely in clear water) compared to dark controls. Inhibition was detectable al 2 m in the dearest lake while in the most humic lake there was no effect belo w 0.2 m. Depth integrated loss of bacterial production in light tubes compared to dark ones decreased with increasing DOC content, ranging f rom 23% (clear lake) to 1-4% (humic lakes). To study the indirect effe cts of solar light on bacteria, due to phototransformation of dissolve d organic matter (DOM), 0.2 pm filtered lake water was exposed from su nrise to sunset at 4 depths (0, 0.2, 0.65 and 2 m) and then inoculated with bacteria, which were allowed to grow in darkness. At stationary phase the abundance and cell volume increased by 23 to 99% and 20 to 1 23%, respectively, related to dark exposed controls, resulting in incr eases in biomass between 83 and 175%. The depth integrated Light media ted increase in bacterial carrying capacity (23 to 34%) showed no rela tion to DOC content. Direct inhibition of growth by sunlight, and indi rect stimulation of growth from increased availability of DOM, were de tectable to depths greater than detectable UV-B penetration, indicatin g that wavelengths >320 nm affect bacterial growth. We suggest that du ring exposure to daylight, bacteria are mainly negatively affected by UV light, especially in clear waters, but a simultaneous stimulation d ue to phototransformation of recalcitrant DOM into more available form s occurs. Time spent by cells and DOM in Light exposed zones during mi xing determines the relative importance of these counteracting process es.