Enhanced bacterial growth in response to photochemical transformation of dissolved organic matter

Abstract

We tested the hypothesis that light, especially UV‐B radiation, increases the availability of dissolved organic matter (DOM) to pelagic bacteria in lake water. Filtered (0.2 µ m) and autoclaved humic lake water (water color 70 mg Pt liter−1, 12 mg dissolved organic C liter−1) was exposed to simulated sunlight (UV‐B, 1.14 W m−2; UV‐A, 3.87 W m−2; and PAR, 20 W m−2) for various periods of time (0–100 h). Irradiated water was then inoculated with a natural bacterial assemblage (0.6‐µ m filtered water) and bacterial yield was measured in the stationary phase of the resulting batch cultures. Both bacterial numbers and cell volumes increased (numbers by 65% and volumes as much as 360%) with increasing UV radiation, resulting in an almost sixfold increase in bacterial biomass. Our experiment shows that light exposure can enhance availability of natural lake DOM to bacteria, possibly through cleavage of macromolecules into smaller units, and may influence both carbon cycling and food webs in lake water.