Photochemically produced carboxylic acids as substrates for freshwater bacterioplankton

High-molecular-weight dissolved organic matter is abundant in humic lakes and is a large potential source of energy for heterotrophic organisms. These substances are hard to degrade enzymatically because of their high aromaticity and complex structure. However, there is increasing evidence that photochemical processes render the material more bioavailable. We demonstrate a substantial photochemical production of four carboxylic acids (oxalic, malonic, formic, and acetic acid) in a humic lake. The combined production rate in the surface water of these four acids was 19 mu g C liter(-1) h(-1) with natural sunlight. Furthermore, based on radiotracer studies, we found that the amount of carbon assimilated and oxidized to CO2 from malonic, formic, and acetic acid exceeded bacterial carbon production, sometimes by more than one order of magnitude. This implies that carboxylic acids were major bacterioplankton substrates. Nevertheless, under natural sunlight at the lake surface, microbial utilization of carboxylic acids was substantially lower than the photochemical production of the acids. Hence, photochemically produced carboxylic acids may accumulate in sunlight exposed environments and may also serve as bacterial substrates after mixing into deeper layers, or during night.