PHOTOCHEMICALLY PRODUCED CARBOXYLIC-ACIDS AS SUBSTRATES FOR FRESH-WATER BACTERIOPLANKTON

High-molecular-weight dissolved organic matter is abundant in humic la kes and is a large potential source of energy for heterotrophic organi sms. These substances are hard to degrade enzymatically because of the ir high aromaticity and complex structure. However, there is increasin g evidence that photochemical processes render the material more bioav ailable. We demonstrate a substantial photochemical production of four carboxylic acids (oxalic, malonic, formic, and acetic acid) in a humi c lake. The combined production rate in the surface water of these fou r acids was 19 mu g C liter(-1) h(-1) with natural sunlight. Furthermo re, based on radiotracer studies, we found that the amount of carbon a ssimilated and oxidized to CO2 from malonic, formic, and acetic acid e xceeded bacterial carbon production, sometimes by more than one order of magnitude. This implies that carboxylic acids were major bacteriopl ankton substrates. Nevertheless, under natural sunlight at the lake su rface, microbial utilization of carboxylic acids was substantially low er than the photochemical production of the acids. Hence, photochemica lly produced carboxylic acids may accumulate in sunlight exposed envir onments and may also serve as bacterial substrates after mixing into d eeper layers, or during night.