Bacterial growth in humic waters exposed to UV-radiation and simulated sunlight

Sterile filtered (0.45 μm) humic lake water was exposed to simulated sunlight (300–800 nm) or UV-radiation (254 nm) for various periods of times and the dissolved organic carbon content, absorbance at 254 and 460 nm and pH were recorded. The irradiated water was inoculated with a natural bacterial assemblage and the number of viable bacteria was estimated 3 and 5 days after the inoculation using the plate count technique. The number of viable bacteria increased with the irradiation time indicating that the chemical changes of the humus macromolecules observed during irradiation resulted in an increased availability of the dissolved organic material as bacterial substrate. No inhibitory effect on the bacterial growth, due to the presence of toxic organic reaction products, was observed.