Decreased biodegradability of algal DOC due to interactive effects of UV radiation and humic matter

The effect of dissolved humic matter (DHM) and UV radiation of intensities similar to solar radiation on the bacterial degradability of algae-derived dissolved organic carbon (DOC) was examined. A C-14-labeled algal extract from a phytoplankton culture was dissolved in organic-free artificial lake water with or without the addition of humic substances. The water was exposed to UV radiation or kept in darkness for 8 h. Subsequently, an inoculum of lake water bacteria was added, and microbial mineralization to (CO2)-C-14 was monitored during 8 d in darkness. DHM and exposure to UV radiation had interactive effects on the susceptibility of algal DOC to microbial mineralization. The DOC became less available to the microbes after UV radiation in the presence of DHM. However, it was not affected by DHM in darkness, and there was also no impact of radiation in the absence of DHM. The interactive effects of DHM and radiation did not occur when DHM was applied as an optical filter outside of the experimental vessel. Thus, the effect was independent of the absorbing properties of the colored humic substances and was probably the result of direct physico-chemical reactions between DHM and DOC freshly derived from algae. Our findings suggest, a pathway for the production of recalcitrant DOC in pelagic waters.