Exposure of dissolved organic matter to UV-radiation increases bacterial growth efficiency in a clear-water Alpine stream and its adjacent groundwater

In many streams, microbial growth largely relies on terrigenous organic carbon that has been initially stored in soils and that is generally believed to be recalcitrant to microbial metabolism. The various mechanisms that underlie the availability of terrigenous organic carbon as it enters streams remain poorly understood. One possible mechanism can be photodegradation of terrigenous dissolved organic carbon (DOC) upon exposure to sunlight in streamwater. To explore this, we experimentally exposed streamwater, shallow groundwater and soil water from a clear-water Alpine headwater stream, and both soil and algal extracts, to UV-radiation and studied the effects on DOC optical properties and implications for microbial growth. Our results on the apparent quantum yield suggest that DOC from groundwater and soil water is more resistant to photodegradation than DOC in the streamwater itself. This would highlight the relevance of the exposure history of DOC to sunlight. Overall, UV-radiation decreased the aromaticity (as SUVA254) of the DOC and reduced its molecular weight as indicated by the slope ratio, S (R) (S (275-295)/S (350-400)). UV-treatment significantly increased bacterial growth rate and bacterial growth efficiency in the streamwater, soil water, groundwater and soil extract but not in algal extract. Our findings suggest photodegradation as one mechanism that contributes to the microbial utilisation of terrigenous DOC even in clear-water streams.