Effects of solar irradiation and nitrate on dissolved organic matter and freshwater bacterioplankton

In surface waters, solar radiation transforms complex dissolved organic compounds into photoproducts that are more readily available for bacterial metabolic use. In this way, photochemical processes can influence both carbon cycling and freshwater bacterioplankton communities. We experimentally studied photochemical transformation of dissolved organic carbon (DOC) in the presence or absence of nitrate added as a photosensitizer and followed how this affected growth and community composition of heterotrophic bacteria. As expected, bleaching of chromophoric dissolved organic matter (cDOM) was strongly positively correlated to total energy content of the solar irradiance. In contrast, the addition of nitrate had no significant impact on neither DOC photodegradation or bleaching. Bacterial growth assays revealed extensive solar-driven transformation of biologically recalcitrant carbon compounds into more labile forms that led to enhanced bacterial growth. Based on 16S rRNA community analyses applied to regrowth cultures, we demonstrate that solar exposure and to a lesser extent also the nitrate-mediated photosensitization, had a significant influence on the taxonomic composition of bacterioplankton communities. All communities were dominated by class Proteobacteria, and notably known bacterivorous taxa such as Bdellovibrio and like organisms (BALOs) within the class of Oligoflexia were strongly favored by DOM exposed to solar irradiation while this effect was not seen when irradiation was combined with nitrate as photosensitizer. In addition, taxa which seem to have a copiotrophic lifestyle such as Acin (Gammaproteobacteria) showed a positive response to the irradiation and nitrate addition while the opposite was found for LD12, acI-A and alfVI that are considered to have a more oligotrophic one. Further, ambiguous responses from the treatments were observed as bal-A1 (FukuN47) belonging to Bacteroidetes only responded positively to the irradiation in the absence of nit