Interactions of photobleaching and inorganic nutrients in determining bacterial growth on colored dissolved organic carbon

Bacteria are key organisms in the processing of dissolved organic carbon (D OC) in aquatic ecosystems. Their growth depends on both organic substrates and inorganic nutrients. The importance of allochthonous DOG, usually highl y colored, as bacterial substrate can be modified by photobleaching. In thi s study, we examined how colored DOC (CDOC) photobleaching, and phosphorus (P) and nitrogen (N) availability, affect bacterial growth. Five experiment s were conducted, manipulating nutrients (P and N) and sunlight exposure. I n almost every case, nutrient additions had a significant, positive effect on bacterial abundance, production, and growth efficiency. Sunlight exposur e (CDOC photobleaching) had a significant, positive effect on bacterial abu ndance and growth efficiency. We also found a significant, positive interac tion between these two factors. Thus, bacterial use of CDOC was accelerated under sunlight exposure and enhanced P and N concentrations. In addition, the accumulation of cells in sunlight treatments was dependent on nutrient availability. More photobleached substrate was converted into bacterial cel ls in P- and N-enriched treatments. These results suggest nutrient availabi lity may affect the biologically-mediated fate (new biomass vs respiration) of CDOC.