Production of microbially-derived fulvic acid from photolysis of quinone-containing extracellular products of phytoplankton

Abstract.

Photochemical and microbial processes in surface waters can result in changes in the chemical quality of dissolved organic matter (DOM), which may in turn influence other ecosystem processes. Microbially-derived non-humic DOM can enter the aquatic ecosystem through extracellular release. Further, it has been hypothesized that microbially-derived fulvic acid can form through condensation reactions involving non-humic DOM. We used spectroscopic measurements and parallel factor analysis (PARAFAC) modeling to characterize whole water and fulvic acid fractions of DOM at the outlet of an alpine lake in the Colorado Rocky Mountains during snowmelt through the summer growing season. Monitoring of the biogeochemical conditions at the outlet of the lake and conservative transport modeling of the PARAFAC components showed that quinone-like moieties of the microbially-derived non-humic DOM became more oxidized than the quinone-like moieties of the microbially-derived fulvic acid. Our findings suggest that while both microbial activity and photochemical processes play an important role in DOM cycling in lakes, some process directly correlated with hydraulic residence time may be particularly important for the redox active fraction of the non-humic DOM. Furthermore, biological monitoring results indicate that temporal changes in DOM chemistry cannot be attributed to changes in the biovolume of any one group of phytoplankton or the total number of bacteria. We present a conceptual model of biogeochemical processes influencing the production and decay of quinone-like moieties associated with the microbially-derived non-humic and fulvic acid fractions of the DOM in the lake. Additionally, we propose that the formation of microbially-derived fulvic acid is driven by photolysis of the microbially derived non-humic DOM pool and possibly associated condensation reactions.