Intriguing changes in molecular size and composition of dissolved organic matter induced by microbial degradation and self-assembly.

Dissolved organic matter (DOM) is ubiquitous and plays an important role in regulating water quality, ecological function, and the fate and transport of trace elements and pollutants in aquatic environments. Both the colloidal precursors (i.e. <1 kDa) and bulk DOM collected from a freshwater estuary were incubated in the dark for 21 days to examine dynamic changes in molecular size and composition induced by microbial degradation and self-assembly. Results showed that the concentrations of total organic carbon, carbohydrates, and protein-like substances decreased by 11-30% during incubation, while those of humic- and fulvic-like substances remained relatively constant, indicating humic substances are more resistant to microbial utilization compared to carbohydrates and protein-like DOM. Despite the different extents in decline, these DOM components had a similar transformation pathway from the <1 kDa to colloids (1 kDa-0.45 μm) and further to microparticles (>0.45 μm). Overall, carbohydrates and protein-like substances, especially the high molecular weight components, were preferentially decomposed by microorganisms whereas humic- and fulvic-like DOM components significantly coagulated through abiotic self-assembly. The contrasting degradation/transformation pathways between the humic-like and protein-like substances along the size continuum, as also characterized by flow field-flow fractionation analysis, demonstrated that the dynamic transformation and degradation of DOM is regulated by both molecular size and organic composition. This finding provides new insights into the biogeochemical cycling pathways of heterogeneous DOM and its environmental fate and ecological role in aquatic systems.