Terrestrial dissolved organic matter inputs drive the temporal dynamics of riverine bacterial ecological networks and assembly processes.

Rivers receive, transport, and are reactors of terrestrial dissolved organic matter (DOM) and are highly influenced by changes in hydrological conditions and anthropogenic disturbances, but the effect of DOM composition on the dynamics of the bacterial community in rivers is poorly understood. We conducted a seasonal field sampling campaign at two eutrophic river mouth sites to examine how DOM composition influences the temporal dynamics of bacterial community networks, assembly processes, and DOM-bacteria associations. DOM composition and seasonal factors explained 34.7% of the variation in bacterial community composition, and 14.4% was explained purely by DOM composition where specific UV absorbance (SUVA254) as an indicator of aromaticity was the most important predictor. Significant correlations were observed between SUVA254 and the topological features of subnetworks of interspecies and DOM-bacteria associations, indicating that high DOM aromaticity results in more complex and connected networks of bacteria. The bipartite networks between bacterial taxa and DOM molecular formulae (identified by ultrahigh-resolution mass spectrometry) further revealed less specialized bacterial processing of DOM molecular formulae under the conditions of high water level and DOM aromaticity in summer than in winter. A shift in community assembly processes from stronger homogeneous selection in summer to higher stochasticity in winter correlated with changes in DOM composition, and more aromatic DOM was associated with greater similarity in bacterial community composition. Our results highlight the importance of DOM aromaticity as a predictor of the temporal dynamics of riverine bacterial community networks and assembly.

Biodegradation of perfluorinated compounds.

The information available in the literature provides evidence for the biodegradation of some poly- and per-fluorinated compounds, but such biodegradation is incomplete and may not result in mineralization. Recent publications have demonstrated that 8:2 fluorotelomer alcohol, for example, can be degraded by bacteria from soil and wastewater treatment plants to perfluorooctanoic acid. Similarly, 2-N-ethyl(perfluorooctane sulfonamido)ethanol can be degraded by wastewater treatment sludge to perfluorooctanesulfonate. It is presently unclear whether these two products are degraded further. Therefore, the question remains as to whether there is a potential for defluorination and biodegradation of PFCs that contributes significantly to their environmental fate. The lack of mineralization observed is probably caused by the stability of the C-F bond, although there are examples of microbially catalyzed defluorination reactions. As is the case with reductive dechlorination or debromination, reductive defluorination is energetically favorable under anaerobic conditions and releases more energy than that available from sulfate reduction or methanogenesis. Consequently, we should consider the possibility that bacteria will adapt to utilize this source of energy, although evolving mechanisms to overcome the kinetic barriers to degradation of these compounds may take some time. The fact that such reactions are absent for some PFCs, to date, may be because too little time has passed for microorganisms to adapt to these potential substrates. Hence, the situation may be comparable to that of chlorinated organic compounds several decades ago. For many years, organochlorine compounds were considered to be catabolically recalcitrant; today, reductive chlorination reactions of many organochlorines, including PCBs and dioxins, are regularly observed in anaerobic environments. Hence, it is opportune and important to continue studying the potential degradation of perfluorinated compounds in carefully designed experiments with either microbial populations from contaminated sites or cultures of bacteria known to dehalogenate chlorinated compounds.