Significant bacterial transformation of riverine humic matter detected by pyrolysis GC–MS in serial chemostat experiments

To examine bacterial degradation of dissolved organic matter (DOM) in an estuarine salinity gradient we set up three-stage chemostats, inoculated by natural brackish (salinity 5 and 15) and coastal marine (salinity 30) bacterial communities. They were fed by freshwater DOM rich in humic substances (HS) adjusted to the respective salinity and run at dilution rates of 0.1 and 0.15 d− 1. Decomposition of the total DOM and fulvic acid (FA), humic acid (HA) and hydrophilic acid fractions (HPAs) of HS was assessed over the incubation time of up to 51 days. The composition of the HS was further analyzed by pyrolysis-GC/MS (py-GC/MS). The DOM was largely dominated by lignin-derived organic matter originating from grass but also contained angiosperm-derived lignins. At salinity 30, the humic-rich DOM was decomposed to more than 60% and all HS fractions were reduced substantially, whereas in the other two experiments no detectable decomposition and only minor changes in the HS fractions occurred. In the salinity 30 experiment, the composition of the HA fraction changed substantially whereas that of the FA and HPA fractions exhibited minor changes. Acid to aldehyde (Ad/Al) ratios of the major phenolic lignin-derived organic compounds indicated a rather high degree of oxidation of the natural HS. However, during decomposition Ad/Al ratios of syringyl and vanillyl phenols decreased, thus indicating a high degree of decomposition, i.e. decarboxylation, of these lignin-derived compounds. The results indicate that decomposition by marine bacterial communities is an important sink of terrestrially derived refractory DOM in the intertidal flat coastal region.