We examined the bacterial decomposition of humic acids (HA) in two flowthrough culture experiments, one inoculated by marine and one by estuarine bacterial communities. In both experiments, the cultures were fed with HA media of salinities of 28 and 14, close to their ambient and a distinctly different, foreign salinity. HA were decomposed to > 60% of the initial concentration within 70 days, and the foreign salinity yielded the highest decomposition. A detrended correspondence analysis of denaturing gradient gel electrophoresis (DGGE) banding patterns showed that during incubation, the bacterial community composition underwent distinct changes. A phylogenetic analysis of DGGE bands excised and bacteria isolated at the end on HA as the sole carbon source showed that Alphaproteobacteria and Gammaproteobacteria largely dominated the communities in the marine flow-through cultures, whereas Gammaproteobacteria, Actinobacteria and Alphaproteobacteria dominated the estuarine communities. Eleven of 13 isolates obtained from both experiments were able to grow on HA as the sole carbon source, seven on phenol and three, affiliated to the Roseobacter clade, on various aromatic acids. The bacteria retrieved from the flow-through cultures were closely (96-99%) affiliated to organisms capable of degrading humic matter, aromatic and aliphatic compounds and also to other bacteria reported previously from the Wadden Sea and Weser estuary.

Humic substances (HS) are the most abundant natural organic compounds in aquatic and terrestrial environments. However, the bacterial degradation of HS in the estuarine salinity gradient and in coastal regions as a sink for HS, entering the open sea, is not well understood. Therefore, we studied the bacterial degradation of humic-rich dissolved organic carbon (DOC) at increasing salinities between 1 and 30 in an estuary of the southern North Sea (Weser). Three-stage chemostats, inoculated with natural brackish (salinity 5 and 15) and coastal marine (salinity 30) bacterial communities fed 0.1 µm filtered natural humic-rich freshwater adjusted to the respective salinity, were run for up to 51 d at dilution rates of 0.1 and 0.15 d -1. DOC concentrations, bacterial numbers, and production were assessed in each stage over the incubation time. In addition, the fulvic acid (FA), humic acid (HA), and hydrophilic acid (HPA) fractions of the HS were determined by XAD fractionation. At a salinity of 30, the humic-rich DOC was decomposed to more than 60%, whereas in the other 2 experiments at salinities of 15 and 5, no detectable decomposition occurred. At the highest salinity condition, all 3 HS fractions were reduced substantially, whereas at a salinity of 5, only the HA and HPA fractions decreased, and at 15, only the HA fraction decreased. In the latter experiments, concentrations of the FA and HPA fractions in some chemostat stages increased despite unchanged DOC concentration. Our study provides evidence that the main bacterial decomposition, i.e. conversion into bacterial biomass and remineralization, of humic-rich DOC occurs in the polyhaline estuarine region and that at lower salinities, only some transformation of the humic fractions takes place, modifying the adsorptive properties of the HS, but no bacterial DOC decomposition occurs. The sequential chemostats were a useful approach to study the complex process of bacterial HS decomposition. KEY WORDS: Humic substances · DOC · Bacteria · Chemostat · Estuary · Marine Resale or republication not permitted without written consent of the publisherAquat Microb Ecol 53: [151][152][153][154][155][156][157][158][159][160] 2008 the lowest molecular weight, i.e. < 600 Da. Riverine inputs of dissolved organic carbon (DOC), of which HS comprise a substantial fraction, constitute the major flux of such compounds into the coastal zone.Flocculation at increasing salinity is one important process of DOC removal during the estuarine passage (Sholkovitz 1976, Sholkovitz et al. 1978, Kerner et al. 2003, but photochemical and bacterial degradation also contribute to this removal (see below). Present estimates imply that terrestrial and thus humic DOC, after mixing with seawater, is degraded rapidly in the coastal zone (Hedges et al. 1997, Cauwet 2002, Mannino & Harvey 2004. Hence, estuaries, the transition zone between freshwater and coastal marine environments, presumably are of prime importance for the degradation and thus for losses of riverine and terrestrial DOC.Deg...

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