Print PageDissolved organic carbon (DOC) is the major source of fixed carbon in aquatic ecosystems and bacteria play a central role in its uptake in streams. DOC is also required for denitrification, a major pathway for returning nitrogen to the atmosphere. DOC in the environment is diverse, ranging from simple amino acids and monosaccharides to complex humic compounds. Previous research suggests that quality of DOC may affect bacterial community composition and denitrification rates. However, the role of DOC composition, in particular molecular diversity, as a structuring force of community composition and denitrification is unknown. This study investigated the effect of simple and complex DOC compounds, administered to microcosms individually and in mixtures, on abundance, community structure and denitrification potential of stream bacteria. Microcosms contained either sand (incubated anaerobically) or unglazed ceramic tiles (incubated aerobically) colonized by stream bacteria. Artificial stream water containing DOC amendments was added to the microcosms followed by a 10-day incubation. Bacterial abundance was determined from DAPI counts while community structure was examined based on PCR amplification of 16S rRNA genes followed by terminal fragment length polymorphism (TRFLP) and quantitative PCR of nosZ gene. Denitrification rates were measured using acetylene block technique followed by gas chromatography.
Results/Conclusions
Dissolved organic carbon concentrations in the microcosms decreased from 12 mg/L to an average of 5 mg/L after incubation revealing uptake of DOC by bacteria. Bacterial abundance at the end of the experiment was ten-fold higher compared to initial abundance. Bacterial numbers differed significantly among treatments (P<0.01) with mixed DOC treatments having higher numbers compared to single treatments. Bacterial abundances in labile DOC treatments were significantly higher than in recalcitrant DOC treatments. Denitrification rates were significantly (P<0.01) higher in glycine compared to other labile DOC treatments (glucose, sodium acetate and a mixture of the three) and to the mixture of recalcitrant compounds (vanillic acid, coumaric acid and syringic acid). Glycine microcosms has a denitrification rate of 0.11 µg N2 O-N/ g DM/ hr which was ten-fold higher than other DOC treatments. Denitrification rates of bacteria treated with leaf leachates and algae exudates were generally below the detection limit. TRFLP data suggested a difference in community fingerprint between the labile and recalcitrant treatments. The data also indicated a change in community structure before and after the experiments. These results suggest that DOC quality and molecular heterogeneity play a role in determining bacterial abundance, community composition, and denitrification rates.
