Characterization of depth-related microbial community activities in freshwater sediment by combined method

Currently there are very few researches on studying the vertical changes of metabolic and thermodynamic properties of microbial communities in freshwater lake sediment. In this work, a multi-channel microcalorimetric system was applied to investigate both the metabolism and thermodynamic properties of 0-35 cm sediment cores from Lake Honghu (Jingzhou, Hubei Province, China). It is suggested that the catastrophic flood in 1998 had changed the structure of the 20-25 cm sediment layer. In this layer, both the physicochemical properties of sediment and the thermodynamic activities of microorganisms exhibit distinct differences from other layers. It displays the highest TOC, TN and C/N values. The power-time curves of microcalorimetric measurement on the sediment samples were plotted to illustrate their microbial activities. The 20-25 cm sediment layer showed the lowest microbial activities with a maximum heat flow rate of 56.97 μW, a growth rate constant of 0.06 h^-1 and the time to reach the peak was 98 h. A positive correlation (r = 0.972, P < 0.001) was found between the cell specific metabolic enthalpy change rate (ΔH₀) and the TOC of the sediment samples. ΔH₀ could indicate the utilization efficiency of carbon source which is not affected by the biomass but relies on the intrinsic properties of sediment. Our work shows that the higher the TOCin sediment; the lower the efficiency in assimilating carbon into biomass by the microbes.