Biomass of Sediment Bacteria by Sedimentation Field-Flow Fractionation

Accurate estimation of sediment microbial biomass is needed for studies in microbial ecology. The most common techniques currently used to estimate biomass in sediments are not only prone to considerable uncertainty, but are also time-consuming and laborintensive. In the present study, a relatively new separation and sizing technique, sedimentation field flow fractionation (SdFFF), previously developed for biomass determination of bacteria in cell cultures and natural waters, was used to determine sediment bacterial biomass. SdFFF together with epifluorescence microscopy for cell counting in separated fractions was used for estimating the biomass in sediment samples from a wetland and a river site. Cell counting was required as yellow autofluorescing particles interfered with the on-line detector signal from the fluorescent tagged bacteria (blue) making this simple method of monitoring cell numbers impossible. Calibration curves were obtained that can be used for calculation of sediment bacterial biomass from cell count data for the two sites under study. This demonstrates that for specific systems the less tedious total counts method can be used to generate quantitative biomass estimates using SdFFF to provide accurate calibration for the conversion of counts to biomass. This approach overcomes the serious problems associated with conventional methods that often assume a constant biomass to cell counts or cell volume ratio between collection sites. The SdFFF method was applied to monitor cell growth in sediment samples after addition of phosphate and several types of carbon sources. On incubation for eight days, carbon-amended sediments showed higher bacterial biomass and contained more aggregated colonies, which needed to be dislodged by more vigorous treatment than those treated with phosphate alone.