Estimation of bacterial production in fresh waters by the simultaneous measurement of [35S]Sulphate and D-[3H]glucose uptake in the dark

Abstract

Sulfate uptake in the dark by phytoplankton constitutes a severe limitation to the determination of bacterial heterotrophic production from sulfate-uptake rates. Consequently a modification to the 35S-method was developed involving size fractionation to separate the algae from the bacteria. Both the whole water sample and the algae-free filtrate are incubated in the dark with trace quantities of [3H]glucose, but the filtrate alone is incubated with 35SO4. The experimental determined ratio (whole sample glucose assimilation:filtrate glucose assimilation) is used to correct the measured sulfate uptake (filtrate) and yields an estimate of bacterial sulfate uptake in the whole sample. A potential filtration artefact was demonstrated in the 35SO4 uptake methodology. Excision of the outer edge of the membrane filter and counting of the inner wetted circle alone eliminated this problem and significantly improved the analytical performance of the method: coefficient of variation .apprx. 5%, detection limit .apprx. 2 ng S l-1 h-1. The modified [35SO4]-[3H]-glucose method was applied to samples from an English chalk stream: bacterial sulfate uptake was higher during the spring diatom maximum (10.6 ng S l-1 h-1) than 3 wk later when detritus dominated the seston (4.9 ng S l-1 h-1). The corresponding rates of formation of particulate (bacterial) C were 0.53 and 0.24 .mu.gC l-1 h-1, respectively.