Glucose uptake and end product formation in an intertidal marine sediment

Glucose uptake was monitored on a seasonal basis, using [6-H]glucose and undisturbed cores collected from an intertidal mud flat. The fate of glucose carbon, including the formation of CO(2) and biomass, was assayed by using undisturbed cores and [U-C]glucose; the production of short-chain fatty acids was monitored with [U-C]glucose and sediment slurries. Rate constants for glucose uptake varied temporally, with temperature accounting for much of the variability; turnover times ranged from about 2 to 10 min. Rate constants decreased with increasing sediment depth and in the following order for several common monosaccharides: glucose>galactose>mannose approximately fucose. Time course analyses of CO(2) production provided evidence of significant isotopic dilution; although pore water glucose turnover times were on the order of minutes, CO(2) did not plateau until after approximately 6 h of incubation. At this time a maximum of about 40% of the added radioglucose had been respired. The extent of respiration varied as a function of sediment depth and season, with the highest values below the surface (4 to 7 cm) and in summer and fall. Incorporation of radiolabelled glucose into biomass also varied seasonally, but the greatest extent of incorporation (about 40%) was observed in the fall and for the 0- to 1-cm depth interval. The production of short-chain fatty acid end products was largely limited to acetate, which accounted for only a small percentage of the added radiolabel. Other organic acids, pyruvate in particular, were observed in pore water and were due to artifacts in the heat-kill procedure used to terminate incubations. An accurate assessment of the distribution and importance of short-chain fatty acids as end products required the use of an enzymatic technique coupled with high-pressure liquid chromatography to verify qualitative identities.