Measurement of hydrolytic activity and incorporation of dissolved organic substrates by microorganisms in marine sediments

A core-injection technique is described which enables the assessment of microbial activities (extracellular enzymatic cleavage of carbohydrates and proteins, incorporation of dissolved organic substrates into microbial biomass) In natural sediment cores. Enzymatic activities were determ~ned by means of fluorogenic substances (4-n~ethylumbelliferyl-P-D-glucoside, L-leucine-4methylcoumarinyl-7-amide HCI); incorporation of dissolved organic substances were measured using tritiated substrates (thymidine, leucine, acetate, glucose). Microliter portions of substrates were injected at 0.5 cm intervals into natural sediment cores, whlch were incubated, quick frozen, dissected and analysed. For a proper application of the core-lnject~on technique different parameters were investigated such as volume of substrate injected, significance of diffusion of substrate, concentration of substrate and incubation time, and treatment of samples to account for the non-biological 'turnover' of substrates. In depth profiles of sedments from the Kiel Bight (Baltic Sea; FRG), microbial extracellular enzymatic activities were highest in the surface horizons and decreased steadily with sediment depth. Incorporation rates of dissolved organic substrates, however, revealed a more complex depthdependent variation pattern indicating a pronounced strabflcatlon of microbial metabolism in the sediment cores.