MICROBIAL UTILIZATION AND TURNOVER OF ORGANIC-CARBON IN SANTA-MONICA BASIN SEDIMENTS

Sediment microbial community biomass and activity in Santa Monica Basin, a nearshore basin in the California Continental Borderland, were examined in October 1985, 1986 and 1987,May 1986,April 1987 and January 1990. Millimeter-scale ATP profiles and incubation of intact cores with H-3-adenine indicated a high-biomass interface microbial population in the low-oxygen central basin, which was absent in samples from the basin slope sediments. A majority of microbial activity and organic matter mineralization occurred in the top centimeter of sediment. Comparison of measured ATP and total organic carbon profiles suggests that the C:ATP ratio (wt:wt) ranges between 47:1 and 77:1 in central basin interfacial populations, substantially lower than reported for other aquatic environments. Carbon production estimated from DNA synthesis measurements via H-3-adenine incorporation was compared with TCO2 fluxes measured by in situ benthic chamber experiments. Within the uncertainty of the C:ATP ratio, an overall microbial carbon assimilation efficiency of 75-90% was indicated. The low C:ATP ratios and high carbon assimilation efficiencies significantly affect H-3-adenine tracer-derived estimates of microbial growth and respiration and are substantially different than those often assumed in the literature. These results suggest that without independent, accurate measurements of these ratios, large uncertainties may exist in H-3-adenine-derived microbial growth and respiration rate estimates.