MARINE ECOLOGY PROGRESS SERIES, 50: 169-176

Dynamics of microbial growth in the surface layers of a coastal marine sediment ecosystem

D. M. Karl¹ and J. A. Novitsky²

¹Department of Oceanography and Hawaii Institute of Geophysics, University of Hawaii, Honolulu, Hawaii 96822, USA

²Department of Biology, Dalhousie University, Halifax, Nova Scotia, Canada B3H 4J1

Growth kinetics of the microbial community of a coastal, sub-tropical sediment were studied by following uptake of ³H-adenine. ³H-adenine was incorporated into RNA and DNA, thus providing a measure of both cell metabolism and growth. Extrapolation from measured values of isotope dilution of the ATP pool at isotopic equhbrium (i.e. SA_max parameter) indicated that a large portion of the adenine required for community nucleic acid synthesis was derived from uptake of exogenous supplies. Microbial community doubling times, estimated from adenine nucleotide pool labeling kinetics, ranged from 26 h at the seawater-sediment interface to 47 to 92 h for samples beneath the interface. RNA synthesis rate consistently exceeded DNA synthesis rate by 1 to 2 orders of magnitude for all horizons examined, in spite of these relatively slow growth rates. The newly synthesized RNA was found to be stable with less than 10% being degraded during a period equivalent to the mean population doubling time. Our results indicate a sediment microbial community with growth rates and metabolism distinctly different from water-column communities. The results strongly suggest net, but unbalanced growth in these sediment communities.