BACTERIAL-ACTIVITY AND PRODUCTION IN NEAR-SURFACE ESTUARINE AND FRESH-WATER SEDIMENTS

Two indices of bacterial production. thymidine incorporation and the f requency of divided and dividing cells were measured, along with a sui te of measurements of aerobic and anaerobic bacterial activity, to inv estigate the relationship between bacterial cell production and organi c carbon mineralisation at three different sediment sites: a sheltered intertidal estuarine mudflat (Kingoodie Bay), a riverside mudbank (As hleworth Quay) and an intertidal mudflat in a hydraulically dynamic es tuary (Aust Warth). Organic carbon mineralisation was dominated by ana erobic processes at all three sites: sulfate reduction at the two estu arine sites (equivalent to 76% and 61% of oxygen uptake) and methanoge nesis at the freshwater site (56%). Although all three sites had simil ar bacterial population sizes, activities in Kingoodie Bay were 2-3 ti mes higher than at Aust Warth or Ashleworth Quay. Thymidine incorporat ion rates and Numbers of Dividing and Divided Cells correlated strongl y at all three sites. Thymidine incorporation rates were spatially unc oupled from zones of principal anaerobic activity, providing in situ e vidence that sulfate-reducing bacteria and methanogens do not incorpor ate radiolabelled thymidine into DNA during growth. Cell yield was low er in the anaerobic zone, as subsurface peaks in anaerobic mineralisat ion were not matched by increases in bacterial productivity. However, as anaerobic degradation processes were so dominant, anaerobic product ivity still accounted for the majority of cell production.