Experiments with simple 14C-labelled substrates have given some insight into the dynamics of bacterial uptake and turnover of the soluble low molecular weight components of the photosynthetically produced dissolved organic carbon (PDOC) which may be primarily responsible
for sustaining microbial growth in pelagic systems. Additional information is, however, required on the rate of degradation and efficiency of conversion of the more refractory components which comprise the bulk of the detrital material found in coastal waters and which are a dominant feature
of salt-marsh wet-land ecosystems and macroalgal communities. Partly for this reason we have recently carried out a series of studies on the biodegradation of plant detritus from a variety of sources including macroalgal debris, phytoplankton and salt-marsh grasses. We have used concentrations
of detritus which are comparable with those found under natural conditions, and have incubated these materials in the presence of the mixed community of microheterotrophs found in seawater at the sites of the detrital supply. The results show some common features in the microbial successions
which characterize the degradation of detrital material from a variety of sources in the sea. Quantitative estimates based on such laboratory microcosms, moreover, agree well with recent field estimates of both bacterial production and associated standing stocks of heterotrophic microflagellates.
Some general inferences can therefore be made on the fate of plant detrital material through the microheterotrophic decomposer pathway. The rate of carbon flow appears to be dependent on the structural complexity of the substrate, small molecular weight compounds of the PDOC pathway being
channelled rapidly through bacteria while the larger molecular weight structural components have a longer residence time. The carbon conversion efficiency is dependent notably on the structural complexity, but also on the utilizable nitrogen available to the bacteria, reaching its highest
value of almost 90% where the C:N ratio of the substrate approaches that of bacteria, but declining to 10% or less for the most refractory components of the plant detritus. Much of the carbon in the structural material entering the decomposer pathway thus appears to be oxidized by the bacteria,
while that from the PDOC pool may be mainly incorporated provided that a source of utilizable nitrogen is available.
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