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1. We examined standing-senescing, standing-dead and recently fallen leaf blades of Carex walteriana in fens of the Okefenokee Swamp to determine the nature of the microbial decomposers in the early stages of decomposition, measuring both standing crops and productivities ([^H]leucine->protein method for bacteria, [^^C]acetate-^ergosterol for fungi). 2. Fungal standing crops (ergosterol) became detectable at the mid-senescence stage (leaves about half yellow-brown) and rose to 14-31 mg living-fungal C g"^ organic mass of the decaying system; bacterial standing crops (direct nucroscopy) were =s 0.2 mgC g"' imtil the fallen-leaf stage, when they rose to as high as 0.9 mgC g"^.3. Potential microbial specific growth rates were similar between fungi and bacteria, at about 0.03-0.06 day"', but potential production of fungal mass was 115-512 )j.gC g"ô rganic mass day-\ compared with 0-22 ^gC g'^ day-^ for bacteria. Rates of fungal production were about 6-foId lower on average than previously found for a saltmarsh grass, perhaps because much lower phosphorus concentrations in the freshwater fen limit fungal activity. 4. There was little change in lignocellulose (LC) percentage of decaying leaves, although net loss of organic mass at the fallen, broken stage was estimated to be 59%, suggesting that LC was lost at rates proportional to those for total organics during decay. Monomers of fungal-wall polymers (glucosamine and mannose) accumulated 2-to 4-fold during leaf decay. This may indicate that an increase found for proximate (aciddetergent) lignin could be at least partially due to accumulation of refractory fungal-wall material, including melanin. 5. A common sequence in decaying aquatic grasses is suggested: principally fungal alteration of LC during standing decay, followed by a trend toward bacterial decomposition of the LC after leaves fall and break into particles.

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It is essential during measurements of aquatic bacterial production with [methyl#x2010;3H]thymidine (Tdr) that only labeled DNA is measured. We found in 12 freshwater and marine systems that DNA labeling represented a variable proportion of total macromolecular labeling. Up to 87% of label appearing in precipitated labeled macromolecules from acid#x2010;base hydrolysis treatments was soluble in ethanol. Reverse#x2010;phase, high#x2010;pressure liquid chromatography showed that the composition of labeled molecules in the ethanol was 78–88% [3H]Tdr. The rate of labeling of the ethanol#x2010;soluble fraction was significantly correlated with the rate of total macromolecular labeling (r = 0.88, n = 40, P < 0.001) and less strongly with the DNA labeling rate (r = 0.49, n = 28, P = 0.005). Experiments in which bacterial cells were labeled with [3H]Tdr or32PO43− showed that above a total macromolecular labeling rate of ∼1 pmol Tdr liter−1 h−1, bacterial cells bind Tdr but do not incorporate it into phospholipids in the cell envelope.

The incorporation of [methyl-3H]thymidine into three macromolecular fractions, designated as DNA, RNA, and protein, by bacteria from Hartbeespoort Dam, South Africa, was measured over 1 year by acid-base hydrolysis procedures. Samples were collected at 10 m, which was at least 5 m beneath the euphotic zone. On four occasions, samples were concurrently collected at the surface. Approximately 80% of the label was incorporated into bacterial DNA in surface samples. At 10 m, total incorporation of label into bacterial macromolecules was correlated to bacterial utilization of glucose (r = 0.913, n = 13, P < 0.001). The labeling of DNA, which ranged between 0 and 78% of total macromolecule incorporation, was inversely related to glucose uptake (r =-0.823), total thymidine incorporation (r =-0.737), and euphotic zone algal production (r =-0.732, n = 13, P < 0.005). With decreased DNA labeling, increasing proportions of label were found in the RNA fraction and proteins. Enzymatic digestion followed by chromatographic separation of macromolecule fragments indicated that DNA and proteins were labeled while RNA was not. The RNA fraction may represent labeled lipids or other macromolecules or both. The data demonstrated a close coupling between phytoplankton production and heterotrophic bacterial activity in this hypertrophic lake but also confirmed the need for the routine extraction and purification of DNA during [methyl-3H]thymidine studies of aquatic bacterial production.