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Acetate is quantitatively the most important substrate for methane production in a freshwater sediment in The Netherlands. In the presence of alternative electron acceptors the conversion of acetate by methanogens was strongly inhibited. By modelling the results, obtained in experiments with and without (13)C-labelled acetate, we could show that the competition for acetate between methanogens and sulfate reducers is the main cause of inhibition of methanogenesis in the sediment. Although nitrate led to a complete inhibition of methanogenesis, acetate-utilising nitrate-reducing bacteria hardly competed with methanogens for the available acetate in the presence of nitrate. Most-probable-number enumerations showed that methanogens (2x10(8) cells cm(-3) sediment) and sulfate reducers (2x10(8) cells cm(-3) sediment) were the dominant acetate-utilising organisms in the sediment, while numbers of acetate-utilising nitrate reducers were very low (5x10(5) cells cm(-3) sediment). However, high numbers of sulfide-oxidising nitrate reducers were detected. Denitrification might result in the formation of toxic products. We speculate that the accumulation of low concentrations of NO (<0.2 mM) may result in an inhibition of methanogenesis.

Kinetic parameters describing growth and decay of mesophilic (30 degrees C) and thermophilic (55 degrees C) aerobic biomass were determined in continuous and batch experiments by using oxygen uptake rate measurements. Biomass was cultivated on a single soluble substrate (acetate) in a mineral medium. The intrinsic maximum growth rate ( micro (max)) at 55 degrees C was 0.71+/-0.09 h(-1), which is 1.5 times higher than the micro (max) at 30 degrees C (0.48+/-0.11 h(-1)). The biomass decay rates increased from 0.004 h(-1) at 30 degrees C to 0.017 h(-1) at 55 degrees C. Monod constants were very low for both types of biomass: 9+/-2 mg chemical oxygen demand (COD) l(-1)at 30 degrees C and 3+/-2 mg COD l(-1)at 55 degrees C. Theoretical biomass yields were similar at 30 and 55 degrees C: 0.5 g biomass COD (g acetate COD)(-1). The observed biomass yields decreased under both temperature conditions as a function of the cell residence time. Under thermophilic conditions, this effect was more pronounced due to the higher decay rates, resulting in lower biomass production at 55 degrees C compared to 30 degrees C.