The substrates used by sulphate-reducing bacteria in sediment slurries from Loch Eil, Loch Etive and the Tay estuary were determined by selectively inhibiting sulphate reduction with 20 mM-molybdate and measuring the resultant substrate accumulation. Substrate accumulation was linear after molybdate addition, and the rate of accumulation closely matched sulphate reduction rates, indicating that metabolic pathways other than those specifically involving sulphate reduction were not affected by the inhibitor. In sediments from all three sites acetate was a major substrate, although the percentage of sulphate reduced due to acetate oxidation varied considerably among the sites (Tay estuary, 35%; Loch Eil, 64%; Loch Etive, 100%). In addition to acctate, 17 individual substrates were shown to be involved in sulphate reduction to varying extents in the Tay estuary and Loch Eil sediments; these included lactate, H, propionate, - and -butyrate, - and -valerate, 2-methylbutyrate and amino acids. At both sites propionate accounted for between 6 and 12% of sulphate reduction. Butyrate (- and -), -valerate and 2-methylbutyrate were of approximately equal importance at each site and together accounted for 13 and 11%, respectively, of the sulphate reduction in the Tay estuary and Loch Eil sediments. Lactate was only importnat in the Tay estuary sediments, where it accounted for 43% of sulphate reduction. The rate of accumulation of amino acids was greatest in the Tay estuary sediments, but the contribution of amino acids to sulphate reduction was higher in the Loch Eil (9%) than in the Tay estuary sediments (2%). Of the 21 individual amino acids that were measured there was a linear increase in nine; the most important of these were serine, glutamate and arginine. In general, when sulphate reduction rates were high the substrates for this process were more varied than when rates were low. Combining the results of two experiments and assuming complete degradation of the individual substrates, almost all the sulphate reduction could be accounted for at each site (Tay estuary, 101%; Loch Eil, 98%; Loch Etive, > 100%).


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