Determination of the Substrates for Sulphate-reducing Bacteria within Marine and Esturaine Sediments with Different Rates of Sulphate Reduction

Abstract

The substrates used by sulphate-reducing bacteria in sediment slurries from Loch Eil, Loch
\nEtive and the Tay estuary were determined by selectively inhibiting sulphate reduction with
\n20 mM-molybdate and measuring the resultant substrate accumulation. Substrate accumulation
\nwas linear after molybdate addition, and the rate of accumulation closely matched sulphate
\nreduction rates, indicating that metabolic pathways other than those specifically involving
\nsulphate reduction were not affected by the inhibitor. In sediments from all three sites acetate
\nwas a major substrate, although the percentage of sulphate reduced due to acetate oxidation
\nvaried considerably among the sites (Tay estuary, 35%; Loch Eil, 64%; Loch Etive, 100%). In
\naddition to acetate, 17 individual substrates were shown to be involved in sulphate reduction to
\nvarying extents in the Tay estuary and Loch Eil sediments; these included lactate, H2,
\npropionate, iso- and n- butyrate, iso- and n-valerate, 2-methylbutyrate and amino acids. At both
\nsites propionate accounted for between 6 and 12% of sulphate reduction. Butyrate (n- and iso-),
\niso-valerate and 2-methylbutyrate were of approximately equal importance at each site and
\ntogether accounted for 13 and 11 %, respectively, of the sulphate reduction in the Tay estuary
\nand Loch Eil sediments. Lactate was only important in the Tay estuary sediments, where it
\naccounted for 43 % of sulphate reduction. The rate of accumulation of amino acids was greatest
\nin the Tay estuary sediments, but the contribution of amino acids to sulphate reduction was
\nhigher in the Loch Eil (9 %) than in the Tay estuary sediments (2 %). Of the 21 individual amino
\nacids that were measured there was a linear increase in nine; the most important of these were
\nserine, glutamate and arginine. In general, when sulphate reduction rates were high the
\nsubstrates for this process were more varied than when rates were low. Combining the results of
\ntwo experiments and assuming complete degradation of the individual substrates, almost all the
\nsulphate reduction could be accounted for at each site (Tay estuary, 101 %; Loch Eil, 98%; Loch
\nEtive, > 100%).