The reduction of tetrazolium salts by the sulfate‐reducing bacteria, Desulfovibrio desulfuricans and Desulfotomaculum orientis, was examined. D. desulfuricans and D. orientis reduced triphenyltetrazolium chloride (TTC) and 2‐(p‐iodophenyl)‐3‐(p‐nitrophenyl)‐5‐phenyltetrazolium chloride (INT) forming intracellular formazan deposits. The reduction rate of INT was higher than that of TTC. INT reduction was not inhibited by the addition of sulfate or molybdate, and sulfate uptake was inhibited by the addition of both INT and molybdate. The ratio of intracellular formazan forming cells to acridine orange direct counts in both strains decreased with culture age and starvation time.
Effects on sulfate respiration of association of sulfate-reducing bacteria (SRB) with solid particles (anion exchange resin and FeS-precipitate) were examined using Desulfovibrio desulfuricans. The rates of sulfide production by resin-and FeS-associated cells were 2-3% and 19-56% of that by free-living ones, respectively, under sulfate-and lactate-rich conditions. On the other hand, under sulfate-poor (less than 50/~M) and lactate-rich conditions the rate by FeS-associated cells was higher than that by free-living ones. The values of K m (/zM), half saturation constant of the Michaelis-Menten model, for sulfate were 244 for free-living cells, 8.96 for resin-associated ones and 8.42 for FeS-associated ones. Under lactate-poor and sulfate-rich conditions the rate by FeS-associated cells was similar to that by free-living ones. These results suggest that FeS-associated SRB are more advantageous than free-living ones under sulfate-poor environments such as freshwater sediments.
Free‐living cells produced colonies more rapidly than particle‐associated ones when a pure culture of Desulfovibrio desulfuricans containing sediment particles was inoculated on an agar medium. The time required for the appearance of the first colony, tr of the first order reation model [4] was 53 h for free‐living cells and 112 h for particle‐associated ones. The value of tr may be determined from the growth rate and sulfate reduction activity because the activity of free‐living cells was much higher than that of aggregating ones.
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