Microdistribution of sulfate‐reducing bacteria in sediments of a hypertrophic lake and their response to the addition of organic matter

To clarify the ecological significance of the association of sulfate‐reducing bacteria (SRB) with sediment particle size, SRB utilizing lactate ( l ‐SRB), propionate ( p ‐SRB) and acetate ( a ‐SRB) were examined with different sizes of sediment particles in a hypertrophic freshwater lake using the anaerobic plate count method. The numbers of l ‐SRB and a ‐SRB were 10 4 –10 5 colony forming units (CFU) per ml in the 0–3 cm layer and 10 2 –10 3 CFU ml −1 in the 10–13 cm layer while the numbers of p ‐SRB were one or two orders lower than those of l ‐SRB and a ‐SRB. A sediment suspension was fractionated into four fractions (<1, 1–10, 10–94 and >94 μm). The highest proportions of l ‐SRB and a ‐SRB were found in the 10–94 μm fraction: 66–97% for l ‐SRB and 53–98% for a ‐SRB. The highest proportion of p ‐SRB was found in the >94 μm fraction (70–74%). These results indicate that most SRB were associated with sediment particles. One isolate from an acetate‐utilizing enrichment culture was similar to Desulfotomaculum acetoxidans , a spore‐forming sulfate‐reducing bacterium. When lactate and sulfate were added to sediment samples, l ‐SRB and a ‐SRB in the <10 μm‐fraction grew more rapidly than those in whole sediment for the first 2 days. This result suggests that nutrients uptake by free‐living and small particle‐associated (<10 μm) SRB is higher than that by SRB associated with larger particles.