Thiobacillus thiophilus became the best competitor in
enriched communities
The community of control sediments was serially diluted and cultured
anaerobically in the NS media where nitrate was the sole electron
acceptor and sulfide was the sole electron donor. This is to enrich
bacteria having a complete or partial respiratory chain from sulfide,
the initial e-donor to nitrate, the terminal e-acceptor. The dominance
of Thiobacillus reappeared in 20 out of 38 communities (Figure
5A) though it was initially sparse (<1.3% relative
abundance). This enrichment suggested the competent metabolic fitness ofThiobacillus . Linear regression also identified positive
correlations between its relative abundance and nitrate consumption,
nitrite production and sulfate production (Figure 5B-5D).
Community enrichments of different dilutabilities were streaked on NS
and NTS plates (anaerobic at room temperature). However, no colony
formed on the NS plates. Using solid NTS medium, 14 strains were
isolated from all community enrichments and all were members of phylum
Proteobacteria (Table 1). Unlike community enrichments, these pure
isolates must a complete respiratory chain from thiosulfate to nitrate
since interspecies cross-feeding was excluded. We then determined their
metabolic characteristics using liquid NS and NTS medium, and found that
strain S5643 (Thiobacillus thiophilus ) was the sole sulfide
oxidizer. PCR of selected functional genes involved in nitrogen and
sulfur cycling found that (i) all strains except T83 (Sphingopyxis
terrae ) and T546 (Rhodanobacter lindaniclasticus ) had nitrate
reductase narG or napA ; (ii) all strains had nitrite
reductase nirK or nirS ; and (iii) thiosulfate oxidasesox B appeared only in 6 strains (Table 1), including S5643. These
results confirmed the metabolic fitness of Thiobacillus, which
was the sole sulfide-oxidizing denitrifier among the 14 isolates.