Abstract:

Utilizing an anaerobic method of isolating microorganisms, we identified and cultured a total of 11 strains of anaerobic
sulfate-reducing bacteria (SRB) from sediment samples collected from 1.2 m-deep mangrove swamps in Hainan. These SRB were classified into 6 genera based on their morphology that was observed under a microscopy and through sequence analysis of their 16S ribosomal DNA (16S rDNA). These include three reported genera: Bacillus, Vibrio, and Clostridium. They also include three newly discovered genera: Burkholderia, Shewanella, and Marinobacterium. The minimum rate of sulfate reduction of these bacteria was 14.71%, and the maximum rate was 56.78%. Moreover, all the 11 strains were able to reduce sulfur S6+ to S2-, which was subsequently bound with iron (Fe2+) to produce black iron (II) monosulfide (FeS) precipitate. FeS precipitate is the precursor of pyrite. The population of SRB in the mangrove swamps decreased with depth. Geochemical analysis of the sediments reveal that the oxidation and reduction interface was located at interface of sediment and water (0 cm), and the periodic input of oxygen partially inhibited the growth of SRB. In contrast, as the depth of sediment increased (10-40 cm), an adequate supply of organic matter, a near-neutral pH, and an enhanced anaerobic environment led to a significant increase in both the number of SRB strains and the population of SRB. However, at depths greater than 60 cm, the total organic carbon (TOC) content in the sediment decreased, which reduced the carbon source available for microorganisms. In addition, both a significant reduction in pH levels and significant increases in sodium cation (Na+ ) and calcium cation (Ca2+) concentrations inhibited SRB growth, greatly decreasing the number of SRB strains and the total SRB population in the deep sediments. The geochemical characteristics of the sediments were the main factors affecting vertical distribution of SRB species in the mangrove swamp samples.

Key words: mangrove swamps, redox condition, culturable SRB, sulfate reduction rate, sulfur cycle, pyrite.