This study investigated the spatiotemporal abundance and diversity of the α-subunit of the dissimilatory sulfite reductase gene (dsrA) in the meromictic Lake Suigetsu for assessing the sulfur-oxidizing bacterial community. The density of dsrA in the chemocline reached up to 3.1 × 10(6) copies ml(-1) in summer by means of quantitative real-time PCR and it was generally higher than deeper layers. Most of the dsrA clones sequenced were related to green sulfur bacteria such as Chlorobium phaeovibrioides, C. limicola, and C. luteolum. Below the chemocline of the lake, we also detected other dsrA clones related to the purple sulfur bacterium Halochromatium salexigens and some branching lineages of diverse sequences that were related to chemotrophic sulfur bacterial species such as Magnetospirillum gryphiswaldense, Candidatus Ruthia magnifica, and Candidatus Thiobios zoothamnicoli. The abundance and community compositions of sulfur-oxidizing bacteria changed depending on the water depth and season. This study indicated that the green sulfur bacteria dominated among sulfur-oxidizing bacterial population in the chemocline of Lake Suigetsu and that certain abiotic environmental variables were important factors that determined sulfur bacterial abundance and community structure.
The abundance and bacterivory of heterotrophic nanoflagellates (HNF) were seasonally followed in the oxic and anoxic layers of the meromictic Lake Suigetsu between May 2008 and November 2010. The HNF abundance in the anoxic layer was always lower than in the oxic layer during the study period. Ingestion of fluorescently labeled 0.5 µm diameter beads by the anaerobic HNF in the anoxic layer indicated bacterivory by HNF. The specific ingestion rates in the anoxic layers were generally similar to those taken from the oxic layer, with some exceptions. Our data thus suggested that anaerobic HNF were bacterial consumers with high potential bacterivory comparable to that of aerobic HNF in Lake Suigetsu. Bacterial turnover rates by HNF grazing in the oxic layer were estimated to be as high as ~10% d −1 of the bacterial standing stock. In contrast, the rates in the anoxic layer were <1% d −1 due to the low density of HNF in the anoxic layer. Our data thus provide a valuable contribution to understanding the structure and function of microbial food webs in anoxic aquatic environments.
The abundance of sulphate-reducing bacteria (SRB) was investigated in the coastal marine sediments along the coast of Japan and South Korea. Sediment samples were collected from fish and shellfish farms between 2006 and 2008. As non-fish farming reference sites, sediments were also collected from highly eutrophic bays, a highly sulphidogenic saline lake, and the deep sea. A quantitative real-time PCR analysis that targeted the gene coding for a portion of the a-subunit of dissimilatory sulphite reductase (dsrA) was performed to assess the abundance of the SRB in the sediments. Between 2.8 9 10 7 and 2.5 9 10 9 copies of the dsrA gene per gram dry sediment were detected. There was no relationship between dsrA gene copy number and total bacterial count in the sediments, whereas organic matter contents (particulate organic carbon and nitrogen, ignition loss and chemical oxygen demand) and acid-volatile sulphide contents were significantly correlated with the dsrA copy number. The data presented demonstrate that organic enrichment of sediment may influence the abundance of SRB communities in coastal marine sediments and that the cell density of SRB may be used as a biological indicator for assessing pollution levels in sediments of marine fish farms.
The functional and numerical responses of the facultative anaerobic heterotrophic nanoflagellate, Suigetsumonas clinomigrationis NIES-3647 to prey density were examined under oxic and anoxic conditions. S. clinomigrationis grew at temperatures between 10 and 30°C and in the salinity range of 3.9–36.9 psu. The maximum specific growth and ingestion rates of S. clinomigrationis were lower under anoxic conditions than under oxic conditions. Half-saturation constants for the growth of S. clinomigrationis were within or greater than the range of bacterial densities in the water column of Lake Suigetsu, suggesting that its growth rate is limited by bacterial prey densities in natural environments.
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