isolation of a marine denitrifying bacterium that could grow using sulfide as the electron donor is possible or not? Please explain
Answers
Explanation:
Molecular approaches have shown that a group of bacteria (called cluster 1 bacteria) affiliated with the ɛ subclass of the class Proteobacteria constituted major populations in underground crude-oil storage cavities. In order to unveil their physiology and ecological niche, this study isolated bacterial strains (exemplified by strain YK-1) affiliated with the cluster 1 bacteria from an oil storage cavity at Kuji in Iwate, Japan. 16S rRNA gene sequence analysis indicated that its closest relative was Thiomicrospira denitrificans (90% identity). Growth experiments under anaerobic conditions showed that strain YK-1 was a sulfur-oxidizing obligate chemolithotroph utilizing sulfide, elemental sulfur, thiosulfate, and hydrogen as electron donors and nitrate as an electron acceptor. Oxygen also supported its growth only under microaerobic conditions. Strain YK-1 could not grow on nitrite, and nitrite was the final product of nitrate reduction. Neither sugars, organic acids (including acetate), nor hydrocarbons could serve as carbon and energy sources. A typical stoichiometry of its energy metabolism followed an equation: S2− + 4NO3− → SO42− + 4NO2− (ΔG0 = −534 kJ mol−1). In a difference from other anaerobic sulfur-oxidizing bacteria, this bacterium was sensitive to NaCl; growth in medium containing more than 1% NaCl was negligible. When YK-1 was grown anaerobically in a sulfur-depleted inorganic medium overlaid with crude oil, sulfate was produced, corresponding to its growth. On the contrary, YK-1 could not utilize crude oil as a carbon source. These results suggest that the cluster 1 bacteria yielded energy for growth in oil storage cavities by oxidizing petroleum sulfur compounds. Based on its physiology, ecological interactions with other members of the groundwater community are discussed.
Underground cavities have been used for long-term storage of crude oil in several countries, including those situated at Kuji in Iwate, Japan. Since these cavities have been constructed in groundwater-rich rocky strata, groundwater migrates into and accumulates at the bottom of these cavities (this groundwater is called cavity groundwater [26]). This flow of groundwater facilitates establishing a continuous culture of microorganisms in cavity groundwater; the cell count was constantly over 106 ml−1, which was 100 times greater than the counts in control groundwater obtained around these cavities (26). This habitat of microorganisms can be characterized by immediate contact with a large quantity of crude oil and by an excess of electron donors (i.e., hydrocarbons) but a shortage of electron acceptors. These characteristics may be similar to those of microbial habitats associated with subterranean oil reservoirs that have recently attracted strong microbiological attention (6, 8, 11, 24). Since cavity groundwater can easily be obtained at any time without contamination by surface water (26, 27), these cavities are considered to represent good models for studying the microbial ecology of subterranean oil fields.
Our previous study applied molecular phylogenetic approaches to analyzing bacterial populations that occurred in the Kuji cavity groundwater (26). It was found that a group of bacteria (called cluster 1 bacteria) affiliated with the Thiovulum subgroup (10) in the ɛ subclass of the class Proteobacteria constituted major populations, sharing 10 to 30% of the total microbial populations. We have also detected cluster 1 bacteria in oil storage cavities at Kushikino in Kagoshima, Japan (unpublished data). The Thiovulum subgroup includes three cultivated sulfur-oxidizing bacteria (SOB) (3, 7, 19) and many environmental clones obtained from hydrothermal vents (5, 12), marine sediments (2), and groundwater (15-17). In this subgroup, the cluster 1 bacteria formed a peculiar assemblage, called the groundwater bacteria assemblage, together with three environmental clones obtained from groundwater at geographically distant sites (15-17, 26). It has thus been suggested that bacteria belonging to this assemblage are widely distributed in the subterranean environment, although their physiology and ecological niche have been unknown.
The present study isolated bacterial strains (exemplified by strain YK-1) affiliated with the cluster 1 bacteria from an oil storage cavity at Kuji. In order to unveil their ecological niche in oil storage cavities, experiments were conducted to investigate physiological features of strain YK-1. In these experiments, especially provocative was the possibility that YK-1 could utilize crude oil for its growth under anaerobic conditions. Based on its physiological features, we discussed ecological interactions of the cluster 1 bacteria with other members of the cavity groundwater community.