Thiomicrorhabdus heinhorstiae sp. nov. and Thiomicrorhabdus cannonii sp. nov.: novel sulphur-oxidizing chemolithoautotrophs isolated from the chemocline of Hospital Hole, an anchialine sinkhole in Spring Hill, Florida, USA

Deciphering the Potential of Sulphur-Oxidizing Bacteria for Sulphate Production Correlating with pH Change

Sulphur, available in the form of sulphate, is one of the essential nutrients that is required by plants. Bacteria capable of oxidizing reduced forms of sulphur to sulphate play an important role in sulphur nutrition for plants. The present study was conducted to isolate, screen, and characterize sulphur-oxidizing bacteria from different soil samples collected from mustard rhizosphere and fly ash mixed soil. A total of 33 sulphur-oxidizing bacterial isolates (HMSOB1-33) were retrieved from soil and further screened for sulphur-oxidizing ability. Maximum solubilization index (3.76), pH reduction (3.93), and sulphate production (173.61 µg/ml) were observed for the isolate HMSOB2 which on the basis of 16S rDNA sequencing was identified as Pantoea dispersa with sequence similarity 98.22%. Four other selected bacterial isolates were identified as Bacillus megaterium, Bacillus tropicus, Bacillus velezensis, and Bacillus cereus. Sulphate solubilization index (SSI) correlated positively (r = 0.91) with sulphate production; however, pH showed negative correlation (r =  - 0.82) with SSI as well as sulphate production after 120 h of incubation. These promising bacterial isolates could be further explored as bioinoculant after assessing plant growth traits.

Physiological and Genomic Characterization of a Novel Obligately Chemolithoautotrophic, Sulfur-Oxidizing Bacterium of Genus Thiomicrorhabdus Isolated from a Coastal Sediment

Thiomicrorhabdus species, belonging to the family Piscirickettsiaceae in the phylum Pseudomonadotav are usually detected in various sulfur-rich marine environments. However, only a few bacteria of Thiomicrorhabdus have been isolated, and their ecological roles and environmental adaptations still require further understanding. Here, we report a novel strain, XGS-01T, isolated from a coastal sediment, which belongs to genus Thiomicrorhabdus and is most closely related to Thiomicrorhabdus hydrogeniphila MAS2T, with a sequence similarity of 97.8%. Phenotypic characterization showed that XGS-01T is a mesophilic, sulfur-oxidizing, obligate chemolithoautotrophy, with carbon dioxide as its sole carbon source and oxygen as its sole electron acceptor. During thiosulfate oxidation, strain XGS-01T can produce extracellular sulfur of elemental α-S8, as confirmed via scanning electron microscopy and Raman spectromicroscopy. Polyphasic taxonomy results indicate that strain XGS-01T represents a novel species of the genus Thiomicrorhabdus, named Thiomicrorhabdus lithotrophica sp. nov. Genomic analysis confirmed that XGS-01T performed thiosulfate oxidation through a sox multienzyme complex, and harbored fcc and sqr genes for sulfide oxidation. Comparative genomics analysis among five available genomes from Thiomicrorhabdus species revealed that carbon fixation via the oxidation of reduced-sulfur compounds coupled with oxygen reduction is conserved metabolic pathways among members of genus Thiomicrorhabdus.

Microbes-mediated sulphur cycling in soil: Impact on soil fertility, crop production and environmental sustainability

Reduction in soil fertility and depletion of natural resources due to current intensive agricultural practices along with climate changes are the major constraints for crop productivity and global food security. Diverse microbial populations' inhabiting the soil and rhizosphere participate in biogeochemical cycling of nutrients and thereby, improve soil fertility and plant health, and reduce the adverse impact of synthetic fertilizers on the environment. Sulphur is 4th most common crucial macronutrient required by all organisms including plants, animals, humans and microorganisms. Effective strategies are required to enhance sulphur content in crops for minimizing adverse effects of sulphur deficiency on plants and humans. Various microorganisms are involved in sulphur cycling in soil through oxidation, reduction, mineralization, and immobilization, and volatalization processes of diverse sulphur compounds. Some microorganisms possess the unique ability to oxidize sulphur compounds into plant utilizable sulphate (SO₄^2-) form. Considering the importance of sulphur as a nutrient for crops, many bacteria and fungi involved in sulphur cycling have been characterized from soil and rhizosphere. Some of these microbes have been found to positively affect plant growth and crop yield through multiple mechanisms including the enhanced mobilization of nutrients in soils (i.e., sulphate, phosphorus and nitrogen), production of growth-promoting hormones, inhibition of phytopathogens, protection against oxidative damage and mitigation of abiotic stresses. Application of these beneficial microbes as biofertilizers may reduce the conventional fertilizer application in soils. However, large-scale, well-designed, and long-term field trials are necessary to recommend the use of these microbes for increasing nutrient availability for growth and yield of crop plants. This review discusses the current knowledge regarding sulphur deficiency symptoms in plants, biogeochemical cycling of sulphur and inoculation effects of sulphur oxidizing microbes in improving plant biomass and crop yield in different crops.

Thiomicrorhabdus immobilis sp. nov., a mesophilic sulfur-oxidizing bacterium isolated from sediment of a brackish lake in northern Japan

A novel sulfur-oxidizing bacterium, strain Am19^T, was isolated from sediment of a brackish lake. Strain Am19^T grew chemolithoautotrophically on inorganic sulfur compounds, and heterotrophic growth was not observed. Cells were rod-shaped with length of 1.1-3.0 μm and diameter of 0.5-0.8 μm. Growth was observed at 5-37 °C with an optimum growth temperature of 30 °C. The pH range for growth was 5.6-8.5 with an optimum pH of 6.6-7.0. Major fatty acids were summed feature 3 (C_{16: 1}ω7c and/or C_{16: 1}ω6c), summed feature 8 (C_{18: 1}ω7c and/or C_{18: 1}ω6c) and C_{16: 0}. The sole respiratory quinone was ubiquinone-8. The complete genome of strain Am19^T is composed of a circular chromosome with length of 2.5 Mbp and G + C content of 42.7 mol%. Phylogenetic analysis based on genomic data indicated that strain Am19^T belongs to the genus Thiomicrorhabdus but is distinct from any existing species. Analysis of the 16S rRNA gene supported creation of a new species to accommodate strain Am19^T. On the basis of genomic and phenotypic characteristics, strain Am19^T (= NBRC 114602 ^T = BCRC 81336 ^T) is proposed as the type strain of a new species, with name of Thiomicrorhabdus immobilis sp. nov.