Mondal, N. | Search bioRxiv

bioRxiv2019

Two pathways for thiosulfate oxidation in the alphaproteobacterial chemolithotroph Paracoccus thiocyanatus SST

Chemolithotrophic bacteria oxidize various sulfur species for energy and electrons, thereby operationalizing biogeochemical sulfur cycles in nature. The best-studied pathway of bacterial sulfur-chemolithotrophy, involving direct oxidation of thiosulfate to sulfate (without any free intermediate) by the SoxXAYZBCD multienzyme system, is apparently the exclusive mechanism of thiosulfate oxidation in facultatively chemolithotrophic alphaproteobacteria. Here we explore the molecular mechanisms of sulfur oxidation in the thiosulfate- and tetrathionate-oxidizing alphaproteobacterium Paracoccus thiocyanatus SST, and compare them with the prototypical Sox process characterized in Paracoccus pantotrophus. Our results revealed the unique case where, an alphaproteobacterium has Sox as its secondary pathway of thiosulfate oxidation, converting [~]10% of the thiosulfate supplied whilst 90% of the substrate is oxidized via a Tetrathionate-Intermediate pathway. Knock-out mutation, followed by the study of sulfur oxidation kinetics, showed that thiosulfate-to-tetrathionate conversion, in SST, is catalyzed by a thiosulfate dehydrogenase (TsdA) homolog that has far-higher substrate-affinity than the Sox system of this bacterium, which, remarkably, is also less efficient than the P. pantotrophus Sox. soxB-deletion in SST abolished sulfate-formation from thiosulfate/tetrathionate while thiosulfate-to-tetrathionate conversion remained unperturbed. Physiological studies revealed the involvement of glutathione in SST tetrathionate oxidation. However, zero impact of the knock-out of a thiol dehydrotransferase (thdT) homolog, together with no production of sulfite as an intermediate, indicated that tetrathionate oxidation in SST is mechanistically novel, and distinct from its betaproteobacterial counterpart mediated by glutathione, ThdT, SoxBCD and sulfite:acceptor oxidoreductase. All the present findings collectively highlight extensive functional diversification of sulfur-oxidizing enzymes across phylogenetically close, as well as distant, bacteria.

microbiology↗

bioRxiv2019

Inorganic salts and compatible solutes help mesophilic bacteria inhabit the high temperature waters of a Trans-Himalayan sulfur-borax spring

While geographically-/geologically-distinct hot springs harbor different levels of microbial diversity, some of them encompass several such taxa which have no strain reported for laboratory growth at >45{degrees}C. We, therefore, hypothesized that native geomicrobial factors could be potent determinants of the microbial habitability of hot spring environments. To test this hypothesis, aquatic microbial communities were revealed metataxonomically, and considered in the context of spring-water chemistry, along the 85-14{degrees}C hydrothermal gradient of a sulfur-boron spring named Lotus Pond located at 4,436 m, within the Puga geothermal area of the Indian Trans-Himalayan region of Ladakh. Water samples were studied from four distinct sites along Lotus Ponds spring-water transit from the vent to an adjacent river called Rulang. Insinuations obtained from geomicrobiological data were tested via pure-culture growth experiments in habitat-inspired media. Microbial diversities were found to be high at all the sample-sites; majority of the genera identified at the 70-85{degrees}C sites were found to have no report of laboratory growth at >45{degrees}C; concurrently, these sample-sites had high concentrations of the kosmotropic solutes boron, lithium, sodium, sulfide, thiosulfate and sulfate, which are known to biophysically stabilize macromolecules. Based on the universal thermodynamic status of these solutes, we conjectured that they may be instrumental in helping mesophiles withstand high in situ temperatures. Corroboratively, growth experiments with a mesophilic, 80{degrees}C-isolate, Paracoccus SMMA_5 showed that at 50{degrees}C and 70{degrees}C, depending on the incubation-time, lithium/boron/sulfate/sodium/glycine-betaine either increases the number of colony-forming units present in the culture or arrests decline of the same. Incubations at 70{degrees}C, followed by fluorescein diacetate staining and flow cytometry, showed that these solutes keep more cells under viable condition than in ready-to-divide state. We concluded that kosmotropes and compatible solutes help mesophiles overcome the chaotropic effects of heat by augmenting such indigenous, entropy-minimizing biophysical mechanisms that apparently trade-off cell division for cell viability.

microbiology↗