Haldar, P. K. | bioRxiv Science

bioRxiv2019

Metabolically-active obligate aerobes in anoxic (sulfidic) marine sediments

Metabolically-active obligate aerobes are unheard-of in tightly-anoxic environments. Present culture-independent and culture-dependent investigations revealed aerobic microbial communities along two, ~3-meter-long sediment-cores underlying the eastern Arabian Sea oxygen minimum zone, where high H2S disallows O2 influx from the water-column. While genes for aerobic respiration by aa3-/cbb3-type cytochrome-c oxidases and cytochrome-bd ubiquinol oxidase, and aerobic oxidation of methane/ammonia/alcohols/thiosulfate/sulfite/organosulfur-compounds, were present across the cores, so were live aerobic, sulfur-chemolithoautotrophs and chemoorganoheterotrophs. The 8820-years-old, highly-sulfidic, methane-containing sediment-sample from 275 cmbsf of 530 mbsl yielded many such obligately-aerobic bacterial-isolates that died upon anaerobic incubation with alternative electron-acceptors/fermentative-substrates. Several metatranscriptomic reads from this sediment-sample matched aerobic-respiration-/oxidase-reaction-/transcription-/translation-/DNA-replication-/membrane-transport-/cell-division-related genes of the obligately-aerobic isolates, thereby corroborating their active aerobic metabolic-status in situ. Metagenomic and metatranscriptomic detection of perchlorate-/chlorate-reduction genes, plus anaerobic growth of an obligately-aerobic Halothiobacillus isolate in the presence of perchlorate and perchlorate-reducing-consortia, suggested that cryptic O2 produced by perchlorate-respirers could be sustaining obligately-aerobes in this environment.

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↗