Sustained presence of metabolically active mesophiles in the boiling vent-water of a low-salinity, slightly-alkaline, sulfate-rich geothermal spring characterized by an inequitable ecology of hyperthermophiles and thermophiles

The slightly-alkaline (pH [~]8.5), boiling ([~]90{degrees}C) vent-water of a Trans-Himalayan geothermal spring, moderately-rich in dissolved solids ([~]1500 ppm), was explored six times over a year. 11 archaeal and 46 bacterial species were detected consistently, while nine bacteria occurred intermittently, in the vent-epicenter featuring a largely-stable physicochemical milieu. All 11 archaea were detected as metagenome-assembled genomes ascribable to Thermoproteota. Of the total 55 bacteria detected, 32 were retrieved as MAGs, 20 as isolates, and three in both forms. Four bacteria could not be classified below the domain-level; three and four belonged to hyperthermophilic (Aquificia) and thermophilic (Thermaceae and Thermoflexaceae) taxa respectively; 27 belonged to taxa having some moderately-thermophilic members; 17 belonged to mesophilic taxa. According to metagenomics, an Aquificia, followed by two Thermoprotei and one Thermoproteales, dominated the microbiome overwhelmingly. Metatranscriptomically, however, the Thermoproteales was most active. Metatranscriptomic signatures envisaged the in situ metabolic status of the 66 species discovered as follows. Among the 18 putative hyperthermophiles and thermophiles identified, 17 rendered wide-ranging activities including growth; one Thermoproteota species had considerable activities sans growth. One new-phylum-level bacterium rendered wide-ranging activities including growth, while three such entities had considerable/minimal activities sans growth. Among the 27 potential moderate-thermophiles discovered, two Armatimonadota and one Thermosynechococcus species rendered wide-ranging activities including growth, 20 had considerable/minimal activities sans growth, whereas four had zero activities. Among the 17 mesophiles identified, 16 rendered considerable/minimal activities sans growth, whereas one had zero activity. Molecular drivers were envisaged from the metatranscriptomic data to explain the trends of inequitable population ecology.