For a holistic understanding of microbial life’s high-temperature adaptation, it is imperative to explore the biology of the phylogenetic relatives of mesophilic bacteria which get stochastically introduced to geographically and geologically diverse hot spring systems by local geodynamic forces. Here, in vitro endurance of high heat up to the extent of growth under special (habitat-inspired) conditions was discovered in a hot-spring-dwelling phylogenetic relative of the mesophilic Paracoccus species. Thermal conditioning, extreme oligotrophy, metabolic deceleration, presence of certain habitat-specific inorganic/organic solutes, and potential genomic specializations were found to be the major enablers of this conditional (acquired) thermophilicity.

Little is known about the relative abundance and metabolisms of microorganisms present in the vent-waters of Trans-Himalayan hot springs. This study revealed a Bacteria-dominated microbiome in the ~84°C vent-water (microbial cell density ~8.5 * 104mL-1; live:dead cell ratio 17:10) of a sulfur-borax spring called Lotus Pond, situated at 4436 m, in Puga Valley. Aquificae accounted for 80% of all 16S rRNA-encoding reads within the metagenomic dataset, while only 14% of such reads were attributed to Proteobacteria. Post assembly, only 25% of all protein-coding genes identified were attributable to Aquificae, and ~41% to Proteobacteria. Annotation of raw metagenomic reads encoding 16S rRNAs, and/or PCR-amplified 16S rRNA genes, identified 163 bacterial genera, out of which 66 had been detected in past investigations of Lotus Pond′s vent-water via 16S amplicon sequencing. Among these 66,Fervidobacterium,Halomonas,Hydrogenobacter,Paracoccus,Sulfurihydrogenibium,Tepidimonas,ThermusandThiofaba(or their close phylogenomic relatives) were presently detected as metagenome-assembled genomes (MAGs). Remarkably, the MAG related to Hydrogenobacter alone accounted for ~56% of the entire metagenome, even as literature survey showed that only 15 out of the 66 genera consistently present in Lotus Pond have strains growing in the laboratory at >45°C, reflecting the continued existence of diverse mesophiles in the habitat. Most genes annotated within the MAGs/assembled-metagenome rendered bioenergetic functions (especially oxidative phosphorylation and rTCA/TCA cycles), while many (especially from Aquificae) conferred virulence functions and antibiotics synthesis/resistance. These could help the Aquificae outcompete other native thermophiles, and contain the proliferation of the bacterial mesophiles that infiltrate the ecosystem.

High temperature growth/survival was revealed in a phylogenetic relative (strain SMMA_5) of the mesophilic Paracoccus isolated from the 78-85°C water of a Trans- Himalayan sulfur-borax spring. After 12 h at 50°C, or 45 minutes at 70°C, in mineral salts thiosulfate (MST) medium, SMMA_5 retained ∼2% colony-forming units (CFUs), whereas comparator Paracoccus had 1.5% and 0% CFU left at 50°C and 70°C respectively. After 12 h at 50°C, the thermally-conditioned sibling SMMA_5_TC exhibited ∼1.5 time increase in CFU-count; after 45 minutes at 70°C, SMMA_5_TC had 7% of the initial CFU-count intact. 1000-times diluted Reasoner’s 2A medium, and MST supplemented with lithium, boron or glycine-betaine (solutes typical of the SMMA_5 habitat), supported higher CFU-retention/CFU-growth than MST. With or without lithium/boron/glycine-betaine in MST, a higher percentage of cells always remained viable (cytometry data), compared with what percentage remained capable of forming single colonies (CFU data). SMMA_5, compared with other Paracoccus, contained 335 unique genes, mostly for DNA replication/recombination/repair, transcription, secondary metabolites biosynthesis/transport/catabolism, and inorganic ion transport/metabolism. It’s also exclusively enriched in cell wall/membrane/envelope biogenesis, and amino acid metabolism, genes. SMMA_5 and SMMA_5_TC mutually possessed 43 nucleotide polymorphisms, of which 18 were in protein-coding genes with 13 nonsynonymous and seven radical amino acid replacements. Such biochemical and biophysical mechanisms could be involved in thermal stress mitigation which streamline the cells’ energy and resources towards system-maintenance and macromolecule-stabilization, thereby relinquishing cell-division for cell-viability. Thermal conditioning apparently helped memorize those potential metabolic states which are crucial for cell-system maintenance, while environmental solutes augmented the indigenous stability-conferring mechanisms.IMPORTANCEFor a holistic understanding of microbial life’s high-temperature adaptation it is imperative to explore the biology of the phylogenetic relatives of mesophilic bacteria which get stochastically introduced to geographically and geologically diverse hot spring systems by local geodynamic forces. Here, in vitro endurance of high heat up to the extent of growth under special (habitat-inspired) conditions was discovered in a hot- spring-dwelling phylogenetic relative of the mesophilic Paracoccus species. Thermal conditioning, extreme oligotrophy, metabolic deceleration, presence of certain habitat- specific inorganic/organic solutes, and typical genomic specializations were found to be the major enablers of this conditional (acquired) thermophilicity. Feasibility of such phenomena across the taxonomic spectrum can well be paradigm-changing for the established scopes of microbial adaptation to the physicochemical extremes. Applications of conditional thermophilicity in microbial process biotechnology may be far reaching and multi-faceted.