Ecophysiological adaptation in cryoconite bacteria and the relationship to horizontal gene transfer

Abstract

Cryoconite, a bio-mineral aggregate found on glacial surfaces, covers a considerable portion of the terrestrial cryosphere, the largest freshwater reservoir on Earth, and hosts microbial inhabitants that play crucial roles in these environments. Despite the threat of extinction from glacial loss, bacterial heterotrophs in cryoconite remain relatively unexplored. Few studies have isolated, characterised, and fully genome sequenced cryoconite bacterial representatives, and no studies have specifically examined plasmids within these communities. This study aims to capture and characterize ecophysiological adaptations in cryoconite bacteria, supported by horizontal gene transfer (HGT). Cryoconite samples from distinct geographic regions were incorporated for local and cross-regional comparative analysis, including three Arctic glaciers (Svalbard), two Subarctic glaciers (Sweden), and three glaciers of the Austrian Alps. Metaplasmidomes (community plasmidomes) were generated via enrichment of samples followed by plasmid DNA extraction. An array of environment-specific stress response genes were revealed, including previously undocumented (in any cryospheric plasmids) light sensing and bacteriochlorophyll genes. Isolates were tested for temperature preference, under light versus dark conditions, and for UV radiation tolerance. Most isolates showed optimal growth at 2°C, indicating their psychrophily. Several isolates exhibited enhanced growth under light, presenting the first account of photoheterotrophy in cryoconite isolates. Sequencing of isolate genomes uncovered numerous ecophysiological adaptation genes, stress response and photosynthesis genes. Notably, rhodopsin and / or bacteriochlorophyll genes were found in most genomes (>83%), across all regions and in all isolated taxonomic classes, highlighting the widespread photoheterotrophic potential of cryoconite bacteria. A potential latitudinal trend emerged of increasing photoheterotrophic potential and HGT abundance in the genomes. Arctic representatives exhibited the highest levels of both photoheterotrophic potential and HGT abundance, followed by Subarctic and finally Alpine representatives. This study underscores the importance of further exploring cryoconite bacterial communities to understand their ecophysiology and the role of HGT in adaptation in these extreme environments.

Date of Award	2024
Original language	English
Awarding Institution	Aberystwyth University
Supervisor	Arwyn Edwards (Supervisor) & Andy Mitchell (Supervisor)