Horizontal gene transfer drives microbial adaptation in extreme environments

Horizontal gene transfer (HGT) is a key mechanism driving microbial evolution, enabling diverse lineages to rapidly acquire adaptive traits. However, the forces governing the retention and functional impact of transferred genes remain poorly understood, particularly in natural communities where gene exchange is frequent but strongly shaped by environmental selection. In extreme environments, high stress and nutrient limitation amplifies the role of HGT in microbial adaptation and community structure. Here, we synthesise evidence that HGT is strongly environment-dependent and often enriched for genes involved in core metabolic functions. We propose that genome streamlining, metabolic plasticity and mobile genetic element enrichment are hallmarks of frequent HGT in microbial communities inhabiting extreme environments. These traits contribute to metabolic interdependence that collectively shapes community-level gene-sharing networks. However, current metagenomic and phylogenetic approaches have limited resolution to capture the diversity and dynamics of HGT in extreme environments. We propose that integrating ecological and evolutionary perspectives is essential to better understand how HGT drives functional innovation and microbial adaptation in these ecosystems.