Microdiverse bacterial clades prevail across Antarctic wetlands

Antarctica's extreme environmental conditions impose selection pressures on microbial communities. Indeed, a previous study revealed that bacterial assemblages at the Cierva Point Wetland Complex (CPWC) are shaped by strong homogeneous selection. Yet which bacterial phylogenetic clades are shaped by selection processes and their ecological strategies to thrive in such extreme conditions remain unknown. Here, we applied the phyloscore and feature-level beta NTI indexes coupled with phylofactorization to successfully detect bacterial monophyletic clades subjected to homogeneous (HoS) and heterogenous (HeS) selection. Remarkably, only the HoS clades showed high relative abundance across all samples and signs of putative microdiversity. The majority of the amplicon sequence variants (ASVs) within each HoS clade clustered into a unique 97% sequence similarity operational taxonomic unit (OTU) and inhabited a specific environment (lotic, lentic or terrestrial). Our findings suggest the existence of microdiversification leading to sub-taxa niche differentiation, with putative distinct ecotypes (consisting of groups of ASVs) adapted to a specific environment. We hypothesize that HoS clades thriving in the CPWC have phylogenetically conserved traits that accelerate their rate of evolution, enabling them to adapt to strong spatio-temporally variable selection pressures. Variable selection appears to operate within clades to cause very rapid microdiversification without losing key traits that lead to high abundance. Variable and homogeneous selection, therefore, operate simultaneously but on different aspects of organismal ecology. The result is an overall signal of homogeneous selection due to rapid within-clade microdiversification caused by variable selection. It is unknown whether other systems experience this dynamic, and we encourage future work evaluating the transferability of our results.

Automated summary

Antarctica's extreme environmental conditions impose selection pressures on microbial communities, but the specific bacterial clades affected by selection processes and their ecological strategies in such conditions remain unknown. This study successfully detected bacterial clades subjected to homogeneous and heterogeneous selection and found that only the homogeneous selection clades showed high abundance and signs of microdiversity across all samples. The majority of the ASVs within each homogeneous selection clade clustered into a unique operational taxonomic unit and inhabited a specific environment. The findings suggest the existence of microdiversification leading to sub-taxa niche differentiation, with putative distinct ecotypes adapted to a specific environment. Homogeneous selection clades in the study site may have phylogenetically conserved traits that accelerate their rate of evolution and enable them to adapt to strong spatio-temporally variable selection pressures.