PNNL

Abstract

Extreme environmental conditions in Antarctic ecosystems impose strong selection pressures. 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 ??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 ASVs within each HoS clade clustered into a unique 97% sequence similarity OTU and inhabited a specific environment. Our findings suggest the existence of microevolutionary processes leading to sub-taxa niche differentiation, with putative distinct ecotypes (consisting of groups of ASVs) adapted to lotic, lentic or terrestrial environments. 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 pressu1res. In turn, 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.