Genome streamlining in CPR bacteria transitioning from soil to groundwater

To better understand the influence of habitat on the genetic content of Candidate Phyla Radiation (CPR) bacteria, we studied the effects of transitioning from soil to groundwater on genomic divergences of these organisms. Bacterial metagenome-assembled genomes (318 total, 35 of CPR bacteria) were generated from seepage waters and compared directly to groundwater counterparts. Seepage water CPR bacteria exhibited 1.24-fold greater mean genome size, while their inferred mean replication rate was 21.1% lower than groundwater lineages. While exploring gene loss and adaptive gains in closely related lineages in groundwater, we identified a zinc transporter, a surface protein, and a lipogylcopeptide resistance gene unique to a seepage Parcubacterium. A nitrite reductase gene unique to the groundwater Parcubacterium, likely acquired from pelagic microbes via horizontal gene transfer, was also identified. Groundwater Parcubacteria harbored nearly double the fraction (9.4%) of pseudogenes than their seepage kin (4.9%), suggesting further genome streamlining.