Long-term incubation of lake water enables genomic sampling of consortia involving Planctomycetes and Candidate Phyla Radiation bacteria

Microbial communities in lakes can profoundly impact biogeochemical processes through their individual activities and collective interactions. However, the complexity of these communities poses challenges, particularly for studying rare members. Laboratory enrichments can select for subsystems of interacting organisms and enable genome recovery for enriched populations. Here, a reactor inoculated with water from Lake Fargette, France, and maintained under dark conditions at 4 C for 31 months enriched for diverse Planctomycetes and Candidate Phyla Radiation (CPR) bacteria. We reconstructed draft genomes and predicted metabolic traits for 12 diverse Planctomycetes and 9 CPR bacteria, some of which are likely representatives of undescribed families or genera. One CPR genome representing the little-studied lineage Peribacter (1.239 Mbp) was curated to completion, and unexpectedly, encodes the full gluconeogenesis pathway. Metatranscriptomic data indicate that some Planctomycetes and CPR bacteria were active under the culture conditions. We also reconstructed genomes and obtained transmission electron microscope images for numerous phages, including one with a >300 kbp genome and several predicted to infect Planctomycetes. Together, our analyses suggest that freshwater Planctomycetes may act as hubs for interaction networks that include symbiotic CPR bacteria and phages.