Reverse Osmosis in an Advanced Water Treatment Train Produces a Simple, Consistent Microbial Community

Potable water reuse has become a key component of water sustainability planning in arid regions. Many advanced water purification facilities use reverse osmosis (RO) as part of treatment, including as a barrier for microorganisms; however, regrowth after RO treatment has been observed. Questions remain about the identity, source, and survival mechanisms of microorganisms in RO permeate, but the extremely low biomass of this water is a limitation for common microbiological methods. Here, we performed high-throughput sequencing on samples collected throughout a potable reuse train, including samples collected by filtering large volumes of RO permeate and biomass collected from RO membranes during an autopsy. We observed a stable, consistent microbial community across three months and in two parallel RO trains. RO permeate samples contained Burkholderiaceae at high relative abundance, including one Aquabacterium sp. that accounted for 29% of the community, on average. Like most other RO permeate microorganisms, this sequence was not seen in upstream samples and we suggest that biofilm growing on unit process infrastructure, rather than active treatment breakthrough, was the primary source. A metagenome-assembled genome corresponding to Aquabacterium sp. from RO permeate was found to lack most sugar-utilization pathways and to be able to consume low molecular weight organic molecules, potentially those that pass through RO.