Patescibacteria are a very large group of enigmatic bacteria that comprise an entire superphylum of “microbial dark matter”. This huge microbial diversity has been little explored. No Patescibacteria have been isolated in pure culture although a few species have been successfully enriched in co-culture. There are a few metagenomic sequences available for Patescibacteria and the common themes that are emerging in these genomes are that Patescibacteria have very small genomes less than 1 Mb, limited metabolic capabilities, are generally unable to synthesize amino acids, and lack the genes needed for electron transport and the TCA cycle. These characteristics strongly suggest that Patescibacteriamay be obligate symbionts or parasites. Intriguingly, despite their small genomes and limited metabolism, Patescibacteria have been found in metagenomic datasets from many marine samples. The role of Patescibacteria in coastal and open ocean microbial communities is completely unknown. In a serendipitous discovery, we found that cultures of the microalga Nannochloropsis oceanica IMET1 that were grown under high carbon dioxide conditions specifically to sequester carbon dioxide from power plant flue gases were very highly enriched in Patescibacteria. We took advantage of this discovery to obtain high-quality full genome sequences from two distinct novel genera of Patescibacteria and revealed new insights into these bacteria, with implications for marine microbial ecology and use of microalgae for carbon sequestration.