How Big Is Big? The Effective Population Size of Marine Bacteria

Genome-reduced bacteria constitute most of the cells in surface-ocean bacterioplankton communities. Their extremely large census population sizes (Nc) have been unfoundedly translated to huge effective population sizes (Ne)—the size of an ideal population carrying as much neutral genetic diversity as the actual population. As Ne scales inversely with the strength of genetic drift, constraining the magnitude of Ne is key to evaluating whether natural selection can overcome the power of genetic drift to drive evolutionary events. Determining the Ne of extant species requires measuring the genomic mutation rate, a challenging step for most genome-reduced bacterioplankton lineages. Results for genome-reduced Prochlorococcus and CHUG are surprising—their Ne values are an order of magnitude lower than those of less abundant lineages carrying large genomes, such as Ruegeria and Vibrio. As bacterioplankton genome reduction commonly occurred in the distant past, appreciating their population genetic mechanisms requires constraining their ancient Ne values by other methods.