The pelagic lifestyle is a minimalist one

Pelagic microbes are small and possess streamlined genomes as an adaptation to their oligotrophic surroundings. Salcher et al. (2019) provided insight into the process of genome streamlining. They studied published and newly sequenced genomes of bacteria within the family Methylophilaceae (class Betaproteobacteria), which inhabits a series of environments, ranging from terrestrial soil to the pelagic ocean. Genome size, guanine-cytosine content and arginine usage were found to decrease with a transition from terrestrial soil over freshwater sediment to pelagic environments. Analysis of protein-encoding gene sequences revealed a loss of motility and nitrogen utilisation except for ammonia uptake. Interestingly, the adaptation to a pelagic lifestyle also involved the acquisition of photoheterotrophy-enabling rhodopsins through horizontal gene transfer.

Although it is well known that pelagic microbes have streamlined genomes, this study traces the evolutionary path for the first time. Nitrogen limitation in the pelagic likely led to the observed reduction in nitrogen-containing compounds. Other environmental changes, such as the absence of substrata and nitrogen sources other than ammonia as well as increased insolation, seem to have led to more streamlining. Salcher et al. (2019) may have been able to make even more precise inferences about functional adaptation, had they used transcriptomic and proteomic methods.

Salcher, M. M., Schaefle, D., Kaspar, M., Neuenschwander, S. M., & Ghai, R. (2019). Evolution in action: habitat transition from sediment to the pelagial leads to genome streamlining in Methylophilaceae. The ISME Journal, 13(11), 2764-2777.