Special Session 9: Microbial ecological processes and marine carbon cycle

A new evolutionary mechanism giving rise to highly reduced genomes of dominant marine bacterioplankton lineages
Tuesday 10th @ 1110-1130
Room 1
Haiwei Luo* , The Chinese University of Hong Kong
Presenter Email:
Surface ocean bacterioplankton communities are dominated by cells with highly reduced genomes. Typical examples are carbon-fixing Prochlorococcus, and carbon- remineralizing SAR11 and SAR86, which together represent over 50% of the bacterial cells and drive carbon cycles. The primary mechanism theorized to support these tiny genomes has been selection for low metabolic costs in the oligotrophic ocean environment. While various genomic traits of these cells in today¡¯s ocean support this hypothesis, massive loss of genomic DNA often occurred in the ancient past and selective pressure underlying these ancient events has not been assessed. Here we develop new software to compute conservative and radical nonsynonymous nucleotide substitution rate, which is useful to probe the selective pressure on ancient diversifications. We find an excess of radical changes in several of these highly reduced lineages in comparison with related bacteria with larger genomes. Because radical replacements are more likely to be deleterious than conservative changes, accelerated fixation of radical changes is evidence for a primary role of genetic drift driving ancient genome reduction of marine bacterioplankton lineages. As drift is often caused by environmental disasters, an important implication of this study is that oceanic carbon cycles may have undergone major shifts upon the massive losses of genomic DNA of these bacteria.