Day six of our Microbial Advent Calendar – today we look at two cousins from two poles: Octadecabacter arcticus and Octadecabacter antarcticus. They belong to the Roseobacter family within the Alphaproteobacteria class. These rod-shaped bacteria are found, like their name suggests, in icy water in the Arctic and Antarctica. They have the particularity of having in their cytoplasms gas vacuoles. These vesicles are thought to adjust the buoyancy of the bacteria and this plays a role in their dispersal and/or position in the water column. (Organism description paper is available here)
Something interesting with those guys is that when you look at the 16S rRNA sequences, the two species are 99% identical, although being present on the two opposite side of the planet. John Vollmers et al., (paper here) investigated this curious similarity in a paper in 2013 by comparing the genomes of the two close cousins. Previous DNA-DNA hybridization methods already showed that although very similar on the 16S rRNA level, the genomes are actually less similar.
The genome comparison showed that overall the two genomes exhibit significant differences in organization and content. They only share 2% of genes exclusively together in comparison to other members of their Roseobacter family but have a large fraction of unique genes, 20% in O. antarcticus and 23% in O. arcticus, indicating a high potential for individual adaptations. These differences are reflected in some metabolic adaptations, such as the cyanophycin ligase of O. arcticus that could be an alternative metabolic way to salvage nitrogen in a limited environment. Additionally, the two genomes show a large number of mobile elements and potential horizontal gene transfers showing that the two Octadecabacter probably picked up what they needed to survive from more adapted neighbours.
The very close similarity of the 16S rRNA genes shows a clear phylogenetic link between the two species, but their genetic differences indicate that their genomes have the ability to be quite plastic and adapt to different environments.
The world map was designed by Freepik