Friday, 21 March 2014

Bacterial water-to-land transition

Terrestrial ecosystems are considered to be a hostile environment and the land colonization is thought to be one of the most challenging transitions for bacteria. Soil bacteria represent the majority of biodiversity on land and participate in many important processes, such as, carbon sequestration, nitrogen fixation and element cycling. Therefore, the colonization of land is considered a fundamental step for evolution and ecosystem functioning. Studies have mainly focused on environment-specific genes involved in metabolic pathways which are hypothesized to be responsible for bacterial niche-specific adaptations. This study investigates the extent to which DnaE2 plays a role in the bacterial water-to-land transition.

Taxonomic structure analysis, metagenomic data sets and phylogenetic tree constructions were carried out to test the hypothesis. 68% of terrestrial bacteria were found to contain the DnaE2 gene with the exception of the Cyanobacteria group that belong to the terrabacteria. The current marine habitant of Cyanobacteria may actually be a back-to-the-sea event.

Genome expansion stimulated by guanine-cytosine (GC) content increase is also thought to contribute to bacterial land colonization by enabling a higher rate of horizontal gene transfers between bacterium. Evidence to suggest that GC increase is required for genome expansion includes a linear correlation between the two factors; GC content is also one of the important barriers for horizontal gene transfer and bacteria can selectively silence foreign genes whose GC content is lower than the hosts. Finally, phylogenetic analysis of dnaE2 suggests that this gene may first appear in terrabacteria, with estimated time of land colonization to be as early as 3.54 to 2.8 billion years ago.

Wu et al., (2014) deduced a possible model for bacterial-land colonization. Firstly, the marine ancestral dnaE1-containing bacteria gain an active copy of dnaE2, followed by GC increase allowing gene expansion and land colonization to occur. However, there are three possible evolutionary scenarios which could occur from this stage. Some bacteria will experience niche-restricted adaptations, lose dnaE2 and reduce GC content, therefore creating a smaller genome, this would be typical of a bacteria living in a host. The bacteria could also experience extreme genome reduction, but still possess dnaE2 thus keeping GC content. Finally, they could re-invade the marine environment and further spread to marine borne-hosts.

In summary, the bacterial-land transition is attributed to a genome with a higher GC content and the gain of the dnaE2 gene, through horizontal gene transfer or recombination. This has enabled bacteria to have a greater ability to deal with strange, hostile soil environments and considered a fitness advantage. Therefore, dnaE2 is of great relevance to the success of terrabacterial land colonization and is still occurring today.

Wu, H., Fang, Y., Yu, J. and Zhang, Z. (2014) The quest for a unified view of bacterial land colonization. International Society for Microbial Ecology.

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