On Becoming a Chemosynthetic Symbiont

The common perception of symbioses between chemosynthetic bacteria and invertebrates from hydrothermal vents and cold seep is that these have evolved only rarely from a few bacterial lineages that were uniquely adapted to a symbiotic lifestyle. Our phylogenetic analyses of the symbionts of deep-sea bathymodiolin mussels from hot vents, cold seeps and other chemosynthetic environments indicate that bathymodiolin mussels were colonized multiple times by many different lineages of bacteria (Petersen et al. 2012). At least four different lineages of free-living sulfur-oxidizing bacteria and six different lineages of free-living methane-oxidizing bacteria have established symbioses with bathymodiolin mussels. Given that free-living bacteria have been able to establish themselves as intracellular symbionts so often in the course of evolution, it appears as if it is evolutionarily "easy" for free-living bacteria to become symbiotic. To better understand the genetic attributes of chemosynthetic symbionts we are currently comparing the genomes of the sulfur-oxidizing symbionts of bathymodiolin mussels with their closest free-living relatives, highly ubiquitous pelagic sulfur-oxidizers called SUP05 or GSO bacteria that dominate oxygen minimum zones worldwide, and have also been found in hydrothermal vent plumes (Anantharaman et al. 2013). Whole genome alignments and bioinformatic analyses show that the Bathymodiolus symbiont has undergone massive rearrangements, and that as much as 38% of its genes are potentially of foreign origin. Genes and regions known to play a role in DNA transfer such as transposases and integron insertion sites are present in the Bathymodiolus symbiont and SUP05 bacteria, revealing a possible mechanism for horizontal gene transfer (HGT) into these lineages. Genes of potentially foreign origin in the Bathymodiolus sulfur-oxidizing symbiont include genes for the use of hydrogen as an energy source (Petersen et al. 2011), and a highly diverse array of toxins, which could be used for beneficial interactions with the host. The Bathymodiolus sulfur-oxidizing symbiont genome consists of a patchwork of horizontally acquired genes, and many of these foreign genes may have played a key role in enabling these bacteria to establish beneficial associations with their hosts and make optimal use of the energy sources available in their environment.

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Trophic relations (including symbiosis)
Evolutionary history (fossil and molecular data)
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