Diverse symbionts may enhance survival in the deep sea

Symbiont intraspecific diversity is thought to be restricted in nature because hosts tend to select an optimal symbiont. Ansorge et al. (2019) showed that this does not hold true for Bathymodiolus spp. and their sulfur-oxidising symbionts at Mid-Atlantic Ridge hydrothermal vents. Metagenome sequencing revealed a high frequency of symbiont SNPs (5-11 per kilobase pair). More specifically, up to 16 strains were estimated per host. Intra- and interhost nucleotide diversity (π) was high but similar for each vent site. In addition to low within-site population isolation (F_ST) this indicated intermixing of symbiont strains between hosts. Metatranscriptome sequencing and direct-geneFISH revealed that only some strains, clustered in specific bacteriocytes, were able to perform hydrogen oxidation. Furthermore, the large variation in the ability to perform the various steps of nitrate respiration suggests labour division among strains.

This study infers that symbiont strain diversity may be widespread in nature, at least in low-cost associations where symbionts harvest energy from the environment. Benefits of high and intermixed strain diversity are (i) optimally adapted symbionts through natural selection, (ii) faster adaptation to new environments, (iii) labour division and (iv) resilience to disease. Perhaps the ecological meaning of host and ecosystem biodiversity is more similar after all?

Ansorge, R., Romano, S., Sayavedra, L., Porras, M. Á. G., Kupczok, A., Tegetmeyer, H. E., Dubilier, N. & Petersen, J. (2019). Functional diversity enables multiple symbiont strains to coexist in deep-sea mussels. Nature Microbiology, 4(12), 2487-2497.