Where there's a whale, there's a way!

Symbiosis has enabled and facilitated some of the world's most fascinating and unique relationships for millions of years. Symbiotic associations between microbes and invertebrates have resulted in an unusual diversity of morphological and physiological adaptations, which have permitted life to thrive in areas originally thought of as barren and inhospitable for many organisms. 2891 m below the ocean surface,  in the axis of the Monterey canyon, one of the deepest grey whale carcasses was found in 2002.  Here, a species of Polychaete worm is able to survive of the nutrients derived from whale bones thanks to a 'nutritional bridge' created by a unique endosymbiont.

Many species of deep-sea worms have been found to contain symbiotic relationships with bacteriocytes such as the famous vestimentiferans and pogonophorans.  The morphology of these worms have adapted throughout evolution as a result of their symbionts, and their internal organs (e.g the gut) have been replaced by a morphological structure termed the  trophosome.  However, molecular evidence revealed that  the newly discovered polychaete species ( identified as belonging to the genus Osedax) which was found in high densities at the whale-carcass site, lacked a trophosome. Instead, the role of the trophosome was being carried out by the protrusion of the posterior ovisac, which similarly to the trophosome was found to be highly vascularised. This 'root' system invades the bone marrow, allowing the Osedax worms to obtain nutrition from the decomposing mammalian bones.  These root systems are home to a group of bacteriocytes which house intracellular symbionts, and it is these symbionts which facilitate the survival of the Osedax polychaete worm. The role of these endosymbionts is crucial, as the metabolic and physiological capabilities required to survive in these deep-sea environments are generally not possible for the majority of metazoans.

This newly discovered morphological apparatus is thought to enable the Osedax to exploit the decomposing mammalian bones, explaining how such large communities of these newly discovered species are able to survive in the deep-sea.  

Histology, epifluorescence microscopy and transmission electron microscopy of the ovisac and root system of Osedax frankpressi revealed rod-shaped bacteriocytes within the root structures. These symbionts were enclosed in secondary vacuoles, with up to 5 bacteria within each. 16S ribosomal DNA sequencing was used to identify the bacteria within the ovisac and roots, and results showed that the symbiont phylotypes differed greatly from all other known chemoautotrophic symbionts found in other siboglinid worms. The Osedax microbes were placed within a well-supported clade of the gamma-proteobacteria, which consists of heterotrophic members of the Oceanospirillales. The free-living, hydrocarbon-degrading Neptunomonas naphthovorans is the closest cultured relative to the Osedax symbionts, and an  environmental relative with 96-97% 16S sequence similarity has also been identified.

16S sequences from the whale bones (free of Osedax tissues) were sampled to determine which of these microbial components might be specific to the worm. FISH revealed that results might not have been hugely reliable, as the bone samples were likely to contain Osedax tissue (even though they worked to mitigate this). Despite cross-contamination, marked enrichment of the Osedax-sym1 and sym2 phylotypes in root tissue of the two Osedax species, compared with bone samples, suggesting that these phylotypes are the endosymbionts.  FISH microscopy with an oligonucleotide probe (sym435_I) targeting against a specific region of the O.frankpressi bacterial phylotype, showed strong hybridisation with bacteria that densely populated the ovisac and root tissues, appearing to be concentrated in bacteriocytes, with their intracellular location confirmed via transmission electron microscopy (TEM). This dense internal population of bacteria coupled with the lack of gut suggested the existence of a nutritional endosymbiosis within these worms. Bulk stable carbon and nitrogen isotope values were observed to support this hypothesis, looking at the values for symbiont-free and symbiont-containing tissues. Symbiont-containing tissue samples were similar to values observed for whale bones, in this study and in other studies on modern and fossil whale bones. This again suggested a potentially heterotrophic reliance on the bone for nutrients. Failure to detect the gene that codes for RubBPCo in the symbiont-containing tissue also supported this idea.

Further methods such as lipid composition analyses were also used to strengthen this hypothesis, however the authors recognise that more studies are needed to understand the specific nutritional integration between Osedax worms and their endosymbionts, The findings of this paper are especially exciting, as the vast majority of bacteria known to form obligate nutritional symbiosis are autotrophic, and the potential discovery of a heterotrophic endosymbiont emphasis the success and potential of marine organisms and bacteria when working symbiotically. 

Goffredi, S.K., Orphan, V.J., Rouse, G.W., Jahnke, L., Embaye, T., Turk, K., Lee, R. and Vrijenhoek, R.C., 2005. Evolutionary innovation: a bone‐eating marine symbiosis. Environmental Microbiology, 7(9), pp.1369-1378.

Further reading:

Goffredi, S.K., Johnson, S.B. and Vrijenhoek, R.C., 2007. Genetic diversity and potential function of microbial symbionts associated with newly discovered species of Osedax polychaete worms. Applied and environmental microbiology, 73(7), pp.2314-2323.

Vrijenhoek, R.C., Johnson, S.B. and Rouse, G.W., 2009. A remarkable diversity of bone-eating worms (Osedax; Siboglinidae; Annelida). BMC biology, 7(1), p.1.

Waits, D.S., Santos, S.R., Thornhill, D.J., Li, Y. and Halanych, K.M., 2016. Evolution of Sulfur Binding by Hemoglobin in Siboglinidae (Annelida) with Special Reference to Bone-Eating Worms, Osedax. Journal of molecular evolution, 82(4-5), pp.219-229.