Slim pickings in the deep results in novel symbiosis

The deep sea is characterised by a scarcity of organic material available its inhabitants. As a result organisms of the deep have evolved novel ways of solving this problem. One of which being the symbiosis seen in hydrothermal vents communities where the conversion of inorganic compounds by chemolithotrophic bacteria inhabiting vestimentiferan tube worms is the source of primary production driving the community. In many cases the symbiotic relationship between invertebrates and microorganisms involves the production of organic material by symbiont autotrophy, as in the example highlighted. The study of Godfreddi et al (2005) shows a novel symbiosis between a polycheate worm, Osedax frankpressi, and its symbiotic bacteria of the family Oceanospirillales, where the degradation of whale bone is the result of the heterotrophic lifestyle of Oceanospirillales producing fatty acids for acquisition by the host.

In February 2002 dense assemblages of invertebrates were discovered on a Grey whale carcus (Eschrichtius robustus) at 2891m depth in the Monterey canyon, California. Between 2002-2004 two polychaete worms were collected Osedax frankpressi and O. rubiplumus, from the Siboglidinae family, the deep sea tube worms (determined by morphological and phylogenetic analysis). The Osedax are mouth less, gutless worms that do not possess a trophosome like other Siboglinids, but instead possess elaborate posterior root like extrusions that contain bacteriocytes (50µm in diameter) which house the pleomorphic rod shaped bacteria. 16s ribosomal DNA sequencing revealed the identity of bacteria as being members of the Oceanospirillales, of the family γ-proteobacteria. In O.frankpressi the dominate phylotype (Osedax SYM_1) consisted of 89% of clones and O.rubiplumas dominant phylotype (Osedax SYM_2) consisted of 81%. The fact that the symbionts show phylogenetic relationships to the genus Oceanospirralis is highly significant because this group is involved in the heterotrophic degradation of organic compounds. Evidence for the distribution of Osedax SYM_1  in O.frankpressi came from in-situ FISH microscopy where an oligonucleotide probe (SYM 435_1) hybridized strongly with the symbiont and determined dense populations in the ovisac and root tissue, confirmed by TEM.  After Godfreddi and colleagues determined the phylogentics and the presence of Osedax SYM_1 as the dominant phylotypes in O.frankpressi, investigation continued into the role of the endosymbiont. The dense population in of the symbionts and the lack of trophosome lead Godfreddi and colleagues to suggest that this this symbiosis is nutritionally beneficial to the worm. Evidence for this came from bulk stable carbon and nitrogen values, which showed similarities in worm symbiont/symbiont free tissue and whale bone, suggesting a potentially heterotrophic reliance on bone for nutrition. Heterotrophy of symbionts was further supported by the absence of RuBPCo gene. RuBPCo is a key microbial enzyme in the autotrophic calvin benson cycle, commonly observed in chemolithotrophic bacteria. Examination of O.frankpressi tissue revealed the presence of the bacterial specific biomarker vaccenic acid, a common product of fatty acid metabolism and eicosapentaenic acid (involved in the maintenance of membrane fluidity) which lead the authors to the conclusion that the symbionts were responsible for the synthesis of these compounds which were transferred to the host. The composition of wax etsers in the tissue conformed in carbon number and bond position of vaccenic acid as shown by C¹³ analysis providing further evidence for the synthesis of compounds by Osedax SYM_1.

In conclusion, this study highlights the possible heterotrophic symbiosis between Osedax frankpressi  with Osedax SYM_1. The study conclusively shows the symbiont is a member of the heterotrophic Oceanospirillales and its position and density in host. Similar stable Carbon and nitrogen analysis between whale bone and host, lack of RuBPCo gene and presence of bacterial specific biomarker Vaccinnic acid in host tissue provide evidence for the heterotrophic lifestyle of the endosymbiont and acquisition of symbiont compounds by the host.  Questions still remain into the mechanisms bone degradation and transport to and from the bacteria, as well as in the host.

Godfreddi S.K, Orphan.V.J, Rouse.G.W, Janke. L, Tsegeria. E, Kendru.T, Lee.R Vrijenhoek.R.,(2005). Evolutionary innovation: A bone eating marine symbiosis. Environmental microbiology. (7) 9. 1369-1378.

   

 http://www.mbari.org/staff/vrijen/pdfs/goffredi_2005em.pdf