Eating whale bone requires some serious symbiote diversity

Since whales do not cremate their dead, there exists a surprising community of extreme specialists who depend on sunken cetacean carcasses for sustenance and habitation. Within this group is the bone-eating polychaete worm, Osedax, an especially specific specialist that permeates whale bones with root-like structures. These structures are brimming with bacterial symbionts hypothesized to break bone down into nutrients which their host can use. Osedax depends entirely on these symbionts for nutrition, as it lacks a mouth and gut, like its fellow Siboglinidae member, the hydrothermal vent worm Riftia pachptila. Due the rarity of whalefall events, only 5 Osedax species have had their symbionts identified and so far all fall within the Oceanospirillales of the Gammaproteobacteria. This study aimed to fill in the knowledge gap regarding Osedax endosymbiont diversity and distribution within individuals, using O. mucofloris, comparative 16S rRNA sequencing and fluorescence in situ hybridization (FISH).

16S rRNA gene analysis found a paraphylectic group of eight phylogenetically distinct clusters (labelled A-H) with 99.5-99.7 % similarity; most O. mucofloris individuals were dominated by a single Oceanospirillales cluster. Endosymbiont microdiversity was impressive, with 61 of 76 full rRNA phylotype sequences being unique (differing by at least one nucleotide from all other endosymbiont sequences) and the majority of individual hosts had phylotypes unique to them, sometimes as many as nine. Geographic patterns of endosymbionts distribution showed no clear pattern, contrasting with clear depth patterns, as statistical and 16S rRNA analyses demonstrated strong phylogenetic distinctions between Osedax species found in shallow and deep water (<500-1000m<). Given the close relatedness of the host species studied, endosymbiote phylogeny is a more likely cause of depth differences, since endosymbionts likely have a free-living stage during which they infect a host.

FISH confirmed endosymbiont presence in O. mucofloris root tissues and revealed further diversity undetected by 16S rRNA sequencing, as general FISH probes for Osedax endosymbionts hybridized to novel endosymbionts which did not belong to any of the eight known phylogenetic clusters. FISH demonstrated dominance of cluster A across most hosts, closely associated with clusters B, C, D and E. FISH analysis of small sections of root tissue suggested low endosymbiont co-occurrence within bacteriocytes, with a single cluster dominating most and minor clusters occupying peripheral root tissue. Endosymbionts also occurred occasionally in epithelial cells and often in the root surface mucus layer, also dominated by a single cluster.

Apart from endosymbionts, other O. mucofloris associated bacteria included Bacteroidetes, Epsilonproteobacteria and Alphaproteobacteria, both of which were never found in worm tissue, had way less diverse sequence libraries than the Oceanospirillales and included whalefall-associated members of these groups. These bacteria are present as epibiota on all Osedax species and their role is unknown.

Endosymbiont and host genetic similarities did not correlate and bacteria were absent from eggs or sperm, strongly indicating horizontal transmission as the means of endosymbiont acquisition. 68% of endosymbiont variability was explained by the effect of host individuality, resulting from heterogeneity in the environment, the greater size and diversity of water column bacteria compared to that of the hosts and competition between endosymbionts.

Despite its unique and unusual nature, this symbiosis lends itself well to being a model for understanding symbiosis formation, symbiont transmission and the influence of genomic associations between host and symbiont on evolution. Particularly prominent is the compartmentalisation of distinct endosymbionts, suggesting a strong requirement for the separation of their respective metabolic roles. The mysterious role of the bacterial epibiota could well be one involving endosymbiont acquirement, depending on whether the skin is the route of uptake or not and also the relative timing of epibiota and endosymbiont acquirement during development. Much is unanswered regarding these bone-worms; how do their symbioses change between growing on whale bone and waiting for the next whalefall? Is all whale bone equally usable by all symbioses? Do shallow water Siboglinid worms also favour horizontal endosymbiont transmission, or is is vertical transmission just too conservative to succeed in the harsh, nutrient scarce deep ocean? How significant is the role of bone being broken down by these endosymbionts in terms of carbon, nitrogen and phosphorus cycling?

Verna, C., Ramette, A., Wiklund, H., Dahlgren, T. G., Glover, A. G., Gaill, F., & Dubilier, N. (2010). High symbiont diversity in the bone‐eating worm Osedax mucofloris from shallow whale‐falls in the North Atlantic. Environmental Microbiology, 12(8), 2355-2370.