| Summary: | <p>Abstract</p> <p>Background</p> <p>Poorly understood but highly diverse microbial communities exist within anoxic and oxygen-depleted marine sediments. These communities often harbour single-celled eukaryotes that form symbiotic associations with different prokaryotes. During low tides in South-western British Columbia, Canada, vast areas of marine sand become exposed, forming tidal pools. Oxygen-depleted sediments within these pools are distinctively black at only 2-3 cm depth; these layers contain a rich variety of microorganisms, many of which are undescribed. We discovered and characterized a novel (uncultivated) lineage of heterotrophic euglenozoan within these environments using light microscopy, scanning and transmission electron microscopy, serial sectioning and ultrastructural reconstruction, and molecular phylogenetic analyses of small subunit rDNA sequences.</p> <p>Results</p> <p><it>Bihospites bacati </it>n. gen. et sp. is a biflagellated microbial eukaryote that lives within low-oxygen intertidal sands and dies within a few hours of exposure to atmospheric oxygen. The cells are enveloped by two different prokaryotic episymbionts: (1) rod-shaped bacteria and (2) longitudinal strings of spherical bacteria, capable of ejecting an internal, tightly wound thread. Ultrastructural data showed that <it>B. bacati </it>possesses all of the euglenozoan synapomorphies. Moreover, phylogenetic analyses of SSU rDNA sequences demonstrated that <it>B. bacati </it>groups strongly with the Symbiontida: a newly established subclade within the Euglenozoa that includes <it>Calkinsia aureus </it>and other unidentified organisms living in low-oxygen sediments. <it>B. bacati </it>also possessed novel features, such as a compact C-shaped rod apparatus encircling the nucleus, a cytostomal funnel and a distinctive cell surface organization reminiscent of the pellicle strips in phagotrophic euglenids.</p> <p>Conclusions</p> <p>We characterized the ultrastructure and molecular phylogenetic position of <it>B. bacati </it>n. gen. et sp. Molecular phylogenetic analyses demonstrated that this species belongs to the Euglenozoa and currently branches as the earliest diverging member of the Symbiontida. This is concordant with ultrastructural features of <it>B. bacati </it>that are intermediate between <it>C. aureus </it>and phagotrophic euglenids, indicating that the most recent ancestor of the Symbiontida descended from phagotrophic euglenids. Additionally, the extrusive episymbionts in <it>B. bacati </it>are strikingly similar to so-called "epixenosomes", prokaryotes previously described in a ciliate species and identified as members of the Verrucomicrobia. These parallel symbioses increase the comparative context for understanding the origin(s) of extrusive organelles in eukaryotes and underscores how little we know about the symbiotic communities of marine benthic environments.</p>
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