Morphology and molecular phylogeny of a marine interstitial tetraflagellate with putative endosymbionts: Auranticordis quadriverberis n. gen. et sp. (Cercozoa)

A rare tetraflagellate, Auranticordis quadriverberis n. gen. et sp., was isolated from marine sand samples. Uncultured cells were in low abundance and were individually prepared for electron microscopy and DNA sequencing. These flagellates possessed several novel features, such as (1) gliding motility associated with four bundled recurrent flagella, (2) heart-shaped cells about 35–75 μm in diam., and (3) bright orange coloration caused by linear arrays of muciferous bodies. Each cell also possessed about 2–30 pale orange bodies (usually 4–5 μm in diam.) that were enveloped by two membranes and sac-like vesicles. The innermost membrane invaginated to form unstacked thylakoids that extended towards a central pyrenoid containing tailed viral particles. Although to our knowledge, these bodies have never been described in any other eukaryote, the ultrastructure was most consistent with photosynthetic endosymbionts of cyanobacterial origin. This combination of morphological features did not allow us to assign A. quadriverberis to any known eukaryotic supergroup. Thus, we sequenced the small subunit rDNA sequence from two different isolates and demonstrated that this lineage evolved from within the Cercozoa.Our discovery and characterization of A. quadriverberis underscores how poorly we understand the diversity of cercozoans and, potentially, represents one of the few independent cases of primary endosymbiosis within the Cercozoa and beyond.Marine benthic environments contain a diverse assortment of microorganisms that are still just beginning to be explored and characterized [1,2]. The challenges associated with extracting and enumerating benthic microorganisms and the extreme variation of physical and chemical factors associated with the benthos have limited our understanding of these ecosystems [2]. Nonetheless, both comparative morphological studies and environmental sequencing surveys have revealed a great deal of microeukaryotic diversity within the interstitial spa