Ultrastructure and molecular phylogeny of <it>Calkinsia aureus</it>: cellular identity of a novel clade of deep-sea euglenozoans with epibiotic bacteria

<p>Abstract</p> <p>Background</p> <p>The Euglenozoa is a large group of eukaryotic flagellates with diverse modes of nutrition. The group consists of three main subclades – euglenids, kinetoplastids and diplonemids – that have been confirmed with both molecular phylogenetic analyses and a combination of shared ultrastructural characteristics. Several poorly understood lineages of putative euglenozoans live in anoxic environments, such as <it>Calkinsia aureus</it>, and have yet to be characterized at the molecular and ultrastructural levels. Improved understanding of these lineages is expected to shed considerable light onto the ultrastructure of prokaryote-eukaryote symbioses and the associated cellular innovations found within the Euglenozoa and beyond.</p> <p>Results</p> <p>We collected <it>Calkinsia aureus </it>from core samples taken from the low-oxygen seafloor of the Santa Barbara Basin (580 – 592 m depth), California. These biflagellates were distinctively orange in color and covered with a dense array of elongated epibiotic bacteria. Serial TEM sections through individually prepared cells demonstrated that <it>C. aureus </it>shares derived ultrastructural features with other members of the Euglenozoa (e.g. the same paraxonemal rods, microtubular root system and extrusomes). However, <it>C. aureus </it>also possessed several novel ultrastructural systems, such as modified mitochondria (i.e. hydrogenosome-like), an "extrusomal pocket", a highly organized extracellular matrix beneath epibiotic bacteria and a complex flagellar transition zone. Molecular phylogenies inferred from SSU rDNA sequences demonstrated that <it>C. aureus </it>grouped strongly within the Euglenozoa and with several environmental sequences taken from low-oxygen sediments in various locations around the world.</p> <p>Conclusion</p> <p><it>Calkinsia aureus </it>possesses all of the synapomorphies for the Euglenozoa, but lacks traits that are specific to any of the three previously recognized euglenozoan subgroups. Molecular phylogenetic analyses of <it>C. aureus </it>demonstrate that this lineage is a member of a novel euglenozoan subclade consisting of uncharacterized cells living in low-oxygen environments. Our ultrastructural description of <it>C. aureus </it>establishes the cellular identity of a fourth group of euglenozoans, referred to as the "Symbiontida".</p>

Abstract licence: CC BY-NC-ND

• Euglenozoa: taxonomy, diversity and ecology, symbioses and viruses · 2021

A person counts once a day per paper. We keep no cookie and no address: a reader is a hash that changes every day. Any month under five reads shows no number.