The ultrastructure and systematic position of the euglenozoon Postgaardi mariagerensis, Fenchel et al.

The ultrastructure of Postgaardi mariagerensis is reported for the first time. This biflagellated protist, which is covered by closely and regularly arranged rod-shaped bacteria, was obtained from the sulphidic zone of Burton Lake, Antarctica. There is a large nucleus containing apparently permanently condensed chromatin. The flagella have paraxial rods and emerge from a deep, anteriorly opening pocket. The basal bodies are associated with three asymmetrically arranged microtubular roots. Near the opening of the flagella pocket there is an MTR structure that is continuous with the microtubules of the feeding apparatus. The cell membrane is supported by a corset of evenly spaced and linked parallel microtubules subtended by an electron-dense layer. The cell has batteries of tubular, thick-walled extrusomes with a cruciform internal structure. These ultrastructural features indicate that Postgaardi mariagerensis cannot be regarded as a euglenid (as had been suggested previously) but that it is a member of the Glade Euglenozoa. On current evidence, Posfgaardi cannot be placed in any established subgroup of the Euglenozoa, nor can any subgroup be nominated as a sister taxon to Postgaardi. We therefore suggest that Postgaardi be regarded as Euglenozoa incertae sedis.