New phagotrophic euglenids from deep sea and surface waters of the Atlantic Ocean (Keelungia nitschei, Petalomonas acorensis, Ploeotia costaversata).

New phagotrophic euglenoid species from marine surface waters and the deep sea were isolated and described by light and scanning electron microscopy and 18S rDNA sequencing: Keelungia nitschei, Petalomonas acorensis and Ploeotia costaversata. The morphological characteristics of Keelungia nitschei agree with Keelungia pulex besides the slightly truncated anterior front of the cell of our strain. Phylogenetic analysis indicated low sequence similarity between K. nitschei and K. pulex (87.3%). Ploeotia costaversata clustered within the Ploeotia costata clade with a sequence similarity of 96.1% to P. costata strain Tam. Ultrastructural characteristics of our strain revealed helically twisted strips towards both poles of the protoplast. 18S rDNA phylogenies showed that Petalomonas acorensis is related to the clade of Petalomonas cantuscygni/Scytomonas saepesedens with the highest sequence similarity of 81.2% to P. cantuscygni. Six pellicle strips are visible, while two of them reach only the middle of the cell and four (two longitudinal, two helically twisted) join at the posterior front of the cell. Pressure experiments showed that the deep-sea strain K. nitschei was better adapted to high hydrostatic pressures (up to 500 bar) at 4 °C than the two surface water strains. All three strains increased the database (18S rDNA) of the underrepresented group of phagotrophic euglenids.