Morphology and ultrastructure of marine bacterivorous bicosoecid Hirugamonas aperieos gen. et sp. nov.

Heterotrophic nanoflagellates (HNF) are recognized as active bacterivores in marine environments. We report the isolation and culture of a strain of HNF obtained from the seawater of Lake Hiruga. The organism has two flagella of different length, a long anterior flagellum with tripartite mastigonemes and a short smooth posterior flagellum; a spiral fiber was observed in the transition zone of the flagella. The spherical or D-shaped cell has a groove with one stretchable edge and is supported by 6-8 microtubule bundle that extends from the basal bodies and forms a cytostome. Additionally, the isolate lacked lorica, body scales, cytopharynx, or acronematic structure of flagella, and thus it could not be assigned to any known genera in the order Bicosoecida. Phylogenetic analysis of 18S rRNA gene sequences from Bicosoecida confirmed an independent clade at the genus level with the closest relative genus being Bicosoeca. The optimal salinity and temperature ranges for growth was 21 %-30 % and 25-31 °C, respectively, indicating a marine mesophilic HNF. Thus, based on the morphological features and phylogenetic analysis of 18S rRNA gene sequences, we propose a novel genus for the isolate: Hirugamonas, n. gen. The species name Hirugamonas aperieos sp. nov. is proposed.