Morphological, phylogenetic and biogeographic characterizations of three heterotrophic nanoflagellates isolated from coastal areas of Korea

Heterotrophic nanoflagellates (HNFs, 2–20 μm in size) are important bacterivores in aquatic environments. We isolated and distinguished three HNF strains from Korean coastal samples: Ancyromonas kenti strain KM086, Cafeteria mylnikovii strain JS001 and Multimonas media strain JS004. Their 18S rDNA sequences aligned with previously known counterparts. Under light microscopy, all strains match their previous respective species descriptions. However, under scanning electron microscopy, strain JS004 shows some features at odds with prior observations of M. media, including different putative extrusomes, an ‘unfrilled’ skirt margin, and a putative ‘tusk’. We also used the V4 region of 18S rDNA to study species distribution globally using Malaspina-2010 data from 122 surface stations and 13 various depth stations. Intriguingly, we found that A. kenti was most abundant in low-oxygen conditions of the bathypelagic zones (1000–4000 m deep) in the Pacific Ocean, suggesting a specialization for thriving in low-oxygen habitats. In contrast, we only found M. media represented at one surface site, and C. mylnikovii was not found at all. Thus, A. kenti may be one of the most important feeders on bacteria in the bathypelagic zones with low-oxygen concentrations, while the other two isolates appear to be rare species in marine systems.