Distribution and Feeding of Hexatilemonas jangsaensis, a Novel Cosmopolitan Member of the Uncultured Marine Apusomonad Clade.

The perplexing apusomonads, a sister lineage to Opisthokonta (including animals and fungi), are bacterivorous heterotrophic nanoflagellates whose diversity and ecological role remain poorly understood. Members of the large APU-30 clade are found exclusively in marine environments and mostly comprise uncultured lineages. Here, we isolated a novel lineage within an uncultured subclade of APU-30 from Korean coastal waters. Although the new isolate shares key morphological features with Chelonemonas in APU-30, it possesses a differently segmented dorsal pellicle. Phylogenetic analyses placed this organism closest to the genetically distinct 'Thecamonas' sp. Bamfield. Based on a combination of morphological and genetic features, we propose a novel genus and species for this organism: Hexatilemonas jangsaenesis gen. et sp. n. This novel apusomonad captures bacteria with its lateral pseudopodia, showing a sit-and-wait feeding strategy, which probably provides an efficient way for utilising bacterial assemblages. Interestingly, environmental DNA surveys showed a widespread distribution of Hexatilemonas-like sequences across global marine environments, occurring in 29.2% of epipelagic and 47.7% of mesopelagic samples, suggesting that this genus is cosmopolitan. Our findings expand the known diversity of apusomonads by describing a novel lineage and provide insights into previously uncharacterised lineages and their ecological roles in marine ecosystems.