Phylogenomic position of a genetically diverse phagotrophic flagellates: sediment-associated MAST-6 lineage and a potentially halotolerant placididean within the stramenopiles

MAST-6 is a phylogenetically distinct group that has been known to be associated with sediments, but little else is known about it. Indeed, until the present study, only a single species from this group has been described and associated with genomic information. Here, we describe three new species of MAST-6, including two new genera, Mastravomonas tehuelche gen. et sp. nov., Mastramonada tlaamin gen. et sp. nov., and one undescribed Pseudophyllomitus sp., BSC2. All new MAST-6 were found in sediments. Additionally, we describe a new species, Haloplacidia sinai sp. nov., from a related lineage, the Placididea, and provide a transcriptome along with two additional bikosians transcriptomes from a public culture collection, to allow for better phylogenetic reconstructions of deep-branching stramenopiles. Based on SSU sequence searches among published sediment amplicon sequencing datasets, MAST-6 related to M. tlaamin were particularly high in relative abundance in samples across various depths and geographical locations. A multi-gene phylogenomic tree was inferred using both Maximum-likelihood and Bayesian phylogenetic frameworks, both of which recovered robust monophylies of MAST-6 and MAST-4, and paraphyletic Bigyra, although some discrepancies remain to be resolved.