INTEGRATIVE MULTI‑MARKER BARCODING AND ITS PHYLOGENY RESOLVE VIETNAMESE STRAIN CCAH013/2 AS Mychonastes rotundus (Sphaeropleales, Chlorophyceae)

A coccoid green microalga (strain CCAH013/2) isolated from a mangrove edge freshwater site in Can Gio, Viet Nam was identified through an integrative workflow combining morphology with multi marker DNA evidence. Nuclear 18S rRNA and plastid 23S rRNA sequences showed very high similarity to named Mychonastes taxa, supporting placement at the genus level but offering limited interspecific resolution due to locus conservatism. The ITS region (ITS1–5.8S–ITS2) provided species level power: BLAST results reached ≥ 99% identity to Mychonastes rotundus and dropped markedly for other congeners (≤~ 95%). A maximum likelihood tree inferred from ITS recovered CCAH013/2 within the M. rotundus clade with strong bootstrap support, mirroring the pairwise identity pattern. Morphological characters-small, spherical to slightly oval, non motile cells occurring in minute clusters and reproducing by autospores-were congruent with the emended generic concept and the species diagnosis. Together, these data support a robust assignment of CCAH013/2 to Mychonastes rotundus and provide a basis for applied studies under local conditions, including CO₂ enrichment tolerance, lipid and carotenoid productivity, and small scale pilot tests in tropical coastal freshwater niches.