Resolving taxonomic uncertainties in tropical diatoms: ultrastructure and rbcL barcoding of Coscinodiscus from Viet Nam

Coscinodiscus is one of the most species-rich centric diatom genera, yet its taxonomy remains challenged due to its high morphological similarity, historical misidentifications, phenotypic plasticity, and scarcity of molecular data, particularly for tropical species. To address this, we conducted the first integrative taxonomic reassessment of Coscinodiscus in Vietnamese coastal waters by combining analysis of high-resolution scanning electron microscopy with rbcL gene sequences of isolated strains. We provided detailed morphology of nine species, documenting C. alboranii and C. concinniformis as new records for the Vietnamese diatom flora. Phylogenetic analyses, based on a dataset of 42 sequences (with 22 new isolates), demonstrated that internal macro-rimoportula ultrastructure is a phylogenetically conserved diagnostic trait at the genus level, whereas the central area structure exhibits significant ontogenetic variability. This study provides the first verified rbcL reference barcodes for five taxa: C. alboranii, C. asteromphalus, C. concinniformis, C. gigas var. praetextus, and C. oculus-iridis. Preliminary molecular evidence raises questions regarding the generic separation of Coscinodiscopsis. Pending further multi-gene studies, we tentatively suggest that C. jonesianus could be retained within Coscinodiscus, positing that its complex external tubes and conical covers might represent species-specific autapomorphies. Furthermore, genetic analysis suggests potential cryptic diversity within C. radiatus and C. granii complexes, as their morphological features remain largely similar despite molecular differences. This study provides molecular evidence supporting Coscinodiscus taxonomy and underscores the necessity of an integrative approach to uncover the hidden biodiversity of marine diatoms.