New epiphytic and epipsammic small diatom species from the intertidal zone of China and the phylogenetic implications for the genus Microcostatus

Two new species of Microcostatus are described based on light microscopy, scanning and transmission electron microscopy, along with SSU rDNA and rbcL gene sequencing. Microcostatus hainanensis sp. nov., an epipsammic marine species, was collected from a sandy beach in Hainan, South China. Microcostatus witkowskii sp. nov., an epiphytic brackish species, was obtained from an estuarine mangrove area in Fujian, Southeastern China. These two species are small (valve length 2–13 µm) with rounded ends and a slightly curved filiform raphe and possess two axial depressions on both sides of the raphe sternum. Striae that form macroareolae are externally occluded by perforated hymenes. Striae exhibit a distinct radial pattern but are absent at the centre of the valve in M. hainanensis, whereas they are nearly parallel and continuous with microcostae in M. witkowskii. Microcostae are scarcely discernible, and a conopeum is absent in M. hainanensis, while microcostae and an unperforated conopeum are clearly present in M. witkowskii. In addition, M. hainanensis is the first reported species of the genus Microcostatus discovered from a marine environment. Molecular phylogenetic analyses have indicated that these two species are members of the family Sellaphoraceae, rather than the family Naviculaceae. On the other hand, the paired, lateral plastids in M. hainanensis and M. witkowskii differ from the single H-shaped plastid found in the Sellaphoraceae, suggesting a more extensive phylogenetic relationship.