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Abstract
The genus Microcostatus was erected in 1998 by Johansen & Sray to accommodate a small number of tiny naviculoid species belonging to the former section Naviculae minusculae, characterized by the presence of the typical microcostae, a series of external transapical costae near the axial area, the possession of a simple raphe system and the lack of a conopeum, typical for the genus Fallacia.
During a survey of the Antarctic non-marine diatom flora, two Microcostatus species were found in several localities in the southern Atlantic and Indian Ocean. Comparison with type material and with the sparse literature that exists on the genus Microcostatus, the Antarctic species were identified as Microcostatus naumannii (Hustedt) Lange-Bertalot and M. maceria (Schimanski) Lange-Bertalot. The species showed the typical northern distribution although some older records of M. naumannii were known from the Antarctic Region. The purpose of this study was the morphological characterization of both species using detailed light and scanning electron microscopy observations. Their ecology and worldwide biogeographical distribution was discussed in the view of the debate on the global distribution of microorganisms.