Diversity and distribution of nanoplanktonic Minidiscus (Bacillariophyta) in shelf waters of the Southwest Atlantic Ocean

Minidiscus Hasle is a genus of small sized diatoms comprising about eight nanoplanktonic marine species. Renewed interest in this genus has arisen since recent investigations recorded a fairly high contribution of some species to the phytoplankton biomass on a global scale. In Brazil, very few studies have been performed, and only two species of Minidiscus have been reported over the 8500 Km coastline. In this paper, we report five species found in Southeast and South Brazilian waters: Minidiscus alphacrucis sp. nov., M. chilensis, M comicus, M. trioculatus, and M. variabilis (also the variety inornata). One of the tiniest diatom ever described, the new species M. alphacrucis has poroidal areolae, which are organized in radial striae with two rows of poroids, conspicuously limited by the interstriae. Minidiscus alphacrucis is compared with other morphologically similar species. In addition, the world distribution of the extant Minidiscus species is reviewed and discussed.