Molecular phylogeny suggests the affinity of the planktonic diatoms <i>Climacodium</i> and <i>Bellerochea</i> (Lithodesmiales, Mediophyceae)

<i>Climacodium frauenfeldianum</i> is a planktonic chain-forming diatom able to proliferate in severely oligotrophic waters of tropical seas. In this study, a strain of <i>C. frauenfeldianum</i>, the type of <i>Climacodium</i>, was established in culture from the tropical southwestern Atlantic Ocean off Brazil. SEM observations showed weakly silicified cells, without visible surface sculpturing. In the small subunit (SSU) rRNA and ribulose-1,5-bisphosphate carboxylase/oxygenase large subunit (<i>rbc</i>L) gene phylogeny, <i>C. frauenfeldianum</i> clustered with sequences of <i>Bellerochea malleus</i> (the type of <i>Bellerochea</i>) and <i>B. horologicalis</i>. Contrary to previous classifications that placed <i>Climacodium</i> in the Hemiaulaceae, <i>Climacodium</i> and <i>Bellerochea</i> should be placed in the same family, Bellerocheaceae. The species <i>Bellerochea yucatanensis</i> needs further research because sequences of the single strain available clustered with the genus <i>Lithodesmioides</i>. In adaptation to the oligotrophic open ocean environment, <i>C. frauenfeldianum</i> seems to have evolved an appearance closer to <i>Hemiaulus</i>, i.e. very lightly silicified flat cells with long elevations, and with diazotrophic cyanobionts.