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

Climacodium frauenfeldianum is a planktonic chain-forming diatom able to proliferate in severely oligotrophic waters of tropical seas. In this study, a strain of C. frauenfeldianum, the type of Climacodium, 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 (rbcL) gene phylogeny, C. frauenfeldianum clustered with sequences of Bellerochea malleus (the type of Bellerochea) and B. horologicalis. Contrary to previous classifications that placed Climacodium in the Hemiaulaceae, Climacodium and Bellerochea should be placed in the same family, Bellerocheaceae. The species Bellerochea yucatanensis needs further research because sequences of the single strain available clustered with the genus Lithodesmioides. In adaptation to the oligotrophic open ocean environment, C. frauenfeldianum seems to have evolved an appearance closer to Hemiaulus, i.e. very lightly silicified flat cells with long elevations, and with diazotrophic cyanobionts.