Convergent evolution masks extensive biodiversity among marine coccoid picoplankton

Coccoid picoplankters are minute unicellular algae that, when viewed with a light microscope, appear as little balls. They comprise an important component of open-ocean phytoplankton, and, except for colour differences (i.e. red, green, brown), many eukaryotic picoplankters are morphologically similar. To evaluate the biological diversity of the little brown ball subpopulation of little balls, we randomly selected nine undescribed algal strains and compared the nucleotide sequences of their nuclear 18S ribosomal RNA genes. The results indicate that little brown balls have evolved independently in three distinct eukaryotic lineages (heterokont algae, haptophyte algae, and green algae), and at least four taxonomic classes, and that, even within the four classes, considerable genetic diversity exists. These findings suggest that a tiny coccoid morphology confers some adaptive advantage in the open ocean, that repeated convergent evolution has occurred, and that molecular data may be necessary for taxonomic distinction of closely related coccoid picoplankters.