: A Global Enigma.

The genus is globally distributed, with blooms commonly occurring on continental shelves. This unusual phytoplankter has two major morphologies: solitary cells and cells embedded in a gelatinous matrix. Only colonies form blooms. Their large size (commonly 2 mm but up to 3 cm) and mucilaginous envelope allow the colonies to escape predation, but data are inconsistent as to whether colonies are grazed. Cultured can also inhibit the growth of co-occurring phytoplankton or the feeding of potential grazers. Colonies and solitary cells use nitrate as a nitrogen source, although solitary cells can also grow on ammonium. colonies might be a major contributor to carbon flux to depth, but in most cases, colonies are rapidly remineralized in the upper 300 m. The occurrence of large blooms is often associated with environments with low and highly variable light and high nitrate levels, with blooms being linked additionally to high iron availability. Emerging results indicate that different clones of have substantial genetic plasticity, which may explain its appearance in a variety of environments. Given the evidence of appearing in new systems, this trend will likely continue in the near future.