<i>PYCNOCOCCUS PROVASOLII</i> GEN. ET SP. NOV., A COCCOID PRASINOXANTHIN‐CONTAINING PHYTOPLANKTER FROM THE WESTERN NORTH ATLANTIC AND GULF OF MEXICO¹

<jats:title>ABSTRACT</jats:title><jats:p>The new genus <jats:italic>Pycnococcus</jats:italic> Guillard is based on several clones from the western North Atlantic and Gulf of Mexico. The type and only described species, <jats:italic>Pycnococcus provasolii</jats:italic> Guillard, sp. nov., is typified by clone <jats:italic>Ω</jats:italic>48‐23 from the North Atlantic. Cells of <jats:italic>Pycnococcus provasolii</jats:italic> are solitary, spherical, 1.5–4.0 μm in diameter, have a resistant cell wall lacking sporopollenin, and have the ultrastructural characteristics of green algae. With the light microscope they are scarcely distinguishable from cells of other coccoid planktonic organisms. In pigmentation <jats:italic>P. provasolii</jats:italic> resembles <jats:italic>Micromonas pusilla, Mantoniella squamata</jats:italic>, and <jats:italic>Mamiella gilva</jats:italic> in having chl <jats:italic>a</jats:italic>, much chl <jats:italic>b</jats:italic>, Mg 2,4‐divinylphaeoporphyrin a<jats:sub>5</jats:sub> monomethyl ester (presumably), and prasinoxanthin as a major xanthophyll. The pyrenoid of <jats:italic>P. provasolii</jats:italic> has a cytoplasmic channel, which is unique among species closely related to it. Flagellates, occurring rarely in culture, are similar to but distinguishable from known <jats:italic>Pedinomonas</jats:italic> species by size and shape. <jats:italic>Pycnococcus provasolii</jats:italic> is referred to the new family Pycnococcaceae Guillard, in the order Mamiellales of the class Micromonadophyceae (Chlorophyta). Clones of <jats:italic>Pycnococcus provasolii</jats:italic> are oceanic in nutritional characteristics, require only vitamin B<jats:sub>12</jats:sub> in culture, and are well adapted to growth under blue or blue‐violet light of low intensity.</jats:p>