Wikipedia
The Prasinophytes are a paraphyletic class of unicellular green algae in the Division Chlorophyta. Prasinophytes mainly include marine planktonic species, as well as some freshwater representatives. The prasinophytes are morphologically diverse, including flagellates with one to eight flagella and non-motile (coccoid) unicells. The cells of many species are covered with organic body scales; others are naked. One well known genus is Ostreococcus, considered to be the smallest (ca. 0.95 μm) free-living eukaryote, found in marine waters worldwide. Prasinophytes have simple cellular structures, containing a single chloroplast and a single mitochondrion. The genomes are relatively small compared to other eukaryotes (about 12 Mbp for Ostreococcus and 21 Mbp for Micromonas).
Recent studies agree that the prasinophytes are a non-evolutionary grouping ( paraphyletic) of chlorophyte green algae from different clades. Alternative classifications of the chlorophytes have been proposed in which this class is replaced by at least six separate taxa.