A taxonomic review of the genus Phaeocystis

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Phaeocystis is recognized both as a nuisance and as an ecologically
important phytoplankton species. Its polymorphic life cycle with both
colonial and flagellated cells causes many taxonomic problems. Sequence
variation among 22 isolates representing a global distribution of the
genus has been compared using three molecular markers. The
ribulose-1,5-bisphosphate carboxylase/oxygenase (RUBISCO) spacer is too
conserved to resolve species. The most conserved 18S ribosomal
deoxyribonucleic acid (rDNA) analysis suggests that an undescribed
unicellular Phaeocystis sp. (isolate PLY559) is a sister taxon to the
Mediterranean unicellular Phaeocystis jahnii; this clade branched prior
to the divergence of all other Phaeocystis species, including the
colonial ones. The internal transcribed spacer (ITS) region shows
sufficient variation that some spatial population structure can be
recovered, at least in P. antarctica. P. globosa and P. pouchetii have
multiple different ITS copies, suggestive of cryptic species that are
still able to hybridize. A molecular clock has been constructed that
estimates the divergence of the cold water colonial forms from the
warm-water colonial forms to be about 30 Ma and the divergence of P.
antarctica and P. pouchetii to be about 15 Ma. A short description of
the colonial stage and the flagellated stage for each formally
recognized species is provided. Morphological information is also
provided on a number of undescribed species. These include the strain
Ply 559, consisting of non-colonial cells with peculiar tubular
extrusomes, a second non-colonial species from the north western
Mediterranean Sea producing a lot of mucus, and a colonial species with
scale-less flagellates found in Italian waters. In addition, three
flagellated morphotypes with scales different from those of P.
antarctica were reported in the literature from Antarctic waters. The
picture emerging from both molecular and morphological data is that the
number of species in the genus is still underestimated and that cryptic
or pseudocryptic diversity requires a sound assessment in future
research of this genus. Based on all published observations, an emended
description of the genus is provided.

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