GREEN ALGAE

SYSTEMATIC BIOLOGY					THE VIRIDIPLANTAE
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PHYLUM PRAESINOPHYTA

INTRODUCTION TO THE PRAESINOPHYTA

Praesinophyta (pra-se-NA-fa-ta) is formed from two Greek roots that mean dark green (prasinos -πράσινος); and plant (phyto -φυτό). The reference is to the obvious color of the taxa in this phylum. Many of these taxa are grass green, but some are dark green.

The praesinophytes comprise an enigmatic collection of organisms that generally occur as unicells, but some are attached as filaments (Figures A-D). They occur in almost all aquatic environments. Indeed, there is a growing realization that these might be significant contributors to marine plankton primary production. The motile cells usually are covered with cellulosic scales rather than a cell wall. In some the scales are quite complex. However, the fusion of scales in taxa like Pedinomonas might indicate how the more typical cell wall of the kingdom evolved.

Until recently, the group was thought to be asexual. However, Suda et al. (1989) demonstrated that Nephroselmis went through a sexual life history that was isogamous. No doubt, other sexual life histories will be reported.

A. A DIC micrograph of Nephroselmis showing its unequal flagella.

B. An SEM micrograph of Pyramimonas. Note the cell and flagella are covered in scales.

C. TEM micrograph of Micromonas which has a single flagellum that lacks scales.

D. Prasinocladus, which in the motile stage is known as Tetraselmis, lives as an attached form that produces branched gelatinous filaments.

Images taken from:
A: http://www.uio.no/miljoforum/m_alge/art/nephroselmis.htm
B: http://www.aad.gov.au/asset/MME/images/P_motile.jpg

C: http://amsawa.murdoch.edu.au/AMSA_news_0107/dunaliella.jpg
D: http://www.bio.mtu.edu/the_wall/phycodisc/CHLOROPHYTA/
E: http://www.biol.tsukuba.ac.jp/~inouye/ino/g/pr/pr_pic.html

SYNOPTIC DESCRIPTION OF THE PRAESINOPHYTA

The following description came from van den Hoek et al. (1995), Melkonian (1989, 1990), Melkonian and Surek (1995), and Sym and Pienaar (1993).

I. SYNONYMS: praesinophytes,

II. NUMBER: >180 species.

III. PHYLUM CHARACTERISTICS:

A. Structure and Physiology

Cell Form: Generally, unicells; some (Prasinocladus) are benthic filaments.

Flagella: 1-8 flagella, laterally or apically inserted with sparse hairs or scales and anteriorly directed.

Basal Bodies: Basal bodies with roots that are cross-shaped to unilateral; those with the broad roots have a multilayered structure (MLS) some with rhizoplast and broad flagellar roots, others with flagellar roots associated with the cell surface.

Cell Covering: Usually covered with elaborate scales of cellulose. Some naked.

Chloroplasts: Variable; grass green with chlorophylls a and b, B-carotene and various xanthophylls. Eyespot, when present, is always enclosed within the chloroplast.

Food Reserves: True starch that is deposited within the chloroplast in association with a pyrenoid.

Mitochondria: Plate-like cristae.

Golgi: Present.

Nucleus: Presumed haploid in vegetative forms.

Centrioles: Not present

Inclusions and Ejectile Organelles: Not present.

B. Mitosis, Meiosis and Life History

Mitosis: Variable, open to closed; with or without a persistent phycoplast; cytokinesis by furrowing to cell plate formation.

Meiosis: Present at least in Nephroselmis.

Sexual Reproduction and Life History: Confirmed in only one genus, Nephroselmis, which is isogamous and haplontic.

C. Ecology: Found in marine, brackish, and fresh water. Free-living and flagellated, or benthic with quadriflagellate zoospores.

SYSTEMATICS OF THE PRAESINOPHYTA

This system is generally based on van den Hoek et al. (1995) but informed by the work of Melkonian (i.e. Melkonian 1989, 1990; Melkonian and Surek 1995; and Sym and Pienaar 1993). Molecular studies such as Melkonian and Surek (1995) indicate that the praesinophytes likely represent remnants of an early radiation of the "green plants". An example is the extraordinary variation in the type of flagellar insertion (apical or lateral), some from an anterior pit (crypt?). The arrangements of the flagellar roots are equally varied. Some possess a multilayered structure (MLS) at the base of the flagella. This is characteristic of most members of the Streptobionta and all of the embryophytes that produce flagellated cells. Characters such as these confirm that the praesinophytes are artificial and paraphyletic (as are all remnant groups of early radiation events).

HIERARCHICAL CLASSIFICATION OF THE PRAESINOPHYTA

This system is generally based on van den Hoek et al. (1995), but informed by the work of Melkonian (i.e. Melkonian 1989, 1990; Melkonian and Surek 1995; and Sym and Pienaar 1993).

CLASS PRASINOPSIDA

The single class has the characters of the phylum. It is divided into 5 orders. based on ultrastructure and characteristics of cell division (after van den Hoek et al. 1995).

ORDER MAMIELLALES.

Tiny unicells; 1-2 laterally-inserted flagella; 2 microtubular roots; one layer of scales over the cell in a pattern that resembles a spider's web.

Mamiella, Bathycoccus, Micromonas.

ORDER PSEUDOSCOURFELDIALES

Cells with 2 laterally-inserted unequal flagella with three flagellar roots; 2-3 layers of different body scales.

Nephroselmis.

ORDER PYRAMIMONADALES

Cell with a pit where 4-8 flagella are inserted; three layers of body scales; often with a lipid body.

Pyramimonas, Halosphaera, Pachysphaera, Pterosperma, Mesostigma, Dunaliella, Polyblepharides.

ORDER CHLORODENDRALES

Cells compressed; 4 flagella from an apical pit; body scales fused to form a theca.

Pedinomonas.

ORDER PRASINOCLADALES

Main form is a small multicellular benthic alga; reproduction by quadriflagellate zoospores.

Prasinocladus, Tetraselmis.

This page is maintained by Jack R. Holt & Carlos A. Iudica. Last revised 03/14/2008 .