Chlorophyta - The Green Algae

 

Introduction

    Globally distributed, in freshwater, marine, and terrestrial environments

    Land plants (Kingdom Plantae, or the embryophyta) are specialized green algae adapted to life on land

    An ancient group according to the fossil record, green algae are extremely diverse.

    Often dominant algae in freshwater environment.

Structure & metabolism

    Structurally diverse, the ancestral condition is thought to be a unicellular flagellate with two identical flagella, perhaps resembling a member of the modern Prasinophyceae

    Derived conditions include

    Coccoid, sessile cells

    Unbranched filaments

    Branched filaments

    Siphons and coenocytes

    Coenocyte - multiple nuclei, without transverse walls

    Siphon - very large cells with multiple nuclei

    Siphonocladous - multiple cells, each of which is siphonous

    Pluriseriate filaments

    Pseudoparenchyma and Parenchyma

    Flagellate stages (if present) are isokont, typically with two or four similar flagella

    Hairs and scales may be present on the flagella, but mastigonemes are absent

    Stellate transition zone on flagellum

    Chloroplast is primary

    Bound by two membranes

    Thylakoids are stacked into lamellae, pseudograna, or grana; girdle lamellae are not present

    Pyrenoids are often (but not always) present, are not stalked, and often have thylakoids passing through them.

    Chlorophylls a and b (but chlorophyll c has been reported in some prasinophyceae)

    Xanthophylls are principal accessory pigments

    Most common are lutein, zeaxanthin, violaxanthin, antheraxanthin, and neoxanthin

    Bryopsidales have siphonein and siphonoxanthin (the latter is also found outside of the Bryopsidales)

    Reserve polysaccharide is starch, which is deposited inside of the chloroplast

    Sometimes chloroplasts are differentiated to specialize in starch storage; these are called amyloplasts

    DNA is scattered through chloropast in small nucleoids

Reproduction

    Many life histories are known

Classification

    Major groups

    Chlorophyta sensu stricto

    Prasinophyceae (paraphyletic)

    Unicellular, typically scaly flagellates

    Not a natural group; either paraphyletic or polyphyletic assemblage of lineages near the base of green algal diversity.

    Trebouxiophyceae

    Equivalent to the Pleurastrophyceae of Mattox and Stewart (1984), but does not include Pleurastrum insigne

    Centrioles to side of spindle at mitosis

     

    Chlorophyceae

    Directly Opposed (DO) clade

    Chlorococcales

    Clockwise (CW) clade

    Volvocales

    Chaetophorales

    Ulvophyceae sensu stricto

    Ulotrichales

    Ulvales

    Codioales

    Trentepohliales (=Trentepohliophyceae)

    Dasycladales (=Dasycladophyceae)

    Caulerpales (=Bryopsidophyceae)

    Siphonocladales

    (= Cladophorales, Cladophorophyceae)

    Van den Hoek et al., 1995 elevates several orders often placed in the Ulvophyceae to the class level; this is probably justified.

    Charophyta sensu Karol et al., 2001

    This group corresponds to the Charophyceae sensu Mattox and Stewart, 1984 but with the addition of embryophytes (land plants), which were arbitrarily exluded by Mattox and Stewart.

    Mesostigmatales?

    Chlorokybales

    Klebsormidiales

    Zygnematales

    Coleochaetales

    Charales

    Embryophyceae

Ecology

    Both freshwater and marine.

    Chlorophyceae are primarily freshwater

    Ulvophyceae, Dasycladophyceae, Bryopsidophyceae, and Cladophorophyceae are primarily marine

    Charophyceae are primarily freshwater

    Important members of freshwater phytoplankton

    Minor members of marine temperate intertidal communities

    Important members of tropical (reef) intertidal communities

    Dasycladales and Bryopsidales are important reef-building organisms

    Complex life cycles in some

Economic importance

    Plants are of profound economic importance

Required Reading: VdH Chapter 19 (actually, chapters 19-31)

Supplementary Reading:

KR Mattox and KD Stewart. 1984. Classification of the green algae: a concept based on comparative cytology. Pp. 29-72 in DEG Irvine and DM John, "Systematics of the Green Algae", Systematics Association Special Volume #27, Academic Press, London and Orlando.

Pickett-Heaps, J.D. 1975. Green Algae. Sinauer, Sunderland, MA.