The Chlorophyceae are one of the lineages of primarily freshwater green algae.
Meiosis occurs when the zygote germinates, so that most of the lifecycle is haploid (Lee, 1999).
- Motile cells with radial or near-radial external symmetry
- Flagella attached at the anterior end of the cell
- The possibility of a rhizoplast
- No multilayered structure
- Eyespots common
- Glycolate breaddown by glycolate dehydrogenase
- Urea breakdown by urea amidolyase
- Theca covering the cells (some of the flagellates have lost this characteristic)
- Collapsing telophase spindle that brings daughter cells close together, followed by cell-division by a phycoplast*
- Flagellar root system cruciate
*”type of cell division in which the mitotic spindle disperses after nuclear division with the two daughter nuclei coming close together, another set of microtubules arising perpendicular to the former position of the microtubules of the mitotic spindle, and the new cell wall forming along these microtubules.”
There are a variety of thallus types among the chlorophyceans.
- individuals or
- unflagellated except in reproductive state
- with unflagellated asexual reproductive cells
- Sarcinoid aggregations of nonmotile cells in which daughter cells fail to separate after division
- Coenobium, a colony in which the number and arrangement of cells can be constant
- unbranched or
- Multinucleate siphonous
(Graham & Wilcox, 2000)
Primarily freshwater or terrestrial (soils, tree bark, etc.) some in brackish or marine water (Graham & Wilcox, 2000).
Those taxa for which the sexual reproductive process is known produce a dormant zygote, with meiosis usually occurring when the zygote germinates (Lee, 1999).
Evolution and Systematics
Systematics and Taxonomy
Under the old five kingdom system of classification, all the algae were placed within the Kingdom Protista. However, the molecular research of the last decades has resulted in major reclassifications, and at least for now, some uncertainty and disagreement. There is no debate about the placement of the algae in the Domain Eukarya – they have eukaryote cells. And, for the most part, there seems to be agreement that the green algae are members of the plant kingdom. However, classification into lower taxa is less clear.
For example, Lee (1999) defines Chlorophyceae as a taxon within the class(?) Chlorophyta and with the following orders:
- Volvocales – can be unicellular or multicellular; the vegetative cells are flagellated & motile
- Tetrasporales – non-filamentous colonies with immobile vegetative cells capable of cell division; pseudocilia may be present
- Schizogoniales – foliose marine algae with stellate choroplasts
- Chlorococcales – unicellular or non-filamentous colonial algae; if colonial, daughter colonies formed as coenobia; vegetative cells non-motile
- Sphaerophleales – unbranched filaments with new walls formed inside the old filament walls, resulting in H-shaped wall pieces
- Chlorosarcinales – daughter cells retained within parent cell wall; no plasmodesmata present
- Chaetophorales – branched or unbranched filaments; plasmodesmata present
- Oedogoniales – uninucleate filamentous freshwater algae with a unique type of cell division; motile spores and gametes with a whorl of flagella at one pole
In Graham and Wilcox (2000), the Chlorophyceans are a lineage of green algae, encompassing the following clades:
- DO (flagellar basal bodies are directly opposed)
- CW (flagellar basal bodies are displaced in a clockwise direction)
“Chlamydomonas and Volvox are important laboratory model systems, Selensastrum capricornutum is widely recognized for its utility as a bioassay organism, and Dunaliella and Botryococcus can be valuable in production of industrially useful materials.” (Graham & Wilcox, 2000)