Chlorophyta

The Green Algae

Characteristics

Very diverse group of forms: from unicells to colonies to sheetlike forms
Chlorophylls A & B (as in higher plants)
Storage product- starch
Chloroplast contain protein body- the pyrenoid (Rubisco)

Characteristics

Thylakoids in stacks of 2-6 membranes
Flagella usually smooth and of equal length
Cell walls of cellulse (or glycoprotein)

Classification Scheme: A Single Class

Class Chlorophyceae
9 Orders: Separated by vegetative cell form

Orders

Volvocales: vegetative cells actively motile, unicells and colonies, glycoprotein (sporopollenin?) walls

Tetrasporales: non-flagellated palmelloid colonies (pseudoflagella) in a gelatinous matrix
Chlorococcales: nonmotile vegetative cells solitary -colonial, asexual reproduction by spores
Ulotrichales: filamentous plants with uninucleate cells, branching or not.
Ulvales: growth as hollow filaments or sheets
Oedogoniales: branched or unbranched filaments, sperm & zoospores with numerous
flagella, prominent cell division scars remain
Cladophorales: filamentous plants with multinucleate cells
Caulerpales: a single coenocytic cell makes up filament (no cross walls)
Dasycladales: single cell with radial symmetry

Order Volvocales

The ancestral green alga probably similar to members of this order.
Chlamydomonas is representative.
Chlamydomonas: small biflagellate unicell with cup-shaped plastid, pyrenoid, eyespot. ~400 species

Chlamy Life Cycle

Asexual fission common. 1 cell forms 2.
Sex. Induced by stress (ie. low N) Heterothallic (+,-) or Homothallic species exist.
Gametes look like vegetative cells but with "sticky" flagella

Sex in Chlamy...!

Gametes initially clump together (chemical attractant?)
Individuals next pair off by flagellar contact (glycoproteins on tip)
Flagella join along their length
Cells brought together and fuse
Motile zygote(2N) results (4 flagella)
Germination and meiosis brings back 1N vegetative cells

Chlamydomonas: Points of Note

Vegetative cells are haploid (1N), only zygote is diploid (2N)
Sex is in response to stress and results in a dormant zygote stage

Chlorophyta-2 Chlorophyta Diversity: Order Volvocales

flagellated cells (Chlamydomonas)
single cells to colonies. Colonies of cells similar to Chlamy
evolutionary progression seen from single cells to colonies with specialized cells (ie. gonidia)
colonial forms begin as inverted colonies which turn cells outward

Example: Volvox carteri

Colony of hundreds of cells.
Some asexual "gonidia" present which undergo division to form new asexual colonies.
Colonies are male & female
Sex brought on by stress!!

Sex in Volvox

Heat shock (42*C) asexual colonies will induce sexuality
Male colonies release a 30Kd glycoprotein inducer.
Inducer works at 6x10-17 M. One converted male will affect all Volvox in 1000 L.
Males release sperm, females with eggs!!

Order Tetrasporales

Chlamy-like cells in a mucilaginous sheath
Cells non-motile with pseudoflagella (9+0)
Botryococcus braunii, free-floating colony found in oil deposits (?) and possible source of hydrocarbon fuel.

Order Chlorococcales

From single cells to colonies to coenocytes (single cell, many nuclei)
non-motile vegetative cells
Trebouxia. Common phycobiont (algal partner) with fungi to form a lichen.
Fungi secretes a protein that will bind only with Trebouxia.

Trebouxia: Free-Living vs. Phycobiont

Free-living cells produce less sugar
Free-living cells form more polysccharides
Free-living cells develop a cell sheath
Free-living cells release less photosynthate into medium

Chlorella/Prototheca

Chlorella. Possible new food source.
Produces little cellulose, 50% protein, lots of vitamins Prototheca.
Colorless Chlorella lives in soil. Cause of human & animal disease. Often misdiagnosed as yeast infection.

Order Ulotrichales

Uninucleate filaments Ulothrix.
Alga common to freshwater lakes. Unbranched filament with modified basal cell forming a holdfast
Lifecycle is H,h.
Asexual reproduction involves 1N vegetative cell producing quadriflagellate 1N zoospores

Sex in Ulothrix

Gametes formed as in zoospores but are biflagellate.
Isogamous gametes from separate plants fuse to form 2N zygote
Meiosis occurs after dormancy (overwinter) produces germling 1N plant
Order Ulvales

Sheet-like or hollow cylinder 1-2 cells thick Ulva.
Marine intertidal plant. Sheet-like growth lost if plant grows without bacteria (or bacterial extracts).
Ulva life cycle is an alteration of generations.
Alternating between identical sporophyte & gametophyte stages

Order Oedogoniales

Unique cell division forming distinctive scar along cell.
Example: Oedogonium..

Order Cladophorales

Multinucleate filaments which develop from cell mitosis without cell division to form a "siphonous" thallus.
Alternation of Generations
Example: Cladophora. A branching marine form.

Order Caulerpales

Siphonous forms- No cross walls.
Tropical species Derbesia with a life cycle comprising two completely different generations.
A filamentous sporophyte with a bulbous gametophyte.
1N plant termed Halicystis. Thought to be different alga.
Halicystis: Gametophyte of Derbesia

Gamete formation independent of temperature or light. Appears to be an endogenous rhythm.

Order Dasycladales

tropical plants, calcified, radial symmetry
uninucleate plants
Acetabularia "Mermaids wineglass". Single-celled plant
Classic studies on nucleus-cytoplasmic interactions

Acetabularia

Haemmerling 1930's.
Showed that nucleus produces certain morphogenic factors which influence development.

What are these factors?

Acetabularia

Shows circadian rhythms in photosynthesis, chloroplast shape, polysaccharide content...