1 cup-shaped chloroplast (chr),
2 flagella (g),
2 contractile vacuoles (v)
1 pyrenoid (py)
5 µm in diameter ->
You can just about see the full range of "body types"
within the algae by just looking at the green algae.
These algae are single cells, with or without flagella.
| Non-motile unicell - Chlorella | |
| Motile unicell - Chlamydomonas.
1 cup-shaped chloroplast (chr), 2 flagella (g), 2 contractile vacuoles (v) 1 pyrenoid (py) 5 µm in diameter -> |
|
| Colonies comprise single cells which typically
exists as clumps. The key point about colonies is that there is no division of labour and each cell can survive on its own. Oocystis is an example of a colonial green alga. |
The coenobium (plural coenobia) is a colony
with a fixed number of cells. Coenobia may be motile or
non-motile.
 |
Volvox
is an example of a motile coenobium. It comprises a set number of Chlamydomonas-like
cells embedded in a hollow, spherical gelatinous matrix.
Photo by Kazukuki Mikami |
| Scenedesmus is an example of a non-motile coenobium. Typically, this coenobium comprises 4 cells. The 2 end cells have horn-like projections of their walls. |  |
| Algae with this body plan are actually giant unicells. These
algae are coenocytic which means they undergo repeated
nuclear division without the accompanying formation of cell walls. These have a tubular
structure with the multinuclear cytoplasm lining the thallus
(the Greek word for tube is siphon). Bryopsis - a siphonous thallus |
 |
 |
At left is Caulerpa, a siphonous alga from Caribbean waters. Despite its simple internal form, it almost looks like a higher plant. It has frond-like assimilators for photosynthesis, a basal runner by which it spreads and rhizoids which fix it to the substratum. Recent evidence suggests these rhizoids may play a role in uptake of nutrients. |
| Another siphonous alga you
might want to aquaint yourself with is Halimeda, shown at right. This alga is extensively calcified making it more resistant to predation |
 |
FILAMENTS
Filamentous algae result from cell division in one plane.
A single cell of Spirogyra - a familiar filamentous alga. It has a
single spiral chloroplast in each cell. |
| Seaweeds made up of "boxy" cells like those of higher plants are termed
parenchymatous. Others in cross-section appear to be parenchymatous but are in fact really made up of interwoven filaments which give this appearance! These are termed pseudoparenchymatous. They may be membranous like Ulva, the sea lettuce, or have a complex structure with a stem-like portion termed a stipe with leafy appendages as in Sargassum. |
 |
Ulva is a membranous sheet with a holdfast for
attachment. It grows in shallow sea water, often where there is nutrient-rich run off from the land. |
 |
Sargassum is a brown alga with a complex pseudoparenchymatous
structure. The Sargasso Sea gets its name from the extensive mats of this algae found
floating there. This is a plate from Phycologia Barbadensis published in 1908. It is a collection of paintings of Barbadian seaweeds made in the 1890's by a French visitor Anna Vickers. Unfortunately, she died before the book was published! |
| Seaweed morphology has even been classified from an ecological, functional perspective relative to herbivory, wave action etc. In such a scheme, various groups are recognised - sheets, filaments, thick & leathery, jointed & calcareous, crustose, coarsely-branched. |
Algae reproduce asexually by fragmentation and by spores. In the sea, which is such a stable environment, spores are a means of dispersal not a resting stage.
Sexual reproduction involves the fusion of
gametes (syngamy).
In algae three forms are found:
 |
isogamy - equal-sized motile gametes anisogamy - motile gametes oogamy - small motile male |
In the simplest algae, all cells can become gametes while in the more specialised only
some can.
Algae, like other plants show alternation of generations. What this
means is that there is more than one
free-living stage of the organism. Most plants have two recognisable phases - the sporophyte
and the gametophyte.
| The sporophyte phase of the life cycle
produces spores by MEIOSIS. The gametophyte phase produces gametes by MITOSIS. NEVER, EVER FORGET THIS! (Yes, there are exceptions but this is a rule to remember.) |
Let's look at some examples of this.
Ulva lactuca - Sea Lettuce
| Ulva has a membranous thallus and attaches to rocks by a holdfast. The
sporophyte produces motile haploid spores which settle and grow into the next generation,
the gametophyte. This produces anisogametes which fuse
to form a zygote which grows into the sporophyte generation. The sporophyte and
gametophyte generations look exactly alike. For this reason Ulva is said to show isomorphic
alternation of generations. |
|
Life cycle
of Ulva
Laminaria sp. - a kelp
| Laminaria's sporophyte is a complex, leathery, parenchymatous thallus divided
into a blade, a stipe (up to 3 m long) and a holdfast. It is a temperate brown alga belonging to the group known as kelps. These plants form dense underwater forests near the shore in temperate regions. The sporophyte produces motile haploid spores which settle and grow into the next generation, the gametophyte. The gametophyte is a tiny microscopic plant. The sporophyte and gametophyte generations look quite different and for this reason Laminaria is said to show heteromorphic alternation of generations. Photo by Mike Guiry |
 |
Life cycle of Laminaria
Interestingly, Laminaria has separate male and female gametophytes, producing
either male or female gametes.
Reproduction is oogamous with sperm swimming towards the
large, non-motile eggs.
| If you prefer a tropical example of an alga with a heteromorphic alternation of
generations, the siphonous green alga Derbesia is a good example. The filamentous
sporophyte and marble-like gametophyte are so different they were initially put into the
genera Derbesia and Halicystis respectively! We now know they are 2 stages
of one plant. The sporophyte produces multiflagellate spores by meiosis which settle to
form the baloon-shaped gametophyte. Gametes are discharged from these which fuse to
produce the filamentous sporophyte. (see p69, Sze,1986) |
 |
Sargassum sp. - Sea Holly
| Sargassum is a complex, parenchymatous thallus with a stipe
bearing leaf-like appendages and gas-bladders resembling berries. Gametes are produced on
special branches called receptacles. These in term have pits termed conceptacles in which
the eggs or sperm are produced. (It is oogamous.) Sargassum and other Fucoid algae have an animal-like life cycle with no alternation of generations. Look upon this as an exception to the general pattern found in the plant kingdom. Sargassum is shown at right. |
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Life cycle of Sargassum
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Would you like to look at a handout for this part of the course?
If so, click the button. |
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Would you
like to look at the lab for this part of the course? If so, click the button. |
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Would you like to look at some sample questions on this part of the
course? If so, click the button. |
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We have
now completed our look at the Algae. Click the button to move on to "The conquest of the land". |
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© C. M. Sean Carrington 1997
updated 28 October, 2000
