GREEN ALGAE
| The term algae is used to collectively refer to a wide range (20,000-30,000 spp.)
of very simple photosynthetic organisms. While this term is no longer
used as a taxonomic grouping, it is still useful for referring
informally to
these photosynthetic protists. (Protists are diverse eukaryotes which
are neither fungi, animals nor plants.)
The earliest multicellular alga known is the red fossil alga
Bangiomorpha
(at right), found in 1,200 million year old rocks in Arctic
Canada. What's more this is the first known organism to show sexual
reproduction.
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In this course, we are dealing only with Green Algae.
Click here for an overview of other algal
groups.
Key
Algal Features
| There are 3 features which distinguish algae from land plants;- |
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Body plan: There is no specialisation of the algal body into root,
stem etc. The photosynthetic portion of the alga is a thallus
while the attachment portion comprises hair-like rhizoids. For this reason, old
classification systems put the algae into a grouping known as the Thallophytes.
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No Embryo: For most algae, sperm and eggs fuse in the open water and the
zygote develops into a new plant without any protection. For other plant groups the zygote
develops into an embryo within the protection of the parent plant. For this reason,
all other plant groups are termed Embryophytes.
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Reproductive structures: The gametes are produced within a single cell.
There is no jacket of sterile cells protecting the gametes.
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| Release of algal sperm cells from a single cell |
vs. Moss egg cell surrounded by sterile cells |
Where algae live
Being aquatic, algae are
- marine
- freshwater
- terrestrial
Terrestrial algae are effectively surviving in an aquatic environment on
land. Soil algae survive in a film of soil water.
The other major group of terrestrial algae are those in lichen symbioses.
Lichens comprise fungi and algae (or blue-greens) in partnership. The fungus provides an outer weft of
mycelia which creates a humid protected environment for the alga to live and
photosynthesise (and feed the fungus!).
Lichens are very good
sensors of environmental pollution. |
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Fungal layer
Algal layer
Fungal layer
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Lichens have distinctive morphologies and so these associations have
traditionally been given
genus & species names as if they were discrete organisms.
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| Crustose lichen on rock |
Fruticose lichen |
Economic Significance of Algae
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Algae are primary producers, i.e. they are the start of the food chain.
Phytoplankton
are responsible for more than 45% of the Earth's annual primary production.
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Algae under particular nutrient-rich conditions may grow disproportionately causing
potentially harmful algal blooms.
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Seaweeds are used as fertilisers and even food (by the Japanese, Irish, Welsh and
even some of us here in the Caribbean who enjoy "sea moss" ).
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Extracts from the cell walls of algae (typically brown & red,
though!) provide the
polysaccharides agar and carageenan. These are used as thickening agents in food, in
surgical dressings and in microbial media.
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Algae help build reefs. Corals
contain microscopic algae in a
mutualistic relationship which fix carbon dioxide to provide a fuel source
for the animals that build the coral reef. Secondly, the exoskeletons of coralline macro-algae often
become incorporated into a reef after the alga dies.
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The skeletons of one group of algae (the
diatoms - not green algae!) are glass-like and
this material (diatomaceous
earth) is put to a variety of uses, such as abrasives (once used
in
toothpaste!), insecticides, reflective road signs, swimming pool filters. |
Algal Morphology
You can just about see the full range of "body types"
within the algae by just looking at the green algae.
UNICELLS
These algae are single cells, with or without flagella.
COLONIES
| 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.
With permission of the
MCC-NIES, Japan
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COENOBIA
The coenobium (plural coenobia) is a colony
with a fixed number of cells. Coenobia may be motile or
non-motile.
| 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.
Image from Protist
Information Server
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Click here
for more micro-algal images
SIPHONOUS ALGAE
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
© Peter v. Sengbusch
mailto:b-online@botanik.uni-hamburg.de |
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At left is one of many species of Caulerpa, a siphonous
green alga, found in 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.
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Another siphonous alga you
might want to
acquaint yourself with is Halimeda, shown
at right. This alga is extensively calcified
making it more resistant to predation |
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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.
There are also algae with branched filaments. Where there are basal, prostrate filaments
for attachment and erect branches for photosynthesis, this is said to be a heterotrichous
filament. |
PSEUDOPARENCHYMATOUS ALGAE
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Seaweeds made up of "boxy" cells like those of higher plants are termed
parenchymatous. Many red and brown seaweeds are of this type and may be even more
complex structurally with stem-like stipes and leaf-like appendages. Others in cross-section appear to be parenchymatous but are in fact really made up of
interwoven filaments which give this appearance! Several green
macroalgae are of this type and are termed
pseudoparenchymatous.
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Ulva is a membranous sheet with a holdfast for
attachment and a
pseudoparenchymatous substructure. It grows in shallow sea water,
often where there is nutrient-rich run off from the
land. |
| Seaweed morphology has also 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. |
REPRODUCTIVE DIVERSITY
WITHIN THE ALGAE
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:
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isogamy - equal-sized
motile gametes
anisogamy - motile gametes
almost equal-sized
oogamy - small motile male
gamete, large non-motile
female gamete |
In the simplest algae, all cells can become gametes while in the more specialised only
some can.
ALGAL LIFE CYCLES
Algae often 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 main types of algal life cycles are exhibited by
green algae. (Red algae have even more complicated life cycles!)
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.) |
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.
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Life cycle
of Ulva
More
on Ulva life history
Life cycle of
Derbesia
The siphonous green alga Derbesia shows a
heteromorphic alternation of generations. The filamentous
sporophyte and balloon-like gametophyte are so different they were initially put into
two different
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 balloon-shaped gametophyte. Gametes are discharged from these which fuse to
produce a zygote and then the filamentous sporophyte.
(see p69, Sze,1986) |
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Derbesia gametophyte from Taylor
(1960) |
Derbesia sporophyte
(photo by Diane Wilson) |
Caulerpa
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Caulerpa is a
genus of green, marine alga, common in the Tropics. It almost looks like a
higher plant with its creeping stolon, photosynthetic assimilators and
root-like rhizoids. Being siphonous, Caulerpa
is like a hollow tube lined with multinucleate cytoplasm. If the outer
wall is punctured, the protoplasm will leak out and so this alga has a
mechanism for rapidly sealing such injuries.
Caulerpa shows no
alternation of generations. Look upon this as an exception to the general pattern
found in the plant kingdom. The mature plant is diploid, producing anisogametes
by meiosis, which fuse to form a zygote that develops into the mature
diploid plant without an intervening haploid stage.
There are two interesting aspects to reproduction in Caulerpa. Firstly,
the entire protoplast migrates to the periphery, cleaving into motile
gametes and leaving a dead, empty thallus. You can actually see
this with the naked eye as the organism turns from green to white!
Secondly, Caulerpa exhibits mass spawning - synchronized,
simultaneous release of vast numbers of male and female gametes. These are
released as green clouds into the sea over a 5-15 minute period, typically
at dawn (Clifton, 1997)
Caulerpa racemosa in
Vickers
(1908) Phycologia barbadensis |
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In this course we will only be looking at green algae.
You may also come across terminology for describing
algal life cycles;-
monobiontic, dibiontic, haplontic, diplontic.
We will not be using these in this course.
If you are going to use them make sure you do not mix them up;-
haplontic- a life cycle in which the dominant phase is haploid
diplontic - a life cycle in which the dominant phase is
diploid
monobiontic - a life cycle of one free-living phase
diplobiontic - a life cycle of two free-living phases.
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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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Would you like to look at
Learning Objectives for 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 27 July, 2005
