MORPHOLOGICAL DIVERSITY
WITHIN THE ALGAE

 

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.
 

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  

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. 

 

COENOBIA

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. 

 


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

brya141.jpg (16709 bytes)

 

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
halimeda.JPG (32468 bytes)

 

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. 

 

PARENCHYMATOUS & PSEUDOPARENCHYMATOUS ALGAE  

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. 

 



  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:
 

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.
 

 

LIFE CYCLE DIVERSITY WITHIN THE ALGAE  

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.

 

                   Life cycle of Sargassum 


 
 


 
 

 

  
  
  
  
  
  

  
  
  

In this course we will NOT be looking at the very complex life cycles of red algae which have THREE generations.  
  
  

You will also in your reading 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.

   
 

Would you like to look at a handout for this part of the course? 
If so, click the button. 
Would you like to look at the lab for this part of the course? 
If so, click the button. 
Would you like to look at some sample questions on this part of the course? 
If so, click the button. 
We have now completed our look at the Algae. 
Click the button to move on to "The conquest of the land". 

 
   .

© C. M. Sean Carrington 1997

updated 28 October, 2000