Chlorophyta

The Chlorophyta are a division of almost 8000 species, most of which are fresh water species.. However some can be found in the most unexpected places... such as growing on snow packs, in soils, on trees, as symbionts in lichens or symbionts of protozoans & hydras. Calcified green algae are important components of marine sediments and beaches and can be found attached to bedrock over 150 meters deep.

Their fossil record extends back a Billion years ago with fresh water species evolving approximately 450 MYA. .

They are considered to be ancestral to land plants for their following attributes:

  • Like land plants they have chlorophyll a & b, and cartenoids in their chloroplasts... thus appear green though some are more yellowish or black due to the addition of cartenoids ( accessory pigments)..
  • They use starch as their storage product.. just like most of the angiosperm or higher plants you normally consider..
  • Their flagella ( only motile stages) are 2 or more anterior or laterally inserted, of equal size and whiplash
  • Their cell wall is made up of cellulose, hemicellulose and pectins

Within the division, there are 3 classes:

  • Chlorophyceae
  • Charophyceae.. some taxonomists consider them to form their own phyta and designate them as the Charaphyta. For now we keep them downgraded as a subset of the Chlorophyta...
  • Ulvophyceae

Before we go over these classes, let's review some primary concepts:

In the greens, we see to a certain extent convergent evolution occurring, in which single celled organisms give rise to progressively larger, 3D structures. Three basic patterns emerge which are best illustrated by reviewing the diagrams below.

Remember, although motile each 'cell' retains its flagella which move synchronously through the colony, they are moving through a fairly viscous medium. We don't consider water viscous, but the smaller the creature the greater the resistance of the fluid. Also, with increasing size the greater the probability of damage, the greater the synchrony involved and so on. Mentally work through the advantages and disadvantages of large relative to small or single celled creatures.

1. Evolution of colonial lines: Perhaps the best example of this is seen in the Volvocine lineage which we will observe in lab. In its simplest form, a single celled individual in the process of replication does not 'move' away from its sister cell. Instead, they remain together bound in some gelatinous sheath. With such evolution we would expect to see a pattern of 2,4,8,16,32,64,128 and so on, we can actually be found in nature. However there is a limit. In the largest volvoxes there is a limit between 500-1000. Why?

 

 

2. In the parenchymatous line, we see the evolution of structure which we find common in the higher plants.

After cell division, cells adhere tightly to one another.

To form a filament, we just need to continue division in one line.

To form a sheet, cells would divide not only linearly but also laterally. If occasional cells divided so we would end up with branching; of all cells did so then a flat thin sheet.

To form a thicker or parenchyma-like 3D structure, cells would also divide horizontally. Example: Ulva, seen on wave swept beaches- looks like limp lettuce

3. The coenocytic is perhaps the most unusual form of growth. Imagine a cell dividing without producing new walls. Nuclei, protoplasm, organelles keep on forming, but no cellular divisions.

Surprisingly these 'balloon' like structures can grow fairly large - inches to feet. One would assume they would be delicate,yet after viewing Codium or 'dead man's fingers, one is rapidly convinced of the toughness of these structures.

 

Life cycles

The diversity of life cycles found in the algal lines is suprising given the pretty much one type of cycle we see in the higher animals and plants. We're all use to a 2n organism giving rise to 1N gametes, which fuse to produce a new 2N organism.

However, the algae vary, in that in some species the 'adult' phase remains 1N with only the zygote 2N, or they may be 2 phases, a 1N and a 2N both codominant either both looking nearly identiacal or totally different! Why this diversity of patterns? Think it through....would 2 differnt forms allow differential survival in differnent enviroments? Is 1N sufficient genetic variety? Conjecture!

Review of life cycle patterns:

Perhaps the simplest pattern is as follows: the only 2N stage is the zygote...give the advantages and disadvantages..

 


In this second pattern, the main stage is 2N and cells undergo meiosis to produce 1N gametes

 

In the 3d pattern:

Placement of flagella: one of the features that differentiate the algal groups is the placement of the flagella. In some they are placed apically ( at the apex or tip) in others laterally ( to the side) , thus making the cell asymmetric.

What difference would it make if the flagella are placed apically or laterally? Think of when you swim and the different strokes you can use... if your arms were placed before your head and then moved towards your body, you would thrust your body through the water. If you could place your arms to your sides, would it be easier to rotate?

Placements can also indicate to a taxonomist possible relationships between species..

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Cell division:

Phragmoplast : during cell division the phragmoplast is a set of short microtubules oriented parallel to the spindle microtubules... it catches dictyosome vesicles and guides them to the site where the new cell plate is forming.

Phycoplast: a set of microtubules oriented parallel to the plane of the new cell wall and involved in wall formation.


Chlorophyceae

The chlorophyceae are one of the largest groups of the green algae, containing over 2500 species. They come in all sizes and shapes, ranging from single celled through round colonies and in filamentous forms.

They are recognized by the following traits...

  • Cell division is by a phycoplast of microtubules that form parallel to the plane of division
  • The spindle disintegrates after anaphase separation or In some species the there is a furrowing of the cytoplasm membrane oriented by the phycoplast while in still other species vesicles coalesce to form a cell plate that divides the cell.
  • If motile cells exist, then the flagella are inserted anteriorly and from the basal bodies, 4 bundles of microtubules cross out in a 'cross-my-heart' or x configuration
  • Life cylcle is zygotic meiosis

The 3 major groups include:

Volvocales, Chaetophorales, & Chlorococcales - they make up more than half of all chlorophyceans and have an offset flagellar arrangement (1 o'clock-7 o'clock) of flagella.

Chlorellales & Chlorococcales - These algae have opposed flagellae (12 o'clock-6 o'clock), though some have only vestigial flagellae and so have not been definitively associated with this group.

Oedogoniales - Members of this smallest group have a complex multiflagellate crown on their swimming spores. All are filamentous, oogamous, and have net-like chloroplasts.

We will only cover the Volvocales here.

 

Order Volvocales or the Volvox lineage contains some of the most beautiful and commonly recognized species... Volvox shown below is known for its colonial structure. The chlamydomonas -like cells ( the 'original' unicells) bound by a gelantinous matrix, communicate with one another through various chemical / homonal signals. They beat their flagella in synchronous fashion to maintain their fluid rolling motion.

 

Pandorina ( shown below), the next step up: note rapidly dividing colonies resulting in a mixture of colony sizes

lab photo: e. iglich

Gonium: how many cells are included here?

 

Chlamydomonas is the simplest in the line.. the single celled flagellated member

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Division Charaophyta or the Stoneworts.

Charaphyta or charophyceae are freshwater algae, which usually grow attached to some substrate by root-like structures called rhizoids. Unlike some of the earlier algae, they can be fairly 'large' in size, with their shoots forming nodes from which whorls of side branches originate.

 

 

 

image from: http://vis-pc.plantbio.ohiou.edu/algaeimage/pages/Chara.html



Sexually they are pretty advanced, producing reproductive structures similiar to that found in some of the mosses..it has an egg protected by secondary sexual cells.. thus some speculate they may be the ancestor of the moss line.
(Note below the immature oogonium in the center...responsible for egg production and its' surrounding tube cells which spiral around the internal egg cell.)

The most differentiating aspect from the other 2 classes is that the nuclear membrane disappears before metaphase. and the spindle persists until after partitioning is complete

In some species furrowing occurs and in others a phragmoplast and cell plate is formed

Species produce a dormant zygote that undergoes zygotic meiosis so that all multicellular stages are haploid; in higher speices there is a clear case of oogamy = gametes differentiate into large eggs and smaller sperm.

Flagellated species are inserted laterally and and attach to a multilayered structure of microtubules so that the cells are asymmetric.

 

Zygnemataceae - there is argument here where this group belongs - your text places in the Ulvophyceae but other sources list it here....they are known as the conjugating algae as two filaments will meet and form conjugation tubes.See lab notes for more information...

Spirogyra is another well known genera in this group.

Spirogyra

Desmids;can be quite beautiful; symmetrical 1-4+ celled floating phytoplankton.

 

Example of one Order:Ulvales

Ulvales - sea lettuce is commonly found washed up on beaches so you may have encountered it in your wanderings as I did below on a Nova Scotia beach:

In this slide of Ulva ( sea lettuce) you can note the parenchymatous morphology; flat thallus 2 cells thick and up to a sizable meter long