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..
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:
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...
Before we go over
these classes, let's review some primary concepts:
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.
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.
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.
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
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.
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.
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.
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!
life cycle patterns:
Perhaps the simplest pattern is as follows: the
only 2N stage is the zygote...give the advantages and disadvantages..
second pattern, the main stage is 2N and cells undergo meiosis
to produce 1N gametes
In the 3d pattern:
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.
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?
can also indicate to a taxonomist possible relationships between
: 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.
a set of microtubules oriented parallel to the plane of the new
cell wall and involved in wall formation.
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
They are recognized
by the following traits...
- Cell division
is by a phycoplast
of microtubules that form parallel to the plane
- The spindle
disintegrates after anaphase separation or In some species the
there is a furrowing of the cytoplasm membrane oriented by the
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
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.
& 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
- 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
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.
( shown below), the next step up: note rapidly dividing colonies
resulting in a mixture of colony sizes
photo: e. iglich
how many cells are included here?
is the simplest in the line.. the single celled flagellated
Charaophyta or the Stoneworts.
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
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
In some species furrowing occurs and
in others a phragmoplast
and cell plate is formed
Species produce a dormant zygote that
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
- 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...
is another well known genera in this group.
be quite beautiful; symmetrical 1-4+ celled floating phytoplankton.
of one Order: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
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