Chlamydomonas
Above:
a 3D computer model of Chlamydomonas, a single-celled green
alga found in freshwater and soil. The study of such organisms
really should not need justification. They are, like all living
things, highly sophisticated machines and to ignore the inner
workings of such natural machines would be truly ignorant of
humanity. They deserve to be studied on aesthetics alone, but as
people increasingly need financial reasons to do anything at all on
planet Earth (except, paradoxically to waste vast amounts of money
and resources on their own petty pleasures!), it can be pointed out
that these organisms make ideal 'model organisms'. Animal cells can
be very difficult to work with and there is much that can be learned
about the fundamental process of life itself by studying such
organisms. They do also have several potential technological uses.
A schematic of the detailed internal structure (shown in
section) is given below:
There
are many different species of Chlamydomonas and the details differ.
All possess the usual eukaryotic cell organelles (nucleus (G), endoplasmic
reticulum (not shown), Golgi apparatus (H) (usually x 1-4 arrayed
around the nucleus), vesicles (F), lipid droplets and mitochondria
(A)). The mitochondria are often branched, and probably divide and
move around the cell. They can be found inside the cup of the
chloroplast, at the front of the cell, and squeezed between the
chloroplast and the cell-surface membrane. A single cup-shaped
chloroplast (I) fills about the posterior two-thirds of the cell and
this contains one or two pyrenoids. This fixed carbon is converted
into starch, which is stored as starch grains in the chloroplast.
Starch grains are distributed throughout the chloroplast, but are
concentrated around the pyrenoid.
Pyrenoid(s): contain the enzyme
RuBisCO (Ribulose bisphosphate carboxylase/oxygenase) which is
responsible
for fixing carbon into carbohydrate during photosynthesis.
Contractile
vacuoles:
one pair of contractile vacuoles are situated near the front-end of
the cell (topmost in the diagram). These expel excess water which
enters the cell by osmosis when the surrounding solution is more
dilute than the cell cytoplasm (that is when the surrounding water
has a higher water potential).
Flagella (E): one pair of (smooth)
flagella project from the anterior or apical end of the cell. These
enable the cell to swim by executing breast-stroke like movements.
Each flagellum crosses the cell wall through a collar and is rooted
in the cytoplasm by a basal
body.
Two long rootlets of microtubule-bundles extend from each basal body
into the cell (not shown), one bundle contains two microtubules, the
other four. Thus there are four bundles altogether, which meet in a
cross beneath the basal bodies, with the microtubule doublets
opposite one-another (and the quadruple bundles also opposite one
another) in a 4-2-4-2 cruciate (cross) arrangement. A pair of
contractile bundles (rhizoplasts) also connect the two basal bodies
together, maintaining them at an angle of about 80 degrees. These
contractile bundles contain the protein centrin, which contracts by
supercoiling. Contraction of the rhizoplasts, moves the nucleus
forward and detaches the antenna and is a shock response (and as one
of its functions may allow the cell to escape from predators which
have grabbed hold of its flagella?). Another contractile bundle also
joins the basal bodies to the nucleus. If the cells are grown on
solid surfaces like agar (in which the cells cannot swim) flagella
are not synthesised. During mitosis
(cell division occurring in asexual reproduction) the
basal bodies detach from the flagella (which are absorbed and so
disappear) and resume their function as centrioles, organising the
microtubule spindle which separates the chromatid pairs to each
daughter cell.
Eye-spot
(stigma).
The highly reflective red eyespot, found only on one side of the
cell inside the chloroplast is constructed of either a flat or
parabolic plate of granules spaced at a quarter of the wavelength of
the light they reflect. They, therefore, appear to behave as a quarter-wave plate which reflects light of a
certain wavelength in such a way that the waves
combine by constructive interference, intensifying the light. A
parabolic plate (shaped like a satellite-receiver dish antenna)
makes an ideal reflector, since it has the property of focusing the
reflected light. Certain colours of light are thus reflected and
focused onto the light sensor itself, which is situated in the cell
surface membrane above the stigma. This
receptor is tuned to match the wavelength of light reflected by the
stigma. Occurring only one side of the cell enables the cell to
detect the direction of a light source. This is assisted by the fact
that the cells rotate as they swim, tracing out helical paths. Thus,
the light sensor will be periodically exposed to a higher light
intensity if the cell is swimming at an angle to the light
source, but will remain evenly illuminated if the cell swims towards
it. A powerful light microscope will reveal the eyespot very clearly
glinting as the cell rotates in the light.
Cell Wall. Chlamydomonas is surrounded by a rigid
cell wall, but unlike plant cells in which the cell wall is made of
cellulose (a glucose polymer), the Chlamydomonas wall is made of fibrous glycoproteins (in most algae it consists
of carbohydrate polymers) and is
triple-layered. The apical papilla is a small hemispherical or
flattened projection of the cell wall between the two flagella.
Genetics. Microscopy usually
reveals 8 chromosomes in the haploid nucleus, however, genetic
studies suggest that there are 16-17 genetic units or so, which
suggests that each visible chromosome contains two molecules of DNA,
instead of the more usual one.
Stage 1
The mature
vegetative cells are haploid (n). When nutrients especially nitrogen
(ammonia) are depleted and present in low concentrations, Chlamydomonas activates its sexual cycle
and the cells differentiate into gametes, each vegetative cell
becoming a single gamete. Some species and strains also require blue
light as a stimulus for gamete development. The gametes are present
in two genetic polarities, plus and minus, and fertilisation can
only occur between two gametes of opposite polarity. The gametes
shed their cell walls, which are dissolved by an autolysin enzyme. The gametes have
sticky glycoprotein projections on their flagella (visible under the
electron microscope as tiny stalked projections with globular
heads). Plus cells have a glycoprotein which will interact with and
stick to the complementary glycoprotein on minus cells (rather like
velcro). The flagellae thus adhere along their length, expressing
more of these glycoprotein adhesins (which is more
concentrated toward the flagella tips) to strengthen the bond. Some
species are homothallic - meaning that gametes descended from the
same parent can fuse (self-fertilisation) whilst others are
heterothallic (meaning that at least two parental stocks are needed
for cross-fertilisation). Compatible gametes apparently bump into
one another and adhere by chance, as apparently no sex pheromones
are produced to attract gametes of the opposite polarity.
Stage 2
At least
in Chlamydomonas
eugamatos,
the plus gametes pushes back the flagella of the minus gamete. In
all forms a cytoplasmic
bridge
(copulation tube) grows between the two gametes, joining their front
(apical) ends together.
Stage 3
The
adherent flagella disengage from one another. Depending on species,
these coupled cells may swim around for several hours connected in
this manner and in Chlamydomonas
eugamatos
only the plus-cell
flagella remain active during this phase. Other gametes may join
them, adhering to the complementary adhesins on their flagella, and
a clump of cells may form.
Stage 4
The two
adherent gametes, one plus and one minus, will later undergo cell
fusion or plasmogamy (cytogamy) which takes a
few minutes. Light is a necessary stimulus to initiate plasmogamy,
and in the darkness the joined gametes will keep swimming until they
die. In Chlamydomonas
eugamatos
this fusion occurs between the two front ends, but in some forms it
occurs side-on. This fused quadriflagellate cell, called a plasmozygote, continues swimming for
some time.
Stage 5
The two
nuclei fuse, first of all their outer membranes fuse together, and
then the inner membranes break and join together. The cell now has a
diploid (2n) nucleus. The flagella disintegrate from their tips
down. The,
now non-motile, zygote becomes invested in a thick warty wall and
becomes filled with starch and lipid reserves and enters a dormant
stage. This kind of zygote is called a hypnozygote.
Stage 6
After a
few days the hypnozygote spore is able to germinate once suitable conditions
of light and sufficient nitrogen supply return. Germination begins
with a meiotic reduction division, forming four haploid daughter
cells. The spore wall breaks down and the four daughter cells escape
as mature vegetative cells and the cycle is complete.
In the species usually considered, the plus and minus gametes look
identical (isogamy) but in some species the plus gametes (male
gametes) are much smaller than the minus gametes (female gametes)
and these are said to be anisogamous. In the soil-dwelling Chlamydomonas
zimbabwaensis
a single vegetative cell will divide to form many small plus
gametes, contained within the parental cell wall. When these male
gametes escape they release a sex pheromone which induces vegetative
cells to escape from their walls and form (much larger) minus or
female gametes. In some species, the minus gametes are immotile
(eggs) and these forms are said to be oogamous.
Chlamydomonas can also reproduce
asexually as illustrated below:
releasing
4, 8, 16 or more cells from a single parent. Mitosis is closed,
meaning that the nuclear membrane does not break down. Instead
microtubules of the mitotic spindle cross the nuclear envelope
through pores in either end of the nucleus. Chlamydomonas is
haploid and only the zygote is diploid. Such an organism is
described as haplontic. (In contrast, human
beings are diplontic - they are diploid and only the gametes are
haploid).
Circadian
cycles
Like
many motile algae, Chlamydomonas will swim toward the
surface to intercept the light during the day and then swim deeper
down at night (probably in order to disperse and escape predators).
The proteins it needs for photosynthesis, UV protection and nitrogen
assimilation whilst nourishing itself in the day also undergo a
daily cycle as more of these proteins are synthesized during the
daytime. These cycles are not simply passive responses to light,
rather the cells have internal biological clocks, enabling them to
anticipate the dawn and so prepare and make the most of the
available light. This clock is constantly reset by brief exposure to
light, allowing the clock to remain correct throughout the year and
at different latitudes.