Mutualism Mutualism brings two creatures, like clownfish and sea anemones, together.
This is a nice relationship. Right out of one of those "how to achieve
happiness with your partner" books.
Mutualism is everywhere you look on coral reefs. One creature, like
the clownfish, will feed and protect the other in exchange for some kind of protection. If
danger threatens the fish it will race into the tentacles and vanish amid hundreds of
stinging tentacles. The clownfish actually trains the anemone not to sting it. Each
clownfish introduces itself to its partner over a period of months when the fish is very
small. A special protein in the mucous of the scales identifies the particular fish.
Most coral reef fish have a mutual relationship with the corals.
They hide in the branches of the corals if danger threatens and in return the fishy waste
products rain down on the coral's little upturned faces providing much needed phosphates
and nitrates.
Symbiosis.
Ahh, Symbiosis; the mover and shaker of evolution. Symbiosis is the
big special love where two beings come together in perfect harmony. They become one,
living together, changing together, staying together for all time. This is different from
any other kind of relationship. Love like this does not happen often. In a world full of
predation, disease, parasitism, mutualism, commensalism, and stoic, uncaring loneliness,
misunderstandings are more common than total understanding. But when it does happen it
changes the whole world.
The Love Bugs
It must have been instant, full, unrequited, perfect love the first
time the beings came together. A touch, a kind of sudden recognition, a wonder, awe, and
zowie. Forever. Absolutely forever. For two billion years they have evolved together. All the myriad forms and styles of beings of Earth are children of that perfect
love.
The love bugs. There are, naturally, two of them. One is called
chloroplast, the other mitochondrion. Chloroplast is the little
being that makes plants green and converts sunshine, carbon dioxide and water into sugars
and oxygen. Mitochondrion is the little being that takes sugars and oxygen and changes
them to carbon dioxide and ATP, the fuel for life.
The Greatest Love Story
on Earth
Once upon a time, billions of years ago, the world was full of
thousands of different kinds of bacteria and they fought continuously with each other,
inventing highly sophisticated molecular weapons to attack and defend the other.
One strain of bacteria developed a system to manufacture a deadly
poisonous gas, Oxygen. They released this into the sea, killing all of their enemies.
Another strain of bacteria discovered a way to detoxify oxygen by combining it with carbon
to produce carbon dioxide and a high octane fuel called ATP. The creature that invented
this defense became the Mitochondrion love bug.
One day, on the edge of a great continent that no longer exists, a
great battleship of a bacterium captured and devoured a mitochondrion love bug. The
defenses of the love bug prevented the giant from digesting it, but the bigger bacterium
built a force field made of complex lipid molecules, imprisoning, but not harming the love
bug. The love bug loved it. Food was abundant, it had everything it wanted. It was so fat
and happy it divided into two and these into two more. The big cell was also ecstatic with
the relationship as it could easily utilize the ATP for it's own a high octane energy. But
even better, the mitochondrion bug destroyed the highly poisonous gas, oxygen! To get more
ATP, the bacterium had only to provide sugars. And there was plenty of sugar in the
cyanophyte bacteria. And the planet's oceans had become covered with thick mats of them.
Now the big bacterium, inhabited by many love bugs, became a "cell" or organization of individuals. It went right into the masses of blue
green algae that coated Earth's seas, without fear of the poison gas, oxygen. The cell fed
on cyanophyte bacteria and divided again and again, populating the oceans with itself. The
cyanophytes developed many other defenses but the great mats were gradually consumed and
the seas became clear. One of the cyanophyte defenses was a much better defended oxygen
manufacturing nodule called a chloroplast.
The Chloroplast
Perhaps a million years or so later, one of the issues of the
predatory cell engulfed the second love bug, the chloroplast. This creature, like the
mitochondrion, could not be digested and the cell placed it in a force field prison and
would not release it. The chloroplast produced lots of oxygen and sugar. The mitochondrion
love bug took oxygen, sugar and water and transformed it into more energy and carbon
dioxide. The chloroplast took sunlight, carbon dioxide, and water and turned it into
sugars. The two love bugs were all that the big cell needed, other than access to water
and some essential minerals that were common in Sea. Instead of chasing all over the
oceans hunting, the big cell could simply float about in the surface waters and relax
while the chloroplast and the mitochondrion did their harmonious best to make everyone
happy. The big cell didn't have to do much more than protect the two of them from harm,
and it designed bigger and better force fields made from a remarkably tough molecule
called cellulose to fend off attacks by other predators. This union worked well and the
cells evolved into all the plants on Earth.
The big cells with only one love bug - the mitochondria - evolved
into all the animals on Earth, keeping up with the constant improvement in plant defense
by inventing new ways to harvest them.
The love bugs are so perfect that they have not changed very much in
the billions of years the cells have cherished them. The cells have evolved into all the
wonderful display of life on Earth, but the love bugs remain happily engaged in their
relationship.
Love bugs reproduce by dividing. The cells don't make them. They
divide and divide again in perfect harmony and balance with the cells. This means that the
love bugs in every form of life on earth are the original issue love bugs. I mean, the
love bugs in every cell on Earth have never died. Each one is at least two, perhaps three
billion years old.
Love Bug Mark II, Zooxanthellae
There is another love bug. A long snake-like bacteria that is a
powerful swimmer. It is called a flagellum or sometimes a cilia. At some point in time,
many different kinds of cells began using flagella or cilia as whip-like propellers to
speed them through the water. One of these cells, called Gymnodinium, became a
second level love bug, one in great demand by some 650 or so reef building corals, 6
species of giant clams, a scattering of gorgonians and a flatworm.
Corals hunt the love bugs just after they form into microscopic
swimming planulae larvae. When they find the Gymnodinium, they absorb it (or the
Gymnodinium infects the larvae, nobody knows who initiates the relationship). The
Gymnodinium drops the flagellum and becomes a rounded cyst. The cysts are called
zooxanthellae. About 8000 could fit on the period at the end of this sentence. Special
amoeboid coral cells grab hold of the zooxanthellae and carry them through the quickly
growing coral polyp. They shove the cysts between other coral cells, just under the outer
skin cells where they can get the most light. The zooxanthellae divide and divide again,
forming a thick garden of brightly colored cells crammed between the coral cells.
Eventually, the zooxanthellae outweigh the coral cells by about three to one. All of the
rich colors of tropical corals come from the zooxanthellae - the coral cells are
transparent.
If the coral is put into a dark room, it rejects the zooxanthellae,
spewing them out into the sea water with long strands of mucous. The coral continues to
live and, if replaced in the light again, it can become repopulated by zooxanthellae.
The coral does not (at least not normally) eat the zooxanthellae in
its system. The coral polyps use their tentacles to catch other kinds of small creatures
from the sea and they capture pellets of fecal material dropped by fish schooling over the
reefs. The captured food and fecal material is rich in phosphates and nitrates. These
fertilize the zooxanthellae.
The coral cells produce carbon dioxide, nitrate and phosphate as
waste as do all animal cells. The zooxanthellae, nestled in the coral tissue, take these
waste materials and use them as food. Using sunlight, they convert the coral wastes into
carbohydrates and oxygen. Both they and the coral cells use these to energize the
complicated job of rearranging the elements of sea into their form, to maintain vigilance,
move and reproduce as needed for survival.
Relationships depend on different viewpoints
Zooxanthellae as parasites
A century ago, biologists classified corals with plants. When they
discovered the relationship of coral and zooxanthellae they thought of the zooxanthellae
as parasites. Even today, biologists talk about corals becoming infected by zooxanthellae.
This was a perfectly reasonable view. Looked at from the standpoint of the zooxanthellae,
the coral was a host. They entered the host and multiplied, filling every available space.
The coral provided food and shelter. The growth pattern of the coral was modified by the
influence of the zooxanthellae to maximize sunlight exposure for the benefit of the
parasites. If things got tough, the zooxanthellae could escape into the sea.
Zooxanthellae as cultured plants
Other scientists thought the corals farmed the zooxanthellae. Seen
from the perspective of the coral, the zooxanthellae were slaves. The coral cells captured
and arranged them in their tissues. The coral cells obviously farmed the zooxanthellae to
provide themselves with food and oxygen. The method is so successful a coral colony
produces more food and oxygen than it can use by itself. These are released into the sea
water and help support the other reef creatures around them.
Zooxanthellae as partners
Then a third viewpoint was proposed to satisfy both schools of
thought. The coral cells and the zooxanthellae both benefited from the association, a
phenomenon already known in lichens. This mutual association in plants was called
symbiosis (living together) and the concept seemed to fit well with the
coral-zooxanthellae relationship. A great deal has been written about corals and
zooxanthellae from this viewpoint and everyone agrees this is the best way to look at the
association. But there is another viewpoint.
Corals and Zooxanthellae as one concept |
