Life in the microcosmos is tough for its unicellular residents.
Prokaryotes and protists have to compete for food and space while also trying to avoid
organisms that are more than happy to see all of their complex, energy-filled biological
bits as an afternoon snack, and managing those tasks can be even more daunting when you are
very, very small.
So instead of facing all these challenges alone, many of these organisms treat survival
as a team effort.
As cells divide, they adhere to one another, building a colony that becomes larger and
larger with every cell division.
An individual cell can only get so big before its size comes at the expense of other abilities.
But a colony can get much bigger, providing protection against predators and a competitive
advantage when it comes to finding space and food.
And as the cells come together and organize, a colony can enable organisms to do so much
more than they could on their own.
It’s an advantage that became a vital step on the path not just to more complexity in
the microcosmos, but also a vital step on the path to us.
But before we get to that, let’s check out this long green thing cutting across the screen.
Maybe it looks like a blade of grass, rudely floating in everybody’s way.
But if you look closer, you can see thin filaments made up of individual cells of a freshwater
cyanobacteria called Aphanizomenon.
Often times the names of these organisms are really a pain to say, but Aphanizomenon is
just a joy.
If these cells were on their own, they’d basically be little balls of candy just waiting
to get chomped but these long grass-like structures make it harder for filter feeders like water
flea to eat the Aphanizomenon colony.
In lakes and ponds, these grassy structures can
accumulate, creating dense, visible blooms that release toxins into the water.
Nostoc colonies are also made up of filaments, like a string of beads, where the beads are
cyanobacteria.
But where Aphanizomenon likes to keep its strands elongated, Nostoc wraps its filaments
around a structure made mostly of different types of sugar that it excretes to create
a round, gelatinous mass.
These colonies vary in size, but some particularly large ones have been measured with a diameter
of 22 cm.
I realize I said that like 22cm is very big, but it is when you’re talking about a colony
of single-celled organisms.
Nostoc use sunlight to make their food.
But in the process, they also produce oxygen.
This is a problem because Nostoc also likes to fix nitrogen from the air to make necessary
chemicals , and oxygen interferes with a key step in that process.
But living in a colony means that some of the Nostoc cells can specialize, developing
a thicker exterior that prevents oxygen from getting inside the cell.
These specialized cells are called heterocysts, and they can focus on fixing nitrogen without
any worries of intruding oxygen, while the rest of the colony can continue photosynthesizing.
Bacteria aren’t the only species that form colonies.
These spiked, circular creatures are Pseudopediastrum boryanum.
The colonies are flat, and the cells are arranged in concentric circles around each other.
Where the Aphanizomenon and Nostoc colonies we showed earlier seemed to be full of cells
strung together, the Pseudopediastrum colonies are usually made up of 8,16, or 32 cells,
whose walls contain sporopollenin, a hardy material that protects the cells from the
environment.
The gold algae Synura also forms colonies, with individual cells coming together like
little yellow marigolds.
Each cell is encased in silica scales and has two whip-like structures called flagella,
which tumble the colony through water.
When Synuras bloom, they can turn a whole pond yellow, letting off a smell that is somewhat
hard to describe, but even if you can’t see the pond, you’ll definitely be able
to smell it.
This Gonium colony is made up of several flagellated cells arranged in a sphere.
It may look simpler compared to some of the other colonies we’ve seen, but with the
rest of its volvocine algae relatives, it may hold answers to how multicellular life
evolved.
Members of this genus are either unicellular or form colonies that, through evolution,
have increased in complexity from species to species, like this Eudorina colony, whose
structure is more complex when compared to its predecessors.
Like Gonium, the Eudorina colony is made up of only one type of cell, repeated around
its surface.
But these colonies created the structure that would serve as the evolutionary stepping stone
for a species just on the edge of multicellular, Pleodorina, which contains specialized reproductive
cells, and then ultimately to the definitely multi-cellular Volvox algae.
Like the colonies before it, Volvox is built around a gooey transparent matrix.
And like Pleodorina, it contains a set of specialized reproductive cells, just a tad
bit more specialized.
Those smaller green spots are somatic cells, or body cells.
they don’t reproduce or divide, and one of their main jobs is to help move the colony
using their flagella.
But after a few days, when their duty is done, those cells die.
Meanwhile, on the inside are the larger gonidia cells, which drive asexual reproduction and
might be immortal.
Now remember the Nostoc colonies that we talked about before?
They also had elements of specialization, with some cells devoted to nitrogen fixing
and others devoted to photosynthesis.
The difference between Nostoc and Volvox however, is that Volvox reproduces by creating multi-cellular
daughters inside of it.
The gonidia cells divide creating a kind of baby volvox with all of the somatic cells
and gonidia cells it will need as it matures.
It’s an organism inside an organism, and when they’re released, it is not cell division,
it’s is birth.
Another example of how, when you look very very close, every sharp line turns out to
be blurry.
The colonies that assemble in the microcosmos are important, not just for the survival of
the organisms that comprise them, but to us and our own understanding of how our own multicellular
existence came into being.
Whether bacteria or algae, the way these colonies come together and the shapes they take on
can be as distinctive as the individual organisms they’re made of.
They make life in the microcosmos just that much more complex, forming the foundations
for multicellular life both under the microscope and beyond.
Thank you for coming on this journey with us.
If you want to see more from our Master of Microscopes, James
check out Jam and Germs on Instagram.
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