Wednesday, May 7, 2014

The Proterozoic Eon, Part 2

Proterozoic: Stromatolite Shoreline with Bacterial Mats
Adapted from an image CC Paul Harrison

When we last left the Proterozoic Eon, it was 2.5 billion years ago and we were finally approaching the shore. It doesn’t look much like a modern beach, does it?

Those rocky protrusions in the tidal zone are stromatolites. They’re a little like coral, in that they're made by microorganisms and grow over centuries. But where corals are complex, multicellular organisms that build calcified structures to protect themselves, these stromatolites are made by simple bacteria—in fact, some of them are the same cyanobacteria that made those terrestrial desert crusts we saw on the way here.

And they’re not building anything on purpose, the way coral does. Go ahead and feel one of the stromatolites. It's okay, it won't hurt you. It's pretty gross, right? Okay, don't look at me like that. I said it wouldn’t hurt you—I didn’t say it wasn’t disgusting. That goop you're still trying to wipe off your fingers is concentrated bacteria, clinging to whatever surface is available. Atop the stromatolites, that surface just happens to be the compressed remains of their dead ancestors, mixed with silt that’s become cemented in the bacterial goop. When the current crop of bacteria are dead, their descendants will set up shop atop their remains. And so it will go, the stromatolite slowly growing year by year, century by century.

You don’t see a lot of stromatolites in the modern era, because the bacterial mat couldn’t get this thick without some grazing animal—whether large, small, or microscopic—coming along and eating it. In the modern world, you’ll only find living stromatolites in the few places where grazers can’t survive, like hot springs.

But here in the early Proterozoic? There aren’t any grazers at all, anywhere on Earth. In fact, there are no multicellular organisms—no fish, no crustaceans, no animals of any kind. There probably aren’t any complex single-celled organisms either—nothing with a nucleus and organelles, like the cells in our bodies. There are just bacteria.

But for the bacteria in these shallow tidal waters? Life is good.

There’s plenty of sunlight, plenty of minerals, and plenty of water. The sea here is a multicolored soup teeming with cyanobacteria, while the sand and rock is covered in a 7-layer dip of different bacteria species, living together in complex micro-ecosystems called bacterial mats.

Bacterial Mat

The top layer of the mats is mostly cyanobacteria, which convert sunlight to energy in a process similar to modern plants. They take water, carbon dioxide, and sunlight, and turn it into sugar. Oxygen is produced as a byproduct.

Below them, in the middle part of the mat, you'll find purple bacteria. Purple bacteria also photosynthesize, but they don’t use water—though they still need a wet environment to survive. Instead of water, these more ancient bacteria require either hydrogen, sulfur, or sulfide, depending on their species. This more ancient form of photosynthesis doesn’t produce oxygen. In fact, oxygen can interfere with photosynthesis in many modern species of purple bacteria1. That’s why purple bacteria tend to form a separate layer, away from the cyanobacteria. Any purple bacteria growing into the cyanobacterial layer will be stunted by the oxygen there, and won’t grow much further. This effect—not any kind of deliberate migration—is why the bacterial mat forms such distinct layers.

In addition to the color purple, purple bacteria can be orange, red, or brown. There may be multiple species in the mat, and they may all form their own separate layers, making this micro-ecosystem into a goopy, disgusting rainbow of diversity.

Stromatolite Section
Stromatolite Section from

Below them are various species of obligate anaerobes. These anaerobes can’t photosynthesize at all and—unlike the purple bacteria, who merely grow more slowly in the presence of oxygen—these poor little dudes are actually poisoned by it. But they have to get energy somehow, so they flirt with danger by living downstairs from a poison factory. They eke out a living there, feeding off the decaying bodies of the bacteria in the layers above. They’re the scavengers of this tiny little ecosystem, getting rid of the dead biomass and recycling it.

These mats aren’t the only life on earth. If we'd remembered to bring a deep-diving submersible, we could find some hydrothermal vents and visit more obligate anaerobes—cousins to the ones at the bottom of the mats—living in the lightless depths of the oceans, feeding on sulfur coming up from undersea vents.

Up here on the surface, though, cyanobacteria are the stars of the show, and they’re about to cause the greatest environmental catastrophe the world has ever seen.

Well, “about to” is relative. It’s going to take about a hundred million years to really feel the effect of the catastrophe, which we'll cover next time. Don’t worry, I brought Monopoly to pass the time.


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Citations and References
  1. Garrity. Bergay’s Manual of Systematic Bacteriology: Volume Two. Pg. 119

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