Home > Readings and Resources Search Glossary Home Site Index
Readings Web Sources Position Statements

Creatures from the Black Lagoon:
Lessons in the Diversity and
Evolution of Eukaryotes (3 of 5)

by Scott Dawson

prev page

 

II. Life in anoxic worlds

Early Earth
We know from the geologic record that the early Earth was a fireball and the atmosphere was quite different from what it is today — in fact, no oxygen was present. All oxygen was produced biogenically by microorganisms several billion years after the earth was formed. So for roughly half of Earth's history, there was no oxygen atmosphere. When microbes began producing oxygen in such quantities that it began to saturate the atmosphere, it was the biggest environmental catastrophe in Earth's history.

Some of the oldest fossils belong to what look like photosynthetic bacteria — bacteria which breathe carbon dioxide and exhale oxygen. So this process happened early in Earth's history.

Microbes made the world. Think about it — oxygen is actually quite poisonous, generating hydrogen peroxides and oxygen radicals. Many microbes don't have a way of dealing with poisonous oxygen. So they avoided it . . . and they still do today.

Fossil Redbeds
This is a rock from a fossil redbed. The red color derives from oxidized iron, like rust. When the percentage of oxygen in the atmosphere became high enough, iron mineral deposits became oxidized (red). Reduced iron compounds like iron sulfides in contemporary anoxic environments look black. How did this happen? Microbes. Specifically, bacteria called cyanobacteria (or blue-green algae) — they use light as energy, breathe carbon dioxide and generate oxygen.

Cyanobacteria
This is one example of a cyanobacterium, Spirulina. Spirulina is pretty common in many aquatic habitats. If you've ever seen a green scum on the surface of soils/sediments, that is cyanobacteria. Think of how many cells you would need to completely change the composition of the Earth's atmosphere, as happened a few billion years ago. Cyanobacteria are found all over the world. In fact, in Berkeley you can find them in grocery stores as . . . Spirulina Smoothies!

 
Evolution of Oxygen
This graph shows the percentage of oxygen in the atmosphere over time and indicates when red beds would have formed. The first appearance of red beds is at two billion years, which is also the oft-cited age of eukaryotes (cells with a membrane-bound nucleus). Why? The oldest eukaryotic fossils are coincidentally about the same age. But do eukaryotes require oxygen? We were all taught about simple cells like bacteria which use glycolysis or fermentation alone to generate ATP (the energy currency of the cell). Then as evolution progressed, eukaryotes developed the ability to harness oxygen via the mitochondrion, the energy powerhouse of the cell. This is another misconception. First, bacteria invented oxidative respiration — eukaryotes just acquired the ability, by acquiring a bacteria cell by endosymbiosis — this later became the mitochondrion. Also, many eukaryotes do live prosperous lives without oxygen (or without mitochondria for that matter). Here's one below (Giardia), that hopefully no one has had a personal relationship with:

Giardia is an aquatic protist, and a parasite that major cause of dysentery world wide. Hopefully no one here has encountered it while camping. Giardia has no mitochondrion and it likes to avoid oxygen. Why? Oxygen is an abundant source of energy, but in many aquatic environments, it gets quickly used up by microorganisms. Many bacteria, however, can respire or "breathe" alternatives to oxygen — sulfur, hydrogen, nitrogen for example. These bacteria are called anaerobes (more on this later). Environments lacking oxygen (anoxic) harbor many of these types of bacteria &emdash; billions per teaspoon of sediment or muck. In many cases, the anoxic environments harbor more microorgansisms than oxic ones. Giardia survives by eating anaerobic bacteria in these rich anoxic environments. But where on Earth are these "extreme" anoxic environments? Where would one go to find them?

Contemporary
anoxic worlds

Virtually anywhere! Check it out — the Black sea is the largest land-locked anoxic basin. Can we find more organisms like Giardia which might still live in anoxic worlds, and still be unknown to us? How could we do it?
next page

Search · Site Index · Navigation · Copyright · Credits · Contact
Understanding Evolution For Teachers Home · Understanding Evolution Home

Read how others have recognized the Understanding Evolution website