As we all know from watching the Discovery Channel, nature is full of carnage, suffering, and big, snarling animals eating little, fluffy animals. I don’t want to talk about that today. Today is about the peaceful relationships in nature.
Certain pairings of creatures go well together. The lichens you see on exposed rock are pairings of algae and fungi, and if you pull back the tentacles of a sea anemone (which are covered in little needles packed with neurotoxin, so don’t actually pull them back), you may just find a clownfish.
These relationships are called mutualisms (The term “symbiosis” is often used, but actually refers to many types of relationships, including parasitism.) because each member helps the other in some way. The algae in lichen convert sunlight into food, which they share with their fungal partner. The fungus, in turn, shelters the algae from harsh conditions. Similarly, clownfish fight off anemone predators and parasites while their anemones provide shelter from whatever might want to eat the clownfish.
Countless examples of mutualism exist in nature. Here I want to focus on a particular kind, called endosymbiosis, in which one organism lives inside the other. An example of this you may be reluctantly aware of is the thriving ecosystem of bacteria in your intestines (downright flourishing!). Endosymbiotic microbes may also be found within other microbes, and here’s where this article will hopefully get intriguing. I’m certainly excited, in any case.
Okay, go back in your mind to high school biology. Now, mentally flip to the “cell” chapter (Yes, it was chapter 6.) Do you remember the mitochondria – the little bean-shaped organelles responsible for supplying the cell with energy? We all have hundreds or thousands of them in each of our trillions of cells. Well, it turns out they almost certainly originated as free-living bacteria in Earth’s distant past. Check it out:
Billions of years ago, living things were far less complex than they are now. Every organism was an individual cell, possibly abiding pleasantly affixed to a rock, or else floating in Earth’s primordial waters. These cells, while all looking more-or-less like tiny bubbles full of particles and DNA, actually represented some very different life-strategies. Some could harvest energy from sunlight. Others got it from chemicals like sulfur and iron, or simply “hunted” for food in their environment. Still others found a way to make vast amounts of energy, as long as oxygen was around, using a process called cellular respiration.
In the swarming seas of early Earth, “eat or be eaten” was already nature’s motto. That is, cells could engulf and digest each other for food. Now imagine that one day a common “engulfer” consumed a newfangled “cellular respirator” and failed to digest it. That would be a useful accident indeed! You see, the cellular respirator would be able to use the food collected by the engulfer to make much more energy than the engulfer would have made on its own. This energy would have then been available to the engulfer, because hey! The respirator was right there inside it! A lovely mutualism was born! As eons went by, the respirator remained inside organisms of increasing complexity, solidifying its place as what would eventually become the modern mitochondrion.
Of course, our mitochondria don’t look like free-living cells, and indeed they couldn’t survive on their own. A recent article in “Small Things Considered: The Microbe Blog" explains why.
As one microbe exists inside another for generations, it loses the need to make some of the materials it made before because the host cell can make things for it. It also might not need as much structural support or as many defensive abilities inside the controlled environment of another cell. As a result, endosymbionts actually lose DNA over time.
For the mitochondria, this means they can no longer make many of the essential compounds or carry out many of the processes they need to survive. These duties have been transferred to our cells to such an extent that it’s hard to believe mitochondria could ever have been something more.
As evidence that they were, we have the fact that mitochondria possess their own DNA (arranged circularly, like bacterial DNA), separate from ours. They also have different versions of other organelles, called ribosomes, which resemble those of bacteria. The way mitochondria divide also closely resembles the binary fission of dividing bacteria [source].
So there you have it - organisms within organisms becoming organelles! How fascinating life is!
I leave you with this electron microscope image of the bacteria Moranella endobia (white ovals) living inside the bacteria Tremblaya princeps (medium, uniform gray around them), which are in turn living inside the cell of a mealybug.
|John P. McCutcheon & Nancy A. Moran|
From Nature Reviews Microbiology