Tuesday, February 14, 2017

all the world’s a test tube, and all the men and women merely chemoheterotrophs


In a recent Microbiology lecture, I learned that we can classify organisms into four categories, based on what they use as an energy source (light vs chemicals) and what they use as a source of carbon (organic compounds vs carbon dioxide). This is a slide from that lecture (my own notes are in blue):

As the blue note states, “most pathogenic microbes are chemoheterotrophs”, meaning that chemoheterotrophs get both their carbon and their energy from organic chemical compounds.

This slide sheds more light on these categories:

The blue FYI note “Carbon & Energy are usually the same compound” points out that most chemoheterotrophs use the same carbon source both for both energy and for growth. In other words, they take a carbon compound and break the carbon bonds (releasing energy) and then piece together the newly-freed carbon atoms to create more of themselves (either to grow their own bodies or, if there are enough carbon compounds, to reproduce more organisms like themselves).

the growth (and death) curve of chemoheterotrophs

If you have a small number of chemoheterotrophic organisms in a confined space, such as a test tube, and add a rich supply of new energy-containing carbon compounds, those chemoheterotrophs will do something very predictable. Whether they’re an Escherichia coli, a Staphylococcus aureus, or a Clostridium difficile, all chemoheterotrophs follow this growth curve:

At first, after the new energy-rich carbon compound is added (for example, sugar or some more complex carbohydrate), nothing much seems to change for a little while. This is called the “lag phase”. During the lag phase, the chemoheterotrophs are not able to use the new carbon compound very well because they don’t yet know how. In a process called "retooling" they are adjusting themselves to figure out what new enzymes and other tools are needed to breakdown this new carbon compound so that its  energy and carbon are accessible.

Once the retooling is done, watch out, because things are about to go crazy as the chemoheterotrophic organisms enter the “exponential growth phase” and start to reproduce like mad. For example, the growth curve above represents a bacterium that can reproduce in just minutes (provided there’s a good carbon compound available, and it has retooled for that carbon compound). Within minutes every bacterium becomes two; minutes later they’ve doubled again to be four bacteria; then eight, then sixteen. and so on. Within one hour, during the exponential growth phase, the test tube has gone from holding 10 bacteria to 100. An hour later it holds 250; by the third hour, 630. At 5 hours, the tube contains over 6,000 bacteria, and by 10 hours it holds about 63 million. An hour after that (11 hours after the exponential phase began at 10 bacteria) the tube holds about 1 billion bacteria.

Then, rather suddenly, the population growth ceases as the “stationary phase” is entered. The bacteria (or whatever chemoheterotroph is in that closed system) would keep expanding in numbers if it could, but the test tube environment has reached a point where so much of the carbon compound source has been consumed, and so much waste material has built up, that organisms are dying as fast as they are being created and the population in the tube remains level for a while.

Eventually, the chemoheterotrophs are dying faster than they’re growing and the population enters the “death phase”. The death phase, like the growth phase, is exponential. In the above case, once the death phase begins, it takes only about 11 hours for the population to drop from 1 billion to about 100,000.

Microbiologists see this growth curve happen again and again. Time after time, in experiment after experiment, a little while after a new carbon compound is introduced into the test tube or petri dish, the chemoheterotrophic organisms reproduce as fast as they can, even though, every time, it means the population will soon be dying off at a catastrophically exponential rate. They never, ever pace themselves. They’re just stupid bacteria.

a bigger chemoheterotroph in a bigger test tube

Single-celled microorganisms aren’t the only chemoheterotrophs. Most of the known animal species use carbon compounds as both their energy and their carbon source. Take, for example, humans. Throughout our long history, we humans have used the complex carbon compounds that make up plants and animals as our source of both energy (from breaking down complex carbon chains in our food and even firewood) and carbon atoms (the primary building blocks of all we eat, both flora and fauna, which become the basis of most of our cells, enzymes, hormones, fats, proteins, muscles, skin, nails, etc.…).

So, it turns out, humans are chemoheterotrophs, too. “Hey, Staphylococcus aureus, we have something in common!”

A few hundred years ago, humans began to realize that there was a new carbon compound available to us and, like the chemoheterotrophs we are, it took us a while to retool ourselves to use it. But once we retooled… well, just look at our recent logarithmic growth curve:

Does that post-fossil-fuel human growth curve look like part of any other growth curves you might have seen recently? Perhaps you recognize a resemblance to the “lag” and “exponential growth” phases of bacteria in a test tube into which a new carbon compound has just been added? Could it be that this last graph demonstrates just another chemoheterotrophic organism (aka “humans”) in a test tube (aka “Earth”) discovering a new complex carbon compound (aka “fossil fuels”)?

It may have taken a few hundred years for we humans to figure out how best to retool to use the new carbon compound (directly for energy, and directly and indirectly for food [1,2]), but once that retooling happened our growth took off at an exponential rate. The time scale is longer than in the bacteria graphs, because our reproductive period is decades instead of minutes, but other than that the human “lag” and “exponential growth” phases are so far looking pretty much the same as any other chemoheterotroph in a confined environment with a limited new carbon compound.

do universal rules apply universally?

So, chemoheterotrophic organisms, from single-celled bacteria to trillion-celled humans, all appear to act the same. The same universal rules apply. I would like to see a beauty in that universal truth, except in this case I happen to be one of those organisms in the graph, and in this case I know where that growth curve leads again and again, and it always leads to an exponential death phase.

I want the humans-on-earth growth curve to be different than the bacteria-in-test-tube growth curve. A tiny, one-celled bacterium doesn’t know any better than to use up its new sugar source as fast as it can. But we humans have big brains, and society, and history, and writing, and science, and microbiologists. We should be able to see what’s coming if we continue blindly down our fossil-fueled path, and we should be able to do something about it.

We are smarter than bacteria, aren’t we?