Sunday, October 21, 2012

Microbiome Part 5: Decomposers

Note: This program first aired on Saturday October 13, 2012.

No one really knows how many bacteria are on the Earth. People have come up with models and sampling protocols to try to determine how many individual bacterial cells are on Earth (five million trillion trillion), how many species are in any given proverbial teaspoon of soil (five thousand or more), and how many different species exist globally (a few thousand to a billion). Many scientists believe that the bacterial biomass on the planet far exceeds the biomass of all other living things. Needless to say, there is little consensus and much research needs to be and is being done.

Taxonomically, bacteria make up the roots of the tree of life. They had the planet to themselves for nearly 2 billion years, and then single celled eukaryotes and multicellular eukaryotes (us included) came along and stole the spotlight. We’ve talked before about the photosynthetic role of bacteria, but they have another equally or even more important ecological role on the planet. Bacteria, along with fungi are the great decomposers of our world.

Decomposition doesn’t sound like a very fun job, and to our sensibilities, it often doesn’t smell very good. It is incredibly important however, to the cycling of nutrients and raw materials of life. There are two major biochemical reactions that drive life as we know it (with of course, a few exceptions). The first is photosynthesis; plants use the energy of the sun to take simple molecules and turn them into more complex molecules. In doing so, they store some of the sun’s energy in the bonds that hold those complex molecules together. Simple to complex. The second major biochemical reaction is respiration. All organisms respire, meaning, they take large complex molecules and break them down into smaller simpler molecules, in the process gaining the energy that was stored in the bonds of those large molecules. This is what we do when we eat. This is why plants photosynthesize—they don’t do it for us, they do it to make food for themselves (we just happen to get it before they do in some instances).

Organisms that eat are also know as heterotrophic, hetero meaning different, trophic meaning eating, so heterotrophs eat food that is different from themselves (by comparison, photosynthesizers are autotrophic—they self feed). Heterotrophs have essentially two options for eating; they can ingest food and digest the complex molecules internally, or they can keep the food outside of them selves and digest it externally, absorbing the molecules only once they have been at least partially broken down. We are of the former type, we ingest our food, secrete acid and enzymes, and with the help of some bacteria, liberate nutrients and energy from that food. We don’t however, get all of the nutrients or energy from our food, not close. The classical ecological number is 10 percent, as in only 10% of the energy from one trophic level gets passed on to the next one, the other 90% is lost as waste (in the form of heat and feces). The reality is that this number varies quite a bit with different feeding levels, but the take home message here is that most of what we eat doesn’t actually get into us. My personal theory about why this is has to do with the difficulty of digestion. Our body has to work quite hard to chemically break down those big food molecules. If we work too at it, we might just start to digest our own bodily tissue—that is one of the roles of mucus in the digestive track, to protect ourselves from our own digestive enzymes.

Bacteria are of the latter type. They are external digesters, excreting digestive enzymes to break down food molecules (both large and those that are partially digested and excreted as feces) and then absorbing the smaller molecules that result. Again, I am not a microbiologist, but my personal theory on this is that bacteria, being prokaryotic, lack a nuclear membrane and have their DNA loose inside their cells. I expect that it is to their advantage to not in any way jeopardize their genetic material by having digestive enzymes also floating around inside their cells, hence, external digestion. Fungi, the other group of great decomposers also digest externally and this pattern leads me to believe that breaking down these larger molecules into smaller inorganic nutrients may be a rather harsh process, which is why both decomposers do it this way.

We’ll continue this discussion about the ecological role of bacteria next week as we look in more detail at the specific nutrients that bacteria are so vital in keeping in play in the biosphere.


From the master himself: Stephen Jay Gould, "Planet of the Bacteria," Washington Post Horizon, 1996, 119 (344): H1

The factoid about the microbial biomass making up the largest single carbon sink came from this author, but to be honest, I read it on Wikipedia…I’m not proud. Fenchel, Tom (1998). Bacterial biogeochemistry : the ecophysiology of mineral cycling (2nd ed. ed.). San Diego: Academic Press.

Just for fun:

“First-Ever Scientific Estimate Of Total Bacteria On Earth Shows Far Greater Numbers Than Ever Known Before” Science Daily, 1998