Sunday, January 20, 2008

Bacteria - Masters of the Biosphere

Bacteria have been perceived by most people as disease-causing microbes since the germ theory of contagion caught on. Otherwise they have been largely ignored. Yet had bacteria been discovered on Mars, their description would have been much more dramatic and the bizarre quality of their natural history, which often seems like science fiction, would not have been missed.

The greatest division in the kingdom of the living is NOT between plants and animals, but between “bacteria” and “organisms made of nucleated cells”. (1) Bacteria are called “prokaryotes” and “organisms made of nucleated cells” are named “eukaryotes”.

A third category of tough bacteria living still today are called “archaebacteria”. They are considered to be the direct descendants of the earliest life on earth. Archaebacteria include salt-loving “halophiles”, heat-loving “thermophiles”, and methane-producing “methanogens”. The most important thing to remember about archaebacteria is that they despise oxygen. They prefer to live in anaerobic (oxygen-less) environments, such as on the bottom of the ocean, in sewer water, in the hot springs of Yellowstone National Park, and even in the stomachs of cows. (2) It is easy to understand how these ancient bacteria thrived on earth when there was no or very little oxygen, but a lot of carbon dioxide.

So, how did oxygen get into the atmosphere? Oxygen was only released into the atmosphere when “blue-green bacteria” evolved a way to use energy from sunlight to break apart water molecules to grab their precious hydrogen, explains Margulis. (2) “Combining the hydrogen with carbon atoms drawn from then-abundant carbon dioxide, blue-green bacteria were able to manufacture DNA, proteins, sugars, and all their other cell components. These light-needy bacteria quickly expanded to sunny waters everywhere on the Archean Earth. In so doing, they released vast amounts of molecular oxygen left over from their hydrogen mining of water.” (2) These earliest of bacteria were true innovators! They are the predecessors of plastids in plants, which remove carbon dioxide from the atmosphere, using carbon for their bodies and eliminating as waste the oxygen we breathe in fresh air.

Bacteria, to repeat, do NOT have a nucleus, which is why they are prokaryotes (which means “before nuclei”) and NOT eukaryotes. Instead, their DNA is loose within their bodies. As a result of this situation, bacteria NEVER reproduce by mitosis, which evolved AFTER the Archean time (in the Proterozoic). “A parent bacterium simply elongates its DNA, dragged by growing membrane to which it is attached, until the full-grown cell splits to form two offspring identical to it,” explains Margulis (p. 94)

Bacteria may not reproduce by mitosis, but they DO trade their DNA very easily. Margulis imagines the Archean Earth as a “promiscuous place of prodigious growth and rapid genetic transfer that led, by and by, to the genetic restrictions of the Proterozoic descendants known as “protists”. (p. 93) Bacteria will sometimes trade naked pieces of DNA called “plasmids” or as protein-coated pieces called “viruses”. [Hmmmm] The way that bacteria trade their DNA is to grow a cell bridge through which the genes pass. This is called “conjugation”. The offspring is a unique genetic recombinant. Bacterial recombination is the rage among biotechnologists who force the colon bacterium Escherichia coli to produce, for example, human insulin by getting the bacteria to take up a particular human gene.

Prokaryotic bacterial cells NEVER fuse (like an egg and sperm). Their genes instead FLOW. Margulis paints a compelling picture of a world in which human genes behaved like prokaryotes: “Imagine you are a blue-eyed person (perhaps with newly acquired green hair) who, in a swimming pool, gulps the more common gene for brown eyes. Toweling off, you pick up genes from sunflowers and pigeons. Soon the brown-eyed you is sprouting petals and flying—eventually reproducing into gliding brown-eyed, green-haired quintuplets. This fantasy is mundane reality in the world of bacteria, except that most genes traded there are for metabolic and subvisible traits.” (p. 96)

1. Lynn Margulis, Dorion Sagan: “What Is Life?” University of California Press, 1995, p. 113.

2. Ibid, p. 89.

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