Sunday, October 21, 2012

Microbiome Part 4: Photosynthesis

Note: This program first aired on Saturday October 6, 2012.

If all the bacteria on Earth were suddenly to vanish, would you care? They are too small to be seen (unless of course, you’ve left something alone in the back of the refrigerator for way way too long), so would you even know it? In fact, some of you, without thinking it through fully, might think it would be a good thing if all the bacteria on earth were to vanish. But, I can say unequivocably that that would be an unmitigated disaster. They have an important job to do here on Earth, many jobs in fact. If all the bacteria on Earth were suddenly to vanish there are many ways we would miss them.

It is often said that bacteria are responsible for much of the oxygen we breathe. This may come as a surprise but many bacteria are indeed photosynthetic. The only source of the free oxygen gas we breathe is photosynthesis, it always has been. Before there was photosynthesis, there was essentially no free oxygen in the atmosphere, and all life (entirely prokaryotic) was anaerobic. The photosynthetic bacteria are the cyanobacteria (popularly known as blue green algae), the green and purple sulfur bacteria, and the purple non-sulfur bacteria.  In terms of oxygen production, the cyanobacteria are the heavy hitters here. The sulfur bacteria by the way, photosynthesize using  the energy of light from the sun to change CO2 into sugar the same way plants do, except that they use hydrogen sulfide instead of water in the chemical reaction. This sounds crazy until you look at the periodic table and see that oxygen and sulfur are in the same column (and thus behave the same way, so the difference between H2O and H2S actually isn’t that great).

We can attribute essentially all of the oxygen we breathe to bacteria if we recognize that the chloroplasts in plant cells—the grass on your lawn, the leaves on the trees around your house, the algae in the ocean—were originally bacteria. This is the theory of endosymbiosis. The original plant was probably a hungry single celled protist that engulfed a cyanobacteria, but then failed to digest it. The bacteria realized it had a good gig there inside the protist, surrounded by a soup of half digested food and all. The protist had a good deal going as well, now housing its own sugar making factory, which is all that chloroplasts do inside a any cell, they absorb light and use that energy to rearrange some commonly found chemicals and store the energy of the sun in the chemical bonds in the sugar they make. The oxygen we breathe is a byproduct, really a waste product, of that process (note, those sulfur bacteria that don’t use water?  They don’t make oxygen either, their waste product is, unsurprisingly, sulfur).

So, directly (through photosynthesizing mats of cyanobacteria), and indirectly (by being responsible for the existence of chloroplasts in plant cells), bacteria save the day and make life  as we know it possible.  And the best part is, that isn’t even half of all the good stuff they do here on Earth, but we’ll have to save that for next time.

The University of California Museum of Paleontology has a huge and amazing website devoted to education about evolution and the tree of life, as well as the geologic past:

See for yourself, Oxygen and Sulfur ARE in the same column!

“From Endotymbiosis to Synthetic Photosynthetic Life”, Andreas Weber and Katherine Osteryoung (full text available)