There are few things, in my opinion, that look so beautiful under a microscope as cyanobacteria. Formerly known as blue-green algae, this is some truly amazing stuff. It can be found everywhere- from oceans to lakes to soils. We use it in “superfood” smoothies and to make biofuel. Most importantly, life as we know it on this planet probably wouldn’t exist without it.
Cyanobacteria are able to do two really important things: they perform photosynthesis, using the energy of sunlight to split water and make sugars, and they can “fix” atmospheric nitrogen into an organic form. This second skill is especially important. Every living thing needs nitrogen to build DNA, proteins, and other important things found in cells. The atmosphere of the earth is over 70% nitrogen gas, but this gas is made up of pairs of nitrogen atoms that are tightly bound together and very difficult to split apart to make other stuff. Cyanobacteria have an enzyme (nitrogenase) that can split apart the nitrogen pairs and convert them to ammonium, a nitrogen compound that living things can more readily use. There are only a handful of organisms that can do this, and they are all microbes. Every other living thing relies on these guys to make nitrogen available for the rest of us.
Pretty sweet! But it gets even sweeter.
Notice how most of the cells in that picture have little green globs inside, but some of the cells just look empty? Those little green globs are folds in the cell membrane that contain the pigments that capture light for photosynthesis. The energy from the captured light splits water molecules into hydrogen and oxygen. The hydrogen goes on to combine with carbon dioxide to make sugar, and the oxygen is released as waste. When cyanobacteria first figured out how to do this, there was no oxygen in the earth’s atmosphere. All that oxygen slowly built up from years and years of these little guys splitting water to make sugar. About 2.4 billion years ago, Great Oxidation Event occurred: the concentration of oxygen in our atmosphere hit about 1% of what it is today. It doesn’t sound like much, but it was enough to radically change the course of life on this planet.
And speaking of evolution, chloroplasts (the organelles that perform photosynthesis in plant cells) most likely evolved from a cyanobacterium that was engulfed by a bigger cell. We know this because the DNA in chloroplasts is much more similar to cyanobacteria DNA than it is to the DNA found in the nucleus of the plant’s cells. Every plant you can think of contains a cyanobacterial ancestor in each of its cells.
Ok, cool, but what about those empty cells up there? Those are called heterocysts, and they’re basically little nitrogen-fixing factories. It turns out that nitrogenase, that enzyme that fixes nitrogen gas, is really sensitive to oxygen. Photosynthesis produces a ton of oxygen as waste. So, the nitrogenase has to be kept in its own compartment separate from where the photosynthesis happens. This means that while most of the cyanobacteria are capturing sunlight and producing sugars, a couple of them are being fed sugars by the other cyanobacteria, and providing fixed nitrogen in return. This is a pretty cool concept, because remember- each one of those cells up there is a single organism. This means that they have basically formed a community where goods and services are being exchanged, and jobs are being assigned. Sound familiar? It’s basically what happens between the cells in your body, except on a smaller scale.
Is your mind blown yet?
(Somebody make me a necklace that looks like these things, please?)