Cyanobacteria are gram-negative photoautotrophs, and many species actually perform nitrogen fixation. Now they are sometimes termed blue-green algae, but this is a bit of a misnomer, as they are in fact prokaryotes, not eukaryotes at all. They're actually the only bacteria that perform oxygenic photosynthesis, and it's pretty incredible because they're responsible for the origin of oxygen in the atmosphere. And you can see a nice up-close image of some cyanobacteria here. These are cyanobacteria that grow in filaments. Some grow as free-floating little cells and others form colonies. And you can see that this is a satellite photo here. And this green mass here, that is just cyanobacteria. That is a huge bloom of cyanobacteria, obviously generating a bunch of oxygen because it's performing a bunch of photosynthesis. I mean, pretty amazing. This is a satellite photo and there are so many of these tiny microorganisms that we can see it from space. And actually, the oldest fossils we have of life on Earth come from cyanobacteria. These are stromatolites. These blobs of what look like rock, it's actually calcium carbonate, and this is exuded by certain types of cyanobacteria. And, basically, the oldest fossils we have of life on Earth are stromatolites from cyanobacteria. These are modern, you know, living cyanobacteria, but we have stromatolites that look like these from, you know, over a billion years ago. It's amazing. Now, and also, don't forget, lastly, that cyanobacteria are the organisms that were engulfed and eventually became chloroplasts.
Now, actinobacteria are high GC gram-positive bacteria. And what that means is they have a large, or a high percentage of guanine-cytosine, right, GC from DNA in their chromosome. So they have a high GC content, so to speak. They're gram-positive bacteria and they include the genus Streptomyces, which is responsible for many antibiotics. Many antibiotics have come from this genus of actinobacteria. Now, these were initially misclassified as fungi because they have a fungus-like morphology. And as you'll see in the name, it ends with myces. What you'll see once we or when we discuss fungi is that the names, the Latin names for fungi end in 'myces'. So these were initially thought to be fungi, actually, bacteria. Now, the misclassification comes from the fact that chains of cells form these branching mycelia. And it's these structures that caused biologists to think that these were actually fungi.
Lastly, we have the Firmicutes, which are low GC gram-positive bacteria. So, the high GC is basically in comparison to these low GC bacteria. And this group includes the genus Lactobacillus, which is super important to humans. Not only are they responsible for yogurt production, obviously very important, and also they're involved in cheese production, very important. And most important to me, they're involved in sour beer production. If you've never heard of sour beer, you should try it. It's delicious. And it involves fermentation with Lactobacillus inside humans that are super important, for our health. I mean, many species live in our gut and help us with digestion. There are species that live in the vagina that help maintain that environment, and that's actually what this image is depicting. This is a human cell and you can see all these little dark rods. Those are Lactobacillus, and they are a species of Lactobacillus that live in the vagina.