Cyanobacteria are fascinating organisms classified as gram-negative photoautotrophs, known for their ability to perform nitrogen fixation. Often referred to as blue-green algae, this term is misleading since they are prokaryotes rather than eukaryotes. Cyanobacteria are unique in that they are the only bacteria capable of oxygenic photosynthesis, a process that significantly contributed to the rise of oxygen in Earth's atmosphere. Their presence is so abundant that large blooms can be observed from space, as seen in satellite images. These microorganisms are also responsible for the oldest known fossils on Earth, called stromatolites, which are formed from calcium carbonate exuded by cyanobacteria and date back over a billion years.

Additionally, cyanobacteria played a crucial role in the evolution of chloroplasts, as they were engulfed by ancestral eukaryotic cells. In contrast, actinobacteria are high GC gram-positive bacteria characterized by a high guanine-cytosine content in their DNA. This group includes the genus Streptomyces, which is notable for producing many antibiotics. Initially misclassified as fungi due to their filamentous, fungus-like morphology, actinobacteria form branching mycelia that resemble fungal structures.

On the other hand, Firmicutes represent low GC gram-positive bacteria, which include the genus Lactobacillus. These bacteria are essential for various human applications, such as yogurt and cheese production, and they also play a significant role in sour beer fermentation. Beyond their culinary contributions, Lactobacillus species are vital for human health, residing in the gut and vagina, where they aid in digestion and help maintain a healthy microbiome. The presence of these beneficial bacteria is illustrated in human cells, where they can be seen as small dark rods, contributing to overall well-being.