Cyanobacteria are bacteria that obtain their energy from photosynthesis. Also known as blue-green algae or Cyanophyta, they make up a huge part of the marine ecosystem and are assumed to have been responsible for the Earth’s shift to an oxygen rich atmosphere and the extinction of organisms that are oxygen-intolerant.
Cyanobacteria exist primarily as single cell organisms, though many species often clearly show colonial activities. These colony-based species even show some form of cell-differentiation, where some of the cells are primarily photosynthetic, while others become structural to hold the colony together. These provides some evidence of how the first multi-cellular organisms may have developed.
These bacteria can be found in almost every environment in the world. While they are most plentiful in the oceans, they have been found thriving in freshwater, soil, rocks, and even deserts. Many exist as endosymbionts, thriving by coexisting with some larger organism, either on their leaves or in their fur.
As mentioned before, cyanobacteria are generally accepted to be the single greatest contributor to Earth’s oxygen rich atmosphere. Their photosynthetic pathway captures light to catalyze the breakdown of water, which in turn allows for the release of oxygen gas. They also utilize nitrogen and carbon in aerobic conditions, allowing them to thrive before Earth became oxygen rich and long after as well. This adaptation makes them among the most resilient organisms known. They are also considered the oldest primary producers in the world, dating in fossil records to nearly 2 billion years ago.
Cyanobacteria are very similar to the chloroplasts in eukaryotic organisms. Many scientists theorize that eukaryotic organisms that use photosynthesis, including algae and plants, probably evolved as a result of an endosymbiotic relationship between cyanobacteria and another organism. Energy was probably coupled and the organisms depended on the cyanobacteria for some of its energy needs. This relationship developed into a mutual dependence and eventually led to the merger of the organisms into something that resembled today’s photosynthetic eukaryotes.
Many features of cyanobacteria make it difficult to classify them effectively. They show circadian rhythms that were long assumed to only exist in eukaryotic cells. They have are also capable of reducing sulfur in anaerobic, dark conditions the same way that many archaea can.
The high reproduction rates of cyanobacteria make them a serious health risk at times. For instance, algal blooms are often the result of cyanobacteria in an area with perfect growth conditions. Also, many cyanobacteria produce cytotoxins that are very harmful when ingested. When algal blooms occur, they can poison the environment around them.
