Cyanobacteria

Cyanobacteria are a massive group of autotrophic, gram-negative bacteria. They belong to the phylum Cyanobacteriota. These organisms are unique because they perform oxygenic photosynthesis. This means they use sunlight to create biological energy. They are often called blue-green algae because of their cyan color.

Scientists believe they are the most numerous taxon to ever exist on Earth. They are essential to the planet's life-support systems. They act as primary producers in many environments.

The process of photosynthesis in cyanobacteria is highly specialized. It happens inside internal membranes called thylakoids. These are flattened sacs located within the cell.

Photopigments like chlorophyll, carotenoids, and phycobilins absorb sunlight. Specifically, they absorb red- and blue-spectrum frequencies. This absorption allows them to split water molecules. This splitting produces hydrogen ions and oxygen. The oxygen is released into the environment as a byproduct. The hydrogen ions then react with carbon dioxide. This reaction is called carbon fixation. It produces complex organic compounds like carbohydrates. To make this efficient, many species use carboxysomes. These are cage-like protein structures that concentrate carbon dioxide. They hold the enzyme RuBisCO to increase efficiency.

Cyanobacteria show incredible variety in their physical forms, or morphology. Some are unicellular, meaning they are single cells. Others are filamentous, forming long chains of cells.

Some filamentous species show the first signs of multicellularity. They can develop specialized cells for different tasks. Vegetative cells perform normal photosynthesis. Akinetes are thick-walled spores that resist harsh climates. Heterocysts are specialized cells used for nitrogen fixation. These cells are vital because they create an anaerobic environment. This protects the enzyme nitrogenase, which is sensitive to oxygen. Some species also form hormogonia. These are motile filaments that move to form new colonies.

The history of cyanobacteria is tied to the history of Earth itself. They likely originated in freshwater or terrestrial environments. They first appeared during the middle Archean eon. Fossil records show they have existed for at least 2.1 billion years.

Before cyanobacteria, Earth's atmosphere was anoxic. This means it lacked free gaseous oxygen. By releasing oxygen for billions of years, cyanobacteria changed the atmosphere. This led to the Great Oxidation Event. This event caused the "rusting of the Earth" during the early Proterozoic. This change allowed for the evolution of eukaryotes. This happened through endosymbiosis, where one cell lives inside another.

The scale of cyanobacterial populations is staggering. One genus, Prochlorococcus, is the smallest known photosynthetic organism. It measures only 0.5 to 0.8 micrometres across.

Despite their size, they are incredibly abundant. A single milliliter of surface seawater can hold 100,000 cells. There are an estimated several octillion individuals worldwide. This group alone accounts for about 20% of Earth's atmospheric oxygen. In the oceans, cyanobacteria contribute about 25% of global marine primary production. They are a fundamental part of marine food webs.

While many cyanobacteria are helpful, some can be dangerous. Certain species produce powerful toxins called cyanotoxins. Examples include microcystins, saxitoxin, and cylindrospermopsin.

These toxins can cause harmful health effects in humans and animals. When these organisms grow rapidly, they create harmful algal blooms. These blooms can disrupt aquatic ecosystems and cause intoxication in wildlife. However, many species are used in biotechnology. They are studied for bioethanol production and food colorings. They can even serve as dietary supplements or raw materials.

Cyanobacteria are deeply connected to other biological systems. Many eukaryotic organisms, like red and green algae, have ancestors in cyanobacteria. Through endosymbiosis, cyanobacteria evolved into specialized organelles called plastids. These include chloroplasts, chromoplasts, etioplasts, and leucoplasts.

Some cyanobacteria also live in symbiotic relationships with plants. For instance, Anabaena lives with the aquatic fern Azolla. This relationship can provide rice plantations with biofertilizer. Others live in symbiosis with lichen-forming fungi. They are truly global players in biogeochemical cycles.