Cyanobacteria: Definition, Characteristics & Species

Cyanobacteria: Definition, Characteristics & Species
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  • 0:05 Our Oxygen Atmosphere
  • 1:10 Cyanobacteria
  • 2:40 Morphology of Cyanobacteria
  • 3:20 Roles of Cyanobacteria
  • 4:23 Common Cyanobacteria
  • 5:15 Life Without Cyanobacteria
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Lesson Transcript
Instructor: Angela Hartsock

Angela has taught college Microbiology and has a doctoral degree in Microbiology.

Cyanobacteria are a phylum of Bacteria that produce oxygen during photosynthesis. This lesson will examine cyanobacteria and the role they played in establishing our oxygen-rich atmosphere.

Our Oxygen Atmosphere

Take a deep breath. Feel your lungs expand as the oxygen is scavenged from the air and enters your bloodstream. Your body, like that of all animals, has evolved to require oxygen for survival. We now consider Earth to be an oxygen-based planet, but this hasn't always been the case. Three billion years ago, that oxygen you take for granted was not found in Earth's atmosphere.

So where did the oxygen in our atmosphere come from? Your first guess is probably that oxygen comes from plants during photosynthesis. While it is true that plants contribute oxygen to the atmosphere, the first land plants only evolved around 475 million years ago, while analysis of ancient rocks shows evidence of oxygen in the atmosphere as far back as 2.3 billion years ago. Obviously we are missing something. That something is bacteria - most notably, cyanobacteria.


Cyanobacteria is a large and diverse phylum in the kingdom Bacteria. The phylum Cyanobacteria includes many different species, but all these species have one thing in common that makes them different than all other bacteria - they are oxygenic phototrophs.

Let's break this term down further: 'Oxygenic' means that they are able to generate and release oxygen. 'Phototroph' refers to the ability of cyanobacteria to use light photons ('photo-') for nourishment ('-troph').

Putting it together, cyanobacteria use photosynthesis to acquire energy from light, generating oxygen as a byproduct. The energy is then used to enzymatically convert carbon dioxide from the air into the nutrients needed for growth.

The cyanobacteria contain a specific type of chlorophyll called chlorophyll a and other pigments that are used for photosynthesis. The chlorophylls and pigments efficiently capture specific wavelengths of light, transferring the light energy to the cell.

One common pigment is phycocyanin, which is a blue color ('cyan' means dark blue). This is the origin of the 'cyano' in the name 'cyanobacteria,' and this pigment, in conjunction with the green chlorophyll, is the source of the common name 'blue-green algae.' It is important to note, however, that cyanobacteria are not a species of algae.

Morphology of Cyanobacteria

Cyanobacterial species have diverse cell morphologies. Cells can range in size from 0.1 micrometers to 40 micrometers and include a range of cell shapes. Some species live as single cells, while others form colonies of many cells. Many species are filamentous, forming long, straight chains of cells or many branching chains. Some filamentous species have special cells mixed into the chains called heterocysts, which capture nitrogen from the atmosphere and transform it into chemical forms that can be used by the cyanobacteria. This is often referred to as fixing nitrogen.

Roles of Cyanobacteria

Cyanobacteria can be found in nearly all environments. They can generally survive anywhere there is enough sunlight for photosynthesis. Several species live in symbiosis with lichens, ferns, and liverworts. Cyanobacteria can survive in soil or on the surface of rocks, often lying dormant in times of drought and awakening during wet periods to divide and grow.

Other species can form thick mats in warm, shallow marine environments or float freely in nutrient-rich bodies of fresh water. Some species living in water can bloom. This occurs when the cyanobacteria enter a period of rapid cell division. These blooms can be dangerous to other organisms, as some cyanobacteria produce toxic chemicals that can cause illness if consumed.

Cyanobacteria are generally very tolerant of extreme environments, such as hot springs and salt lakes. It is not unusual for cyanobacteria to be the dominant, or only, phototroph in these extreme environments.

Common Genera

There are several common genera of cyanobacteria.

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