Cyanobacteria

The Cyanobacteria are one of several major groupings of bacteria:

• Proteobacteria (mostly Gram-negative)
• Firmicutes and Actinobacteria (Gram-positives)
• Cyanobacteria

They were formerly known as "blue-green algae" - but this is a misleading name! Algae are eukaryotic organisms, not prokaryotes like Cyanobacteria. Cyanobacteria are interesting for a number of reasons:

They are photoautotrophs, able to use CO₂ as their sole carbon source and light as their energy source. However, unlike other photosynthetic bacteria, Cyanobacteria use the same photosynthetic pathway as eukaryotic cells such as algae and higher plants (the "C3" or "Calvin" cycle). (Other photosynthetic bacteria use different light-harvesting pigments (bacteriochlorophyll) and metabolic pathways). Even more interesting is that we now know from DNA sequencing that eukaryotic chloroplasts evolved from Cyanobacteria, presumably after millions of years of symbiotic association.

Microbes are the predominant photosynthetic organisms in most aquatic environments. In aerobic conditions, (e.g. shallow water), algae, diatoms and Cyanobacteria predominate. In anaerobic conditions (polluted or eutrophic waters), other photosynthetic bacteria are dominant.

Spirulina is big business, with thousands of metric tons grown in outdoor ponds and harvested annually in the USA, Mexico, Thailand, India and China. It is used as a food supplement, high in protein, sold in health food stores as a dried powder or in tablet form. However, many other species of Cyanobacteria produce toxins - blue-green pond scums have been linked to the poisoning of cattle, dogs and occasionally people.

Many Cyanobacteria are able to survive in hostile environments, such as African soda lakes. Some Cyanobacteria adjust their buoyancy by means of gas vacuoles, enabling them to adjust their position in the water column, floating near the surface during the day for photosynthesis and sinking deeper at night to harvest nutrients.

Wow! These guys are great - do they have any other tricks?
You bet. Some can also fix atmospheric nitrogen - but they have to be smart to do this. Nitrogen fixation requires anaerobic conditions, but Cyanobacteria are aerobes. They solve this problem by having specialized cells called heterocysts which have thick walls impermeable to oxygen and in which nitrogen fixation can occur. Smart, huh? Hmm... let's review the evidence. Cyanobacteria are:

• Photoautotrophs
• Able to survive in hostile environments
• Able to fix nitrogen

For these reasons, we believe that Cyanobacteria were some of the first living organisms to colonize the Earth - but there's also more direct evidence. Fossils known as stromatolites found in rocks 3 to 3.5 billion years old are layered mounds, columns and sheets found in typically "mushroom-shaped" rocks. These were originally formed by the growth of layer upon layer of Cyanobacteria, but it is another 2 billion years before eukaryotic organisms appear in the fossil record. Stromatolites are still forming today in places such as Shark Bay, Australia, where the salinity of the water is very high and one of the few organisms that can survive are Cyanobacteria.

Did I mention that Cyanobacteria are the reason we exist? Producing oxygen as a by-product of photosynthesis, over millions of years they slowly changed the carbon dioxide-rich atmosphere of the early earth to the present-day oxygen-rich atmosphere we breathe. Around 2.5 billion years ago, stromatolites become more frequent in the fossil record, and about this time the atmosphere changed substantially, allowing the evolution of aerobic organisms (including all eukaryotes, and us) to occur.

Oh yes, one more thing. Another reason that Cyanobacteria are interesting is that they show gliding motility - movement of cells over surfaces without the aid of flagella. The molecular mechanism by which most bacteria propel themselves through liquid media by means of rotating flagella is relatively well understood. Gliding motility is a trait common to many bacteria, yet the mechanism of gliding motility is unknown. The gliding motility apparatus which propels the cells involves a complex of proteins, yet the actual nature of the "motor" and how the components interact is not understood. You can watch Oscillatoria cells gliding in real time in this video:

Microbiology
by L.M.Prescott et al.
A balanced, comprehensive introduction to all major areas of microbiology. The sixth edition has been updated extensively to reflect the latest discoveries in the field.
(Amazon.co.UK)