Cyanobacteria play two important roles on reefs. They carry out photosynthesis which produces food for many organisms and they fix atmospheric nitrogen into organic form, making it available for other organisms. Since the waters around coral reefs are generally low in organic forms of nitrogen (eg nitrate, ammonium) the nitrogen fixing cyanobacteria play a crucial role on most coral reefs. Cyanobacteria are amongst the oldest organisms in the world today. They form stromatolites in restricted regions of the world such as Shark Bay in Western Australia. And in WA fossil stromatolites occur which have been dated as far back as 3.5 billion (one thousand million) years!

• To be a true bacteria (Eubacteria).
• To have chlorophyll a as an important molecule for photosynthesis.
• To have Phycobiliprotein

Blue-green algae, Cyanobacteria

Blue-green algae are microscopic unicellular organisms which also grow in filamentous form up to a few mm. The filaments can grow up into large masses reaching 10-20 cm in diameter and appear red, blue or dark brown.

Blue-green algae are found in all regions of the world, both in the sea and on land. There are two types: free-floating and attached. Free-floating blue-green algae are important members of the phytoplankton. They belong to one genus -Trichodesmium and are found in the deeper waters between reefs. In the Great Barrier Reef blooms of Trichodesmium have been observed from manned satellites stretching for hundreds of miles. These blooms must play an important part in enriching the nutrient-poor waters between reefs. Attached or bottom-dwelling cyanobacteria are important part of the epilithic algal community which is the predominant source of food on coral reefs (see Green Algae)

Blue-green algae make their own food using sunlight (photosynthesis) and nutrients dissolved in the water.

Many herbivorous animals such as fish, crustaceans, worms and gastropods eat blue-green algae. It is particularly a valuable source of food for filter feeding organisms that can trap very small particles from the water. Cyanobacteria are at the base of the food chain and may generate up to 60% of all the food produced on a coral reef.

These very simple organisms grow and reproduce by simple division.

Free-floating blue-green algae are part of the phytoplankton community which includes many other photosynthetic organisms.

Cyanobacteria are mostly beneficial to humans. They produce large amounts of fixed-nitrogen in the sea and on land. Rice paddies are a prime example of a crop which is supported by the nitrogen-fixing activities of cyanobacteria. In a few cases cyanobacteria produce toxins which are lethal to farm animals and to humans. Unfortunately, deaths attributable to blue-green algae have become more frequent as urbanisation and agricultural run-off increase the nutrient content of rivers and lakes.