AGP - Algae
 
Algae growing on soil

Worldwide, there is thought to be several thousand different species of algae ranging in size from a few microns to 10’s of metres. Algae obtain their energy from the sun via photosynthesis and occur as single cells or in chains and typically have their chromosomes enclosed within a membrane and so are classes as eukaryotes. The different species are different colours depending on the type and proportion of chlorophyll(s) they posses. They range from green, yellow-green, red and brown. The Brown algae (Phaeophyta) are commonly seen as kelp or seaweed and are the largest. Red algae (Rhodophyta) can be found in the sea, whilst green algae (Chlorophyta) are found in fresh water and on the surface of some soils. 

In soil, several hundred species form three general groups:  green , yellow-green (Xanthophyta)  and diatoms (Bacillarophyta). Some are motile can be found a few centimetres below the surface although earthworms and rainwater may relocate them to further below the surface.  Algal populations typically range from 10,000 - 100,000 cells per gram of soil. Being photosynthetic, algae play an important part in introducing organic matter into the soil and excrete polysaccharides which increase soil aggregation. Green algae prefer moist, non flooded acidic soils while diatoms prefer well drained land rich in organic matter. When they die or dry up, they form, together with other soil microbes, a crust onto of the soil which can prevent wind erosion and input organic matter into the soil.

Superficially related to the algae are the blue-green algae or cyanobacteria (Chlorophyta). Once thought to be plants, blue-green algae, or Cyanobacteria, do not have a nuclear membrane and possessing some of the features of bacteria such as membrane composition, and are classified as prokaryotes. These organisms, obtain their energy from sunlight via photosynthesis but unlike algae and have been found in very dry areas such as polar and desert regions (Freeman et al., 2009).

Cyanobacteria do, however, share a number of features with algae and higher plants. For example, they release oxygen and have the same chlorophyll pigment (chlorophyll a) as plants. Similar to plants and algae, the photosynthetic prokaryotes produce carbon dioxide although using a different metabolic pathway.  In common with other bacteria, cyanobacteria have been shown to also fix atmospheric nitrogen (Schlegel, 1986).  

Related to the cyanobacteria are the purple bacteria and the green bacteria (the Rhodospiralles) which also posses  photosynthetic pigments but unlike the blue-greens, these bacteria do not metabolise water as part of the photosynthesis pathway but instead, utilise more a reduce form of hydrogen such as H2 or hydrogen sulphide, organic acids and so do not produce oxygen as a product of photosythesis.

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