Biological soil crust near Monitor, Washington.

Cyanobacteria is an organism that is very important in the formation of biological soil crusts. They are photosynthetic, and live within the first ten inches of topsoil. Cyanobacteria help to reduce erosion by helping bind the particles of soil together. When conditions are wet they become active and move through the soil, leaving behind sticky filaments which particles of soil cling to. When the filaments become wet, they absorb water and swell up to ten times their original size which helps to store moisture within the upper layer of soil where many plants root systems and other organisms live. Cyanobacteria also play a more direct role in aiding plant survival and growth. Levels of nitrogen in arid environments are often low, and limit the growth of plants. Cyanobacteria are important because they help to convert atmospheric nitrogen into a form that can be utilized by plants. In this conversion nitrogen, N2, from the air is fixed into an organic form such as nitrate or ammonia. Many cyanobacteria have formed symbiotic relationships with other plants. Cyanobacteria enables plants to photosynthesize and are responsible for the plants green color. This relationship between plant and cyanobacteria is believed to have first occurred during the Precambrian era. Cyanobacteria live in the roots of some plants such as legumes as well, and provide nitrogen directly for plants to use. They have also formed symbiotic relationships with fungi and lichens.