Prokaryotes are so important to the biosphere that if they were to disappear the prospects for any other life surviving would be dim. Prokaryotes are indispensable links in the recycling of chemical elements in ecosystems.
Chemoheterotrophic prokaryotes function as decomposers breaking down corpses dead vegetation and waste products and unlocking supplies of carbon nitrogen and other elements essential for life.
Prokaryotes also mediate the return of elements from the nonliving components of the environment to the pool of organic compounds.
Autotrophic prokaryotes use carbon dioxide to make organic compounds which are then passed up through food chains. They are the only organisms able to metabolize inorganic molecules containing elements such as iron sulfur nitrogen and hydrogen.
Cyanobacteria not only synthesize food and restore oxygen to the atmosphere but they also fix nitrogen. This stocks the soil and water with nitrogenous compounds that other organisms can use to make . When plants and animals die other prokaryotes return the nitrogen to the atmosphere.
[brief info]
(http://www.course-notes.org/Biology/Outlines/Chapter_27_Prokaryotes)