Bacteria and Archaea are prokaryotic organisms, in most cases microscopic and single-celled, except some Cyanobacteria and Actinobacteria that form mycelia-like structures. Despite their relatively simple structure, their diversity and functionality is enormous and they are the most efficient organisms in terms of survival and dispersion.
In fact, all eukaryotes evolved to fit in a world that was and is shaped by bacteria and archaea. Their role in biogeochemical cycles is essential and they contribute to important ecosystem processes including creation, maintenance and functioning of soil.
Bacteria and archaea are the only organisms that can gain energy from redox reactions that do not involve carbon compounds. Redox reactions involve nitrogen (including the greenhouse gas N2O), sulphur, iron and hydrogen (including production and consumption of the greenhouse gas methane).
Some bacteria can fix atmospheric carbon, but almost all soil bacteria have a carbon-based economy dependent on the breakdown of carbon compounds and can, like fungi, be classified in three major functional groups:
- Saprotrophs (decomposers): Consume carbon compounds (root exudates and/ or plant litter, especially the more easily degradable parts) contributing to carbon and nutrient cycling and formation of stable soil organic matter (humus). When nutrients are limited, they also immobilize nutrients in their cells and then reduce plant growth.
- Mutualists: Form beneficial symbioses with plants, like the nitrogen- fixing bacteria that occur on the root nodules of legumes and a few other plants
- Pathogens; may cause important plant diseases