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Microorganisms in terrestrial environments Soil is dominated by inorganic material that is modified by microorganisms and plants Soil microorganisms include bacteria, archaea, fungi, algae, protozoa and viruses Only about 10% of the microorganisms found in soil have been cultured Oxygen concentrations in soil Soil microorganisms tend to be located in thin films of water on the particle surfaces Oxygen diffuses rapidly in this environment Oxygen diffuses very slowly in water-logged soils Oxygen concentrations in soil Less oxygen is available at lower depths, especially in wetter soils Microorganisms in soil Microscopic and macroscopic organisms contribute to the formation and characteristics of soil nematodes, earthworms, protozoa, fungi and bacteria All contribute to the decomposition of plant material and the turnover of nutrients Microorganisms in soil Many important soil bacteria have only recently been cultured Microorganisms and the formation of soils Rate at which nutrients in plant material is released is dependent on activity of soil microorganisms Influences the soil and manner in which plants adapt Microorganisms and the formation of soils Nutrients released rapidly in tropical forests so plant roots do not penetrate far Slow release in temperate grasslands causes deep root penetration fertile soil Microorganisms and the formation of soils Release of acid compounds in temperate coniferous forests results in a bleached zone Accumulation of litter and increased risk of fire (need for controlled burns) Microorganisms and the formation of soils Low oxygen content in bogs results in slow decomposition and the formation of peat Microorganisms and the formation of soils Microorganisms also involved in the formation of soil in cold moist regions, desert regions and geologically heated regions Microorganisms and plants Microorganisms are associated with various parts of plants Some are commensals, some are symbionts and some are pathogens Phyllosphere microorganisms are found on the aerial surfaces of plants (leaves, stems, etc.) Microorganisms and plants Rhizosphere microorganisms are found in the area surrounding and influenced by the plant roots Rhizoplane microorganisms are found on the surface of plant roots Microorganisms and the formation of soils Many bacteria grow within plants and can induce formation of nodules (nitrogen-fixation) These include species of Rhizobium and related bacteria Microorganisms and plants Microorganisms and plants Microorganisms and plants Microorganisms and plants Some nitrogen-fixing bacteria can induce the formation of nodules on the stems of some tropical legumes Plant pathogens A number of fungi and bacteria are capable of causing diseases in plants Plant pathogens Infection of plants with Agrobacterium tumefaciens causes the formation of tumorlike growths on plants Plant pathogens Tumor formation due to the Ti plasmid which integrates into the host chromosome Ti plasmid is being used to genetically modify plants (e.g. insertion of resistance genes) Soils, plants and nutrients Microorganisms have a profound influence on the cycling of nutrients in soils Excessive ammonium in fertilizer is oxidized to nitrite and nitrate These anions enter the surface and ground waters (harmful to animals and humans) Soils, plants and the atmosphere Some soil microorganisms nucleate ice and can increase precipitation and frost damage to crops Plants sprayed with genetically modified “ice-minus” bacteria can protect crops from frost Soil microorganisms can degrade airborne pollutants such as CO, benzene and methane (1 plant/ 100 ft2 of living space) The subsurface biosphere Microbial processes take place in different subsurface regions The shallow subsurface where water flows below the plant root zone Regions where organic matter has been transformed into hydrocarbons Regions where methane is being synthesized The subsurface biosphere Microorganisms appear to be growing in high temperature (60-90 C) oil reserves Soil microorganisms and human health Most soil microorganisms are harmless but a few can cause diseases in humans Species of Clostridium can cause gangrene and tetanus Many fungi (molds) and some bacteria can cause respiratory diseases when they grow in buildings (“sick-building syndrome”) Microbial diversity in the soil Only 1-10% of soil microorganisms can be grown in the laboratory Diversity can be assessed by extracting and analyzing DNA in a soil sample Growth of identified organisms will ultimately be required for a complete understanding of their role in the environment