Symbiotic bacteria

Nitrogen-fixing bacteria (symbiotic organisms that form nodules on the roots of leguminous plants, such as beans and alfalfa) and some free-living microbes and cyanobacteria can fix nitrogen. Plants and animals cannot. 

Soil Nutrient. Molybdenum has been widely used to increase crop productivity in many soils woddwide (see Fertilizers). It is the heaviest element needed for plant productivity and stimulates both nitrogen fixation and nitrate reduction (51,52). The effects are particularly significant in leguminous crops, where symbiotic bacteria responsible for nitrogen fixation provide the principal nitrogen input to the plant. Molybdenum deficiency is usually more prominent in acidic soils, where Mo(VI) is less soluble and more easily reduced to insoluble, and hence unavailable, forms. Above pH 7, the soluble anionic, and hence available, molybdate ion is the principal species. 

Thiobacillus thiooxidans is an aerobic organism that oxidizes various sulfur-containing compounds to form sulfuric acid. These bacteria are sometimes found near the tops of tubercles (see Chap. 3, Tubercu-lation ). There is a symbiotic relationship between Thiobacillus and sulfate reducers Thiobacillus oxidizes sulfide to sulfate, whereas the sulfate reducers convert sulfide to sulfate. It is unclear to what extent Thiobacillus directly influences corrosion processes inside tubercles. It is more likely that they indirectly increase corrosion by accelerating sulfate-reducer activity deep in the tubercles. 

The source of light emission. Each individual animal in a colony of Pyrosoma has two groups of luminous cells at the entrance to the branchial sac (Herring, 1978a). The luminous cells contain tubular inclusions whose identity has been a matter of dispute for nearly a century. Buchner (1914) originally suggested that the inclusions were symbiotic luminous bacteria. However, efforts to cultivate luminous bacteria from the luminous cells of Pyrosoma consistently failed. In... 

Moreover, the lipo-chitooligosaccharides, also known as nod factors, permit nitrogen fixation by which plants and symbiotic Rhizobia bacteria can reduce atmospheric nitrogen to the ammonia that is utihzed by the plant, thus making available nitrogen compounds to other living organisms. 

The presence of microorganisms in the rhizosphere has been shown to increa.se root exudation (58,61-65). This stimulation of exudation has been shown to occur in the pre.sence of free-living bacteria such as Azospirillum spp. and Azotobacter spp. (66,67) and in the presence of symbiotic organisms such as mycorrhizae (68,69). Increased root exudation has also been shown to be species-specific for example Meharg and Killham (65) found that metabolites produced by Pseudomonas aeru/ inosa stimulated a 12-fold increase in C-labeled exudates by perennial ryegrass. However, under the same conditions, metabolites from an Arthro-bacter species had no effect on root exudation. 

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