Soil bacteria are very important in biogeochemical cycles and have been used for crop production for decades. Plant–bacterial interactions in the rhizosphere are the determinants of plant health and soil fertility. Free-living soil bacteria beneficial to plant growth, usually referred to as plant growth promoting rhizobacteria (PGPR), are capable of promoting plant growth by colonizing the plant root. PGPR are also termed plant health promoting rhizobacteria (PHPR) or nodule promoting rhizobacteria (NPR). These are associated with the rhizosphere, which is an important soil ecological environment for plant–microbe interactions. Symbiotic nitrogen-fixing bacteria include the cyanobacteria of the genera Rhizobium, Bradyrhizobium, Azorhizobium, Allorhizobium, Sinorhizobium and Mesorhizobium. Free-living nitrogen-fixing bacteria or associative nitrogen fixers, for example bacteria belonging to the species Azospirillum, Enterobacter, Klebsiella and Pseudomonas, have been shown to attach to the root and efficiently colonize root surfaces. PGPR have the potential to contribute to sustainable plant growth promotion. Generally, PGPR function in three different ways: synthesizing particular compounds for the plants, facilitating the uptake of certain nutrients from the soil, and lessening or preventing the plants from diseases. Plant growth promotion and development can be facilitated both directly and indirectly. Indirect plant growth promotion includes the prevention of the deleterious effects of phytopathogenic organisms. This can be achieved by the production of siderophores, i.e. small metal-binding molecules. Biological control of soil-borne plant pathogens and the synthesis of antibiotics have also been reported in several bacterial species. Another mechanism by which PGPR can inhibit phytopathogens is the production of hydrogen cyanide (HCN) and/or fungal cell wall degrading enzymes, e.g., chitinase and s-1,3-glucanase. Direct plant growth promotion includes symbiotic and non-symbiotic PGPR which function through production of plant hormones such as auxins, cytokinins, gibberellins, ethylene and abscisic acid. Production of indole-3-ethanol or indole-3-acetic acid (IAA), the compounds belonging to auxins, have been reported for several bacterial genera. Some PGPR function as a sink for 1-aminocyclopropane-1-carboxylate (ACC), the immediate precursor of ethylene in higher plants, by hydrolyzing it into α-ketobutyrate and ammonia, and in this way promote root growth by lowering indigenous ethylene levels in the micro-rhizo environment. PGPR also help in solubilization of mineral phosphates and other nutrients, enhance resistance to stress, stabilize soil aggregates, and improve soil structure and organic matter content. PGPR retain more soil organic N, and other nutrients in the plant–soil system, thus reducing the need for fertilizer N and P and enhancing release of the nutrients.

Soil beneficial bacteria and their role in plant growth promotion: a review

The structure

Nitrate reductase assay in intact plant tissues.

http://fac.ksu.edu.sa/sites/default/files/Abdel-Raouf%20et%20al%2C2012.pdf

Microalgae and wastewater treatment.

Urban, industrial and agricultural wastewaters contain up to three magnitudes higher concentrations of total nitrogen and phosphorous, compared with natural water bodies.1 Normal primary and secondary treatment of these wastewaters eliminates the easily settled materials and oxidizes the organic material present, but does not remove the nutrients which will cause eutrophication of the rivers or lakes into which these wastewaters may be discharged. Tertiary treatment of the effluent is therefore required, and both chemical and physical methods which are used are very expensive. Oswald2 estimates that the relative cost of tertiary treatment to remove PO 4 3− , NH 4 + and NO 3 − is about 4 times the cost of primary treatment. Higher orders of treatment, such as quaternary treatment required to remove refractory organics and organic and inorganic toxicants and quinary treatment to remove inorganic salts and heavy metals, are 8 to 16 times as expensive as primary treatment. Algae can be used as a biological alternative tertiary treatment and also for the removal of heavy metals and possibly other toxic substances.3,4 The possibility exists that the algae produced in these systems can be used as animal feed supplements,5,6 or be composted. The use of waste-grown algae may ultimately also have application in closed cycle life-support systems,7,8 or may be used in conjunction with power stations, not only to treat wastewaters, but also to act as a CO2 sink for the amelioration of the impact of greenhouse gases.9–13

Limits to Growth

Ammonium at low concentrations caused a rapid and effective inhibition of nitrate utilization in the light by the cyanobacterium Anacystis nidulans without affecting the cellular level of nitrate reductase activity. The inhibition was reversible, and the ability of the cells to utilize nitrate was restored immediately after ammonium had been exhausted. The inhibitory effect was dependent on consumption by the cells of the added ammonium which was rapidly incorporated into amino acids. In the presence of L-methionine-d,l-sulfoximine (MSX) or azaserine, inhibitors of the glutamine synthetase-glutamate synthase pathway, ammonium did not exhibit any inhibitory effect on nitrate utilization. Ammonium assimilation, rather than ammonium itself, seems to regulate nitrate utilization in A. nidulans. Short-term inhibition by ammonium of nitrate utilization and its prevention by MSX were also demonstrated in the filamentous cyanobacteria Anabaena and Nostoc.

Manuel Losada

Papers

Short-term ammonium inhibition of nitrate utilization by Anacystis nidulans and other cyanobacteria

Properties and function of yeast pyruvate carboxylase.

Ferredoxin-nitrite reductase from spinach.

Photosynthetic nature of nitrate uptake and reduction in the cyanobacterium Anacystis nidulans

Flavin nucleotide nitrate reductase from spinach.