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Microalgae II: Cell Structure, Nutrition and Metabolism
Published in Arun Kumar, Jay Shankar Singh, Microalgae in Waste Water Remediation, 2021
Nitrogen assimilation in eukaryotic microalgae involves four steps in which two are transport steps and two are reduction steps (Guerrero et al. 1981, Fernandez and Galvan 2007, 2008): (a) in the first step, transport of nitrate in to the cell; (b) reduction of nitrate in to nitrite through cytosolic enzyme nitrate reductase (narB); (c) subsequent transport of nitrite in to the chloroplast; (d) reduction of nitrite in to ammonium through the enzyme nitrite reductase (nir) (Fig. 2.3). The incorporation of this reduced ammonium takes place as an amide group of glutamine in a reaction involving glutamate and ATP in the presence of enzyme glutamine synthetase, and then the amide group is transferred reductively to a-oxoglutarate to form two molecules of glutamate, this whole process was earlier known as glutamate synthase cycle (Miflin and Lea 1975). By investigating Chlamydomonas microalgae, three families of proteins NRT1 (NPF), NRT2, and NAR1 are found to be involved in nitrate and/or nitrite transport (Crawford and Glass 1998, Forde 2000, Galvan and Fernández 2001, Forde and Cole 2003).
Performance Characteristics of an Activated Sludge System When Nitrate is the Sole Source of Nitrogen
Published in John W. Bell, Proceedings of the 45th Industrial Waste Conference May 8, 9, 10, 1990, 1991
Nannette Corey, James M. Montgomery, Larry Benefield
The key enzyme in nitrate respiration is nitrate reductase which reduces nitrate to nitrite. Actually two different nitrate reductase enzymes have been found to be involved in nitrate respiration.4 These enzymes have been classified as nitrate reductase A and nitrate reductase B. Nitrate reductase A seems to always play a role in nitrate respiration, but the function of nitrate reductase B is different in different bacteria and little information is available on the basic properties of this particular enzyme. In general, nitrate reductase A is an inducible enzyme which is formed only in the absence of oxygen and in the presence of nitrate. Under aerobic conditions the formation of both nitrate reductase A and nitrate reductase B is repressed.
Nitrogen Cycle Bacteria in Agricultural Soils
Published in Vivek Kumar, Rhizomicrobiome Dynamics in Bioremediation, 2021
Guillermo Bravo, Paulina Vega-Celedón, Constanza Macaya, Ingrid-Nicole Vasconez, Michael Seeger
Denitrification Denitrification is a process in which nitrate is reduced successively to NO, N2O and N2 gases. Nitrate reduction by anaerobic microorganisms uses nitrate as electron acceptor. Nitrate reduction starts with the action of the transmembrane nitrate reductase, whose product is nitrite. Nitrite is then reduced to nitric oxide by nitrite reductase. Nitric oxide is successively reduced to gaseous nitrogen (Rütting et al. 2018). Denitrification can be harmful to soils. It has been shown that 17 Tg of N are lost worldwide every year from soils and 15 Tg at the agroecological level due to the microbial denitrifying activity (Mosier et al. 2004, Mao et al. 2011).
Enhancement of growth and biomolecules (carbohydrates, proteins, and chlorophylls) of isolated Chlorella thermophila using optimization tools
Published in Preparative Biochemistry & Biotechnology, 2022
Sambit Sarkar, Jaivik Mankad, Nitin Padhihar, Mriganka Sekhar Manna, Tridib Kumar Bhowmick, Kalyan Gayen
In this study, nitrate was found to impart a negative effect on biomass synthesis. This result is similar to another study conducted with Asterarcys sp. where 0.375 g/L of nitrate concentration in the media provided higher biomass concentration than other concentrations of nitrates.[16] Similar results were obtained in another study where an increase in nitrate concentration decreased the biomass production and the highest biomass was obtained at the lowest nitrate concentration used in that study (1.5 g/L).[44] However, nitrogen in a form of urea was shown to boost the growth in Chlorella spp.[19] Microalgae cells necessitate nitrogen in a higher amount owing to its role as the critical constituent of proteins, peptides, chlorophylls, enzymes, ATP, RNA, DNA, and other cellular constituents.[45] Nitrogen in form of nitrate, nitrite, and ammonium are directly assimilable in the metabolic activity of microalgae. However, nitrate is comparatively stable thermodynamically than other forms of nitrogen that get oxidized in the aqueous medium. Nitrate after being translocated across plasmalemma requires to be chemically reduced to ammonium for being assimilated into the cell. Chemical reduction of nitrate is governed by two enzymes which are nitrate reductase and nitrite reductase.[46] Nitrate reductase catalyzes the bi-electron transfer in the cytosol with the aid of NADPH. The enzyme nitrate reductase is attached with the pyridine nucleotide oxidation in microalgae.[44] Nitrite reductase reduces nitrite in a reaction of six-election transfer. Nitrite reductase localized in the chloroplast uses ferredoxin which is sourced from the photosynthetic electron flow in microalgae. Ammonium is mostly incorporated into amino acids by the chronological act of glutamine synthetase (GS) and glutamine 2 oxoglutarate aminotransferase (GOGAT). Ammonium is assimilated by GS in an irreversible reaction utilizing glutamate as substrate. GS and GOGAT are usually located in the chloroplast. However, their isoenzymes may also be found in the cytosol. Glutamate synthase facilitates the synthesis of amino acids through transamination after being transported to cytosol from chloroplast.[44]