Sustained Photoproduction of Ammonia from Dinitrogen and Water by the Nitrogen-Fixing Cyanobacterium Anabaena sp. Strain ATCC 33047

Ammonium photo-production by heterocytous cyanobacteria: potentials and constraints

Over the last decades, production of microalgae and cyanobacteria has been developed for several applications, including novel foods, cosmetic ingredients and more recently biofuel. The sustainability of these promising developments can be hindered by some constraints, such as water and nutrient footprints. This review surveys data on N2-fixing cyanobacteria for biomass production and ways to induce and improve the excretion of ammonium within cultures under aerobic conditions. The nitrogenase complex is oxygen sensitive. Nevertheless, nitrogen fixation occurs under oxic conditions due to cyanobacteria-specific characteristics. For instance, in some cyanobacteria, the vegetative cell differentiation in heterocyts provides a well-adapted anaerobic microenvironment for nitrogenase protection. Therefore, cell cultures of oxygenic cyanobacteria have been grown in laboratory and pilot photobioreactors (Dasgupta et al., 2010; Fontes et al., 1987; Moreno et al., 2003; Nayak & Das, 2013). Biomass production under diazotrophic conditions has been shown to be controlled by environmental factors such as light intensity, temperature, aeration rate, and inorganic carbon concentration, also, more specifically, by the concentration of dissolved oxygen in the culture medium. Currently, there is little information regarding the production of extracellular ammonium by heterocytous cyanobacteria. This review compares the available data on maximum ammonium concentrations and analyses the specific rate production in cultures grown as free or immobilized filamentous cyanobacteria. Extracellular production of ammonium could be coupled, as suggested by recent research on non-diazotrophic cyanobacteria, to that of other high value metabolites. There is little information available regarding the possibility for using diazotrophic cyanobacteria as cellular factories may be in regard of the constraints due to nitrogen fixation.

Biotechnological uses of cyanobacteria

Cyanobacteria (blue-green algae) are O(2)-evolving photosynthesizing prokaryotes that have an extensive history of use as a human food source and as a fertilizer in rice fields. They have also been recognized as an excellent source of vitamins and proteins and as such are found in health food stores in North America and elsewhere. Cyanobacteria have a great deal of potential as a source of fine chemicals, as a biofertilizer and as a source of renewable fuel. This potential is being realized as data from research in the areas of the physiology and chemistry of these organisms are gathered and as the knowledge of cyanobacterial genetics and genetic engineering increases. We review, here, the present (and possible future) uses of cyanobacteria and assess the state of the art with respect to the genetic manipulation of cyanobacteria.

Excretion of ammonium by a nifL mutant of Azotobacter vinelandii fixing nitrogen

A mutation in the gene upstream of nifA in Azotobacter vinelandii was introduced into the chromosome to replace the corresponding wild-type region. The resulting mutant, MV376, produced nitrogenase constitutively in the presence of 15 mM ammonium. When introduced into a nifH-lacZ fusion strain, the mutation permitted beta-galactosidase production in the presence of ammonium. The gene upstream of nifA is therefore designated nifL because of its similarity to the Klebsiella pneumoniae nifL gene in proximity to nifA, in mutant phenotype, and in amino acid sequence of the gene product. The A. vinelandii nifL mutant MV376 excreted significant quantities of ammonium (approximately 10 mM) during diazotrophic growth. In contrast, ammonium excretion during diazotrophy was much lower in a K. pneumoniae nifL deletion mutant (maximum, 0.15 mM) but significantly higher than in NifL+ K. pneumoniae. The expression of the A. vinelandii nifA gene, unlike that of K. pneumoniae, was not repressed by ammonium.

Effect of glutamine on growth and heterocyst differentiation in the cyanobacterium Anabaena variabilis

Mutants of the cyanobacterium Anabaena variabilis that were capable of increased uptake of glutamine, as compared with that in the parental strains, were isolated. Growth of these mutants and their parental strains was measured in media containing N2, ammonia, or glutamine as a source of nitrogen. All strains grew well with any one of these sources of fixed nitrogen. Much of the glutamine taken up by the cells was converted to glutamate. The concentrations of glutamine, glutamate, arginine, ornithine, and citrulline in free amino acid pools in glutamine-grown cells were high compared with the concentrations of these amino acids in ammonia-grown or N2-grown cells. All strains capable of heterocyst differentiation, including a strain which produced nonfunctional heterocysts, grew and formed heterocysts in the presence of glutamine. However, nitrogenase activity was repressed in glutamine-grown cells. Glutamine may not be the molecule directly responsible for repression of the differentiation of heterocysts.

CARBON NUTRITION AND THE REGULATION OF UPTAKE HYDROGENASE ACTIVITY IN FREE-LIVING AND SYMBIOTIC ANABAENA CYCADEAE

Anabaena cycadeae was grown with N₂ as nitrogen source, either photoautotrophically in light or with glucose as carbon source in darkness. The rate of growth was much slower in darkness but the heterocyst frequency was much the same; nitrogenase activity (on a chlorophyll basis) was about half that of light-grown cells. Light-grown organisms contained uptake hydrogenase activity but dark-grown organisms did not. The addition of glucose to light-grown organisms was followed by the disappearance of uptake hydrogenase activity over the following 48 hours and the disappearance was independent of light. Heterocyst frequency and nitrogenase activity were much less affected by glucose addition. A. cycadeae growing symbiotically in cycad roots had much higher heterocyst frequency and nitrogenase activity than the free-living form but no detectable uptake hydrogenase activity. It is suggested that the rate of supply of carbohydrate to the heterocyst controls the development of uptake hydrogenase activity and that the absence of this activity in the symbiotic cyanobacteria indicates that the organisms in the cycad roots have an ample supply of carbohydrate.

Isolation and characterization of nitrogenase-derepressed mutant strains of cyanobacterium Anabaena variabilis

A positive selection method for isolation of nitrogenase-derepressed mutant strains of a filamentous cyanobacterium, Anabaena variabilis, is described. Mutant strains that are resistant to a glutamate analog, L-methionine-D,L-sulfoximine, were screened for their ability to produce and excrete NH4+ into medium. Mutant strains capable of producing nitrogenase in the presence of NH4+ were selected from a population of NH4+-excreting mutants. One of the mutant strains (SA-1) studied in detail was found to be a conditional glutamine auxotroph requiring glutamine for growth in media containing N2, NO3-, or low concentrations of NH4+ (less than 0.5 mM). This glutamine requirement is a consequence of a block in the assimilation of NH4+ produced by an enzyme system like nitrogenase. Glutamate and aspartate failed to substitute for glutamine because of a defect in the transport and utilization of these amino acids. Strain SA-1 assimilated NH4+ when the concentration in the medium reached about 0.5 mM, and under these conditions the growth rate was similar to that of the parent. Mutant strain SA-1 produced L-methionine-D,L-sulfoximine-resistant glutamine synthetase activity. Kinetic properties of the enzyme from the parent and mutant were similar. Mutant strain SA-1 can potentially serve as a source of fertilizer nitrogen to support growth of crop plants, since the NH4+ produced by nitrogenase, utilizing sunlight and water as sources of energy and reductant, respectively, is excreted into the environment.