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

Diurnal switches in diazotrophic lifestyle increase nitrogen contribution to cereals

Uncoupling of biological nitrogen fixation from ammonia assimilation is a prerequisite step for engineering ammonia excretion and improvement of plant-associative nitrogen fixation. In this study, we have identified an amino acid substitution in glutamine synthetase, which provides temperature sensitive biosynthesis of glutamine, the intracellular metabolic signal of the nitrogen status. As a consequence, negative feedback regulation of genes and enzymes subject to nitrogen regulation, including nitrogenase is thermally controlled, enabling ammonia excretion in engineered Escherichia coli and the plant-associated diazotroph Klebsiella oxytoca at 23 °C, but not at 30 °C. We demonstrate that this temperature profile can be exploited to provide diurnal oscillation of ammonia excretion when variant bacteria are used to inoculate cereal crops. We provide evidence that diurnal temperature variation improves nitrogen donation to the plant because the inoculant bacteria have the ability to recover and proliferate at higher temperatures during the daytime.

Soil microalgae and cyanobacteria: the biotechnological potential in the maintenance of soil fertility and health

The soil microbiota plays a major role in maintaining the nutrient balance, carbon sink, and soil health. Numerous studies reported on the function of microbiota such as plant growth-promoting bacteria and fungi in soil. Although microalgae and cyanobacteria are ubiquitous in soil, very less attention has been paid on the potential of these microorganisms. The indiscriminate use of various chemicals to enhance agricultural productivity led to serious consequences like structure instability, accumulation of toxic contaminants, etc., leading to an ecological imbalance between soil, plant, and microbiota. However, the significant role of microalgae and cyanobacteria in crop productivity and other potential options has been so far undermined. The intent of the present critical review is to highlight the significance of this unique group of microorganisms in terms of maintaining soil fertility and soil health. Beneficial soil ecological applications of these two groups in enhancing plant growth, establishing interrelationships among other microbes, and detoxifying chemical agents such as insecticides, herbicides, etc. through mutualistic cooperation by synthesizing enzymes and phytohormones are presented. Since recombinant technology involving genomic integration favors the development of useful traits in microalgae and cyanobacteria for their potential application in improvement of soil fertility and health, the merits and demerits of various such advanced methodologies associated in harnessing the biotechnological potential of these photosynthetic microorganisms for sustainable agriculture were also discussed.

Cyanobacterial Toxins of the Laurentian Great Lakes, Their Toxicological Effects, and Numerical Limits in Drinking Water

Cyanobacteria are ubiquitous phototrophic bacteria that inhabit diverse environments across the planet. Seasonally, they dominate many eutrophic lakes impacted by excess nitrogen (N) and phosphorus (P) forming dense accumulations of biomass known as cyanobacterial harmful algal blooms or cyanoHABs. Their dominance in eutrophic lakes is attributed to a variety of unique adaptations including N and P concentrating mechanisms, N₂ fixation, colony formation that inhibits predation, vertical movement via gas vesicles, and the production of toxic or otherwise bioactive molecules. While some of these molecules have been explored for their medicinal benefits, others are potent toxins harmful to humans, animals, and other wildlife known as cyanotoxins. In humans these cyanotoxins affect various tissues, including the liver, central and peripheral nervous system, kidneys, and reproductive organs among others. They induce acute effects at low doses in the parts-per-billion range and some are tumor promoters linked to chronic diseases such as liver and colorectal cancer. The occurrence of cyanoHABs and cyanotoxins in lakes presents challenges for maintaining safe recreational aquatic environments and the production of potable drinking water. CyanoHABs are a growing problem in the North American (Laurentian) Great Lakes basin. This review summarizes information on the occurrence of cyanoHABs in the Great Lakes, toxicological effects of cyanotoxins, and appropriate numerical limits on cyanotoxins in finished drinking water.

Efficient Gene Induction and Endogenous Gene Repression Systems for the Filamentous Cyanobacterium Anabaena sp. PCC 7120

In the last decade, many studies have been conducted to employ genetically engineered cyanobacteria in the production of various metabolites. However, the lack of a strict gene regulation system in cyanobacteria has hampered these attempts. The filamentous cyanobacterium Anabaena sp. PCC 7120 performs both nitrogen and carbon fixation and is, therefore, a good candidate organism for such production. To employ Anabaena cells for this purpose, we intended to develop artificial gene regulation systems to alter the cell metabolic pathways efficiently. We introduced into Anabaena a transcriptional repressor TetR, widely used in diverse organisms, and green fluorescent protein (GFP) as a reporter. We found that anhydrotetracycline (aTc) substantially induced GFP fluorescence in a concentration-dependent manner. By expressing tetR under the nitrate-specific promoter nirA, we successfully reduced the concentration of aTc required for the induction of gfp under nitrogen fixation conditions (to 10% of the concentration needed under nitrate-replete conditions). Further, we succeeded in the overexpression of GFP by depletion of nitrate without the inducer by means of promoter engineering of the nirA promoter. Moreover, we applied these gene regulation systems to a metabolic enzyme in Anabaena and successfully repressed glnA, the gene encoding glutamine synthetase that is essential for nitrogen assimilation in cyanobacteria, by expressing the small antisense RNA for glnA. Consequently, the ammonium production of an ammonium-excreting Anabaena mutant was significantly enhanced. We therefore conclude that the gene regulation systems developed in this study are useful tools for the regulation of metabolic enzymes and will help to increase the production of desired substances in Anabaena.

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.

Ammonia accumulation in culture broth by the novel nitrogen-fixing bacterium, Lysobacter sp. E4

This is the first report that Lysobacter fixes nitrogen under free-living conditions, as shown by its ability to grow on nitrogen-free medium and accumulate relatively high amounts of ammonia in the culture broth. Growth of the E4 Lysobacter strain, isolated in a screen for nitrogen-fixing and ammonia-producing bacteria, resulted in higher ammonia accumulation (0.53 mM ammonium ion concentration) in media containing glucose rather than other tested carbon sources. The optimum glucose concentration was 0.30% at an initial medium pH of 7.0 and incubation temperature of 30°C. From time-course experiments, when the glucose in the culture was exhausted, ammonia began to be accumulated, and maximum ammonia accumulation (∼1.60 mM) was reached after 8 days of incubation. Ammonia accumulation by this strain required molybdenum, manganese, and iron.

Effect on heterocyst differentiation of nitrogen fixation in vegetative cells of the cyanobacterium Anabaena variabilis ATCC 29413

Heterocysts are terminally differentiated cells of some filamentous cyanobacteria that fix nitrogen for the entire filament under oxic growth conditions. Anabaena variabilis ATCC 29413 is unusual in that it has two Mo-dependent nitrogenases; one, called Nif1, functions in heterocysts, while the second, Nif2, functions under anoxic conditions in vegetative cells. Both nitrogenases depended on expression of the global regulatory protein NtcA. It has long been thought that a product of nitrogen fixation in heterocysts plays a role in maintenance of the spaced pattern of heterocyst differentiation. This model assumes that each cell in a filament senses its own environment in terms of nitrogen sufficiency and responds accordingly in terms of differentiation. Expression of the Nif2 nitrogenase under anoxic conditions in vegetative cells was sufficient to support long-term growth of a nif1 mutant; however, that expression did not prevent differentiation of heterocysts and expression of the nif1 nitrogenase in either the nif1 mutant or the wild-type strain. This suggested that the nitrogen sufficiency of individual cells in the filament did not affect the signal that induces heterocyst differentiation. Perhaps there is a global mechanism by which the filament senses nitrogen sufficiency or insufficiency based on the external availability of fixed nitrogen. The filament would then respond by producing heterocyst differentiation signals that affect the entire filament. This does not preclude cell-to-cell signaling in the maintenance of heterocyst pattern but suggests that overall control of the process is not controlled by nitrogen insufficiency of individual cells.

N 2 -fixing cyanobacteria and their potential applications

Cyanobacteria are N 2 -fixing oxygenic phototrophs with potential as sources of nitrogenous biofertilizer independent of fossil fuels. Recently, attention has been paid to their role in this respect, and to strain selection to facilitate enhanced agricultural productivity. The capacities to fix N 2 in the presence of combined nitrogen, to resist herbicides and to tolerate salinity changes and desiccation are of particular importance. Mutant strains of N 2 -fixing cyanobacteria with potential applications have been produced, including strains which photoproduce ammonia and amino acids, strains with altered transport systems and strains which are herbicide-resistant. By using bioreactors and immobilization systems in conjunction with these strains, techniques have been developed for extracellular product-liberation. Gene transfer systems are now available in both unicellular and filamentous cyanobacteria and these offer the possibility of strain selection and modification. The way in which these advances will contribute to the development of strains with desirable attributes for use in the field is discussed.

Photodissimilation of Fructose to H(inf2) and CO(inf2) by a Dinitrogen-Fixing Cyanobacterium, Anabaena variabilis

The ability of cyanobacteria to serve as biocatalysts in the production of H(inf2) as a fuel and chemical feedstock was investigated with Anabaena variabilis. The results show that A. variabilis, when incubated under argon, dissimilated fructose to H(inf2) and CO(inf2) in a light-dependent reaction. The H(inf2) production had an obligate requirement for fructose and was heterocyst dependent, since NH(inf4)(sup+)-grown cultures lacking heterocysts failed to produce H(inf2). Differential inhibition studies with CO showed that nitrogenase is the main enzyme catalyzing the H(inf2) production. Net H(inf2) yield increased with increasing concentrations of fructose up to 10 mM in the medium. The average apparent conversion efficiency of fructose to H(inf2) (net H(inf2) produced/fructose removed from the medium) was about 10, although higher conversion efficiencies of 15 to 17 could be obtained during shorter periods and at optimum fructose concentrations. Under the same conditions, the ratio of CO(inf2) released to fructose removed from the medium was about 3.5, suggesting that only a fraction of the fructose carbon was completely oxidized to CO(inf2). Under conditions of carbon excess, which prevents H(inf2) uptake, the maximum ratio of H(inf2) to CO(inf2) was found to be 3.0. This is higher than the expected value of 2.0, indicating that water was also a source of reductant in this fructose-mediated H(inf2) production. Inhibition of H(inf2) evolution by 3-(3,4-dichlorophenyl)-1,1-dimethylurea confirmed a role for photosystem II in this process. The rate of H(inf2) production by A. variabilis SA1 was 46 ml h(sup-1) g (dry weight)(sup-1). This high rate was maintained for over 15 days. About 30% of this H(inf2) was derived from water (10 ml of H(inf2) h(sup-1) g [dry weight](sup-1)). These results show that filamentous, heterocystous cyanobacteria can serve as biocatalysts in the high-efficiency conversion of biomass-derived sugars to H(inf2) as a fuel source while simultaneously dissimilating water to H(inf2).

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.

Ammonium Excretion by an l -Methionine- dl -Sulfoximine-Resistant Mutant of the Rice Field Cyanobacterium Anabaena siamensis

An ammonium-excreting mutant (SS1) of the rice field nitrogen-fixing cyanobacterium Anabaena siamensis was isolated after ethyl methanesulfonate mutagenesis by selection on 500 μM l -methionine- dl -sulfoximine. SS1 grew in the presence and absence of l -methionine- dl -sulfoximine at a rate comparable to that of the wild-type strain, with a doubling time of 5.6 h. The rate of ammonium release by SS1 depended on cell density; it peaked at the 12th hour of growth with 8.7 μmol mg of chlorophyll −1 h −1 (at a chlorophyll concentration of 5 μg ml −1 ) and slowed down to almost nil at the fourth day of growth. A similar pattern of release by immobilized SS1 was observed between 12 to 20 h after loading alginate beads in packed-bed reactors at the rate of 11.6 μmol mg of chlorophyll −1 h −1 . The rate was later reduced significantly due to the fast growth of SS1 on the substrate. Prolonged release of ammonium at the peak level was achieved only by maintaining SS1 under continuous cultivation at low chlorophyll levels (5 to 7 μg ml −1 ). Under these conditions, nitrogen fixation in the mutant was 30% higher than that in its parent and glutamine synthetase activity was less by 50%. Immunoblot analysis revealed that SS1 and its parent have similar quantities of glutamine synthetase protein under ammonium excretion conditions. In addition, a protein with a molecular weight of about 30,000 seems to have been lost, as seen by electrophoretic separation of total proteins from SS1.

Analysis of drug resistance in the archaebacterium Methanococcus voltae with respect to potential use in genetic engineering

The sensitivity of the methanogenic archaebacterium Methanococcus voltae to 12 inhibitors was tested in liquid medium. Four compounds appeared to be inhibitors of growth. Their MICs were as follows: pseudomonic acid, 0.1 micrograms/ml (0.19 microM); puromycin, 2 micrograms/ml (3.6 microM); methionine sulfoximine, 30 micrograms/ml (170 microM); and fusidic acid, 100 micrograms/ml (170 microM). On solid medium, the MICs were similar and the frequency of spontaneous resistance was found to be 5 X 10(-5) (methionine sulfoximine), 10(-7) (pseudomonic acid), and less than 10(-7) (puromycin and fusidic acid). Pseudomonic acid was found to inhibit isoleucyl-tRNA synthetase activity as measured by the in vitro aminoacylation of M. voltae tRNA with L-[U-14C]isoleucine. Fusidic acid and puromycin were shown to inhibit poly(U)-dependent polyphenylalanine synthesis in S30 extracts. Acetylpuromycin was inhibitory at much higher concentrations both in vivo and in vitro for M. voltae. Thus, the pac gene of Streptomyces alboniger, which is responsible for acetylation of puromycin and which conferred resistance to puromycin when introduced in eubacteria and eucaryotes, is a potential selective marker in gene transfer experiments with M. voltae. The latter was recently shown to be transformable. The same would be true for the cat gene of Tn9, which encodes resistance to fusidic acid in eubacteria in addition to resistance to chloramphenicol.

Physiological conditions for nitrogen fixation in a unicellular marine cyanobacterium, Synechococcus sp. strain SF1

A marine, unicellular, nitrogen-fixing cyanobacterium was isolated from the blades of a brown alga, Sargassum fluitans. This unicellular cyanobacterium, identified as Synechococcus sp. strain SF1, is capable of photoautotrophic growth with bicarbonate as the sole carbon source and dinitrogen as the sole nitrogen source. Among the organic carbon compounds tested, glucose and sucrose supported growth. Of the nitrogen compounds tested, with bicarbonate serving as the carbon source, both ammonia and nitrate produced the highest growth rates. Most amino acids failed to support growth when present as sole sources of nitrogen. Nitrogenase activity in Synechococcus sp. strain SF1 was induced after depletion of ammonia from the medium. This activity required the photosynthetic utilization of bicarbonate, but pyruvate and hydrogen gas were also effective sources of reductant for nitrogenase activity. Glucose, fructose, and sucrose also supported nitrogenase activity but to a lesser extent. Optimum light intensity for nitrogenase activity was found to be 70 microE/m2 per s, while the optimum oxygen concentration in the gas phase for nitrogenase activity was about 1%. A hydrogenase activity was coinduced with nitrogenase activity. It is proposed that this light- and oxygen-insensitive hydrogenase functions in recycling the hydrogen produced by nitrogenase under microaerobic conditions.

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.