Isolation and characterization of heterocysts from Anabaena sp. strain CA

Antenna Modification Leads to Enhanced Nitrogenase Activity in a High Light-Tolerant Cyanobacterium

Biological nitrogen fixation is an energy-intensive process that contributes significantly toward supporting life on this planet. Among nitrogen-fixing organisms, cyanobacteria remain unrivaled in their ability to fuel the energetically expensive nitrogenase reaction with photosynthetically harnessed solar energy. In heterocystous cyanobacteria, light-driven, photosystem I (PSI)-mediated ATP synthesis plays a key role in propelling the nitrogenase reaction. Efficient light transfer to the photosystems relies on phycobilisomes (PBS), the major antenna protein complexes. PBS undergo degradation as a natural response to nitrogen starvation. Upon nitrogen availability, these proteins are resynthesized back to normal levels in vegetative cells, but their occurrence and function in heterocysts remain inconclusive. Anabaena 33047 is a heterocystous cyanobacterium that thrives under high light, harbors larger amounts of PBS in its heterocysts, and fixes nitrogen at higher rates compared to other heterocystous cyanobacteria. To assess the relationship between PBS in heterocysts and nitrogenase function, we engineered a strain that retains large amounts of the antenna proteins in its heterocysts. Intriguingly, under high light intensities, the engineered strain exhibited unusually high rates of nitrogenase activity compared to the wild type. Spectroscopic analysis revealed altered PSI kinetics in the mutant with increased cyclic electron flow around PSI, a route that contributes to ATP generation and nitrogenase activity in heterocysts. Retaining higher levels of PBS in heterocysts appears to be an effective strategy to enhance nitrogenase function in cyanobacteria that are equipped with the machinery to operate under high light intensities. IMPORTANCE The function of phycobilisomes, the large antenna protein complexes in heterocysts has long been debated. This study provides direct evidence of the involvement of these proteins in supporting nitrogenase activity in Anabaena 33047, a heterocystous cyanobacterium that has an affinity for high light intensities. This strain was previously known to be recalcitrant to genetic manipulation and, hence, despite its many appealing traits, remained largely unexplored. We developed a genetic modification system for this strain and generated a ΔnblA mutant that exhibited resistance to phycobilisome degradation upon nitrogen starvation. Physiological characterization of the strain indicated that PBS degradation is not essential for acclimation to nitrogen deficiency and retention of PBS is advantageous for nitrogenase function.

Isolation and characterization of high protein and phycocyanin producing mutants of Arthrospira platensis

Cyanobacteria are known to exhibit their efficiency in producing high concentrations of compounds of commercial value. Arthrospira is one such cyanobacterium which is considered as important source of protein (65%) and other nutrients. In present study, chemical mutagenesis using N-methyl-Ń-Nitro-nitrosoguanidine (NTG), a proven potent mutagen for cyanobacteria was used to bring stable and desirable alteration in Arthrospira platensis ARM 730. Three morphological mutants (G-1, G-2, and SF) were selected and characterized. The G-1 and G-2 were helical, more bluish in pigmentation than the wild type strain where G-1 also showed enlarged cell size. The SF mutant was an altered straight-filament having maximum biomass. Among three mutants, higher protein and phycocyanin contents were observed in G-1 and G-2 mutants whereas chlorophyll was less in these mutants as compared to wild type strain indicating change in the pigment ratio. Carotenoid content was higher in SF mutant as compared to wild type and other mutants. Variation in total sugar content was not observed in comparison to wild type strain. The analysis of amino acid spectrum of all the mutants and wild type showed significant increase in proline content. Overall, it is revealed from the results that G-1 and G-2 mutants showed higher biomass, phycocyanin, and protein contents in comparison to wild type which indicated their great potential to be used in food and pharmaceutical industries.

Complete Genome Sequence of the Cyanobacterium Anabaena sp. 33047

This study presents the complete nucleotide sequence of Anabaena sp. ATCC 33047 (Anabaena CA), a filamentous, nitrogen-fixing marine cyanobacterium, which under salt stress conditions accumulates sucrose internally. The elucidation of the genome will contribute to the understanding of cyanobacterial diversity.

Biomass production potential of a wastewater alga Chlorella vulgaris ARC 1 under elevated levels of CO₂and temperature

The growth response of Chlorella vulgaris was studied under varying concentrations of carbon dioxide (ranging from 0.036 to 20%) and temperature (30, 40 and 50°C). The highest chlorophyll concentration (11 μg mL^–1) and biomass (210 μg mL^–1), which were 60 and 20 times more than that of C. vulgaris at ambient CO₂ (0.036%), were recorded at 6% CO₂ level. At 16% CO₂ level, the concentrations of chlorophyll and biomass values were comparable to those at ambient CO₂ but further increases in the CO₂ level decreased both of them. Results showed that the optimum temperature for biomass production was 30°C under elevated CO₂ (6%). Although increases in temperature above 30°C resulted in concomitant decrease in growth response, their adverse effects were significantly subdued at elevated CO₂. There were also differential responses of the alga, assessed in terms of NaH¹⁴CO₃ uptake and carbonic anhydrase activity, to increases in temperature at elevated CO₂. The results indicated that Chlorella vulgaris grew better at elevated CO₂ level at 30°C, albeit with lesser efficiencies at higher temperatures.

Physiology ofAnabaena khannae andChlorococcum humicola under fluoride stress

Sodium fluoride showed pH-dependent physiological responses in the two test microalgae Anabaena khannae and Chlorococcum humicola. A. khannae showed severe membrane damage with fluoride at low pH with leakage of pigments and electrolytes. Annihilation of photosynthesis along with inhibition in 14C uptake was observed at pH 6 with 50 mg/L fluoride. While respiration was less affected in the cyanobacterium, C. humicola showed 30 % inhibition in respiratory activity. Resistance of C. humicola to fluoride toxicity has been attributed to the hindrance provided by the thick cell envelope, intracellular compartmentation and increase in extracellular pH as a consequence of its metabolism.

Role of white light in reversing UV-B-mediated effects in the N2-fixing cyanobacterium Anabaena BT2

The effects of various irradiances of artificial UV-B (280-315 nm) in the presence or absence of visible light (photosynthetically active radiation) on growth, survival, 14CO2 uptake and ribulose 1,5-bisphosphate carboxylase (RuBISCO) activity were studied in the N2-fixing cyanobacterium Anabaena BT2. We tested the hypothesis whether or not visible radiation offers any protection against UV-B-induced deleterious effects on growth and photosynthesis in Anabaena BT2. Attempts were also made to determine the irradiances of UV-B where inhibitory effects could be mitigated by simultaneous irradiation with visible light. Exposure of cultures to 0.2 W m(-2) or higher irradiance of UV-B caused inhibition of growth and survival and growth ceased above 1.0 W m(-2). 14CO uptake and RuBISCO activity were found to be more sensitive to UV-B and around 60% reduction in 14CO2 uptake and RuBISCO activity occurred after exposure of cultures to 0.4 W m(-2) for 1 h. However, growth, 14CO2 uptake and RuBISCO activity were nearly normal when UV-B (0.4 W m(-2)) and visible light (14.4 W m(-2)) were given simultaneously. Blue radiation (450 nm) was found to be the most effective in photoreactivation against UV-B, better than UV-A or any other light wavelength band. Our results demonstrate that the studied cyanobacterium possesses active photoreactivation mechanism(s) against UV-B-mediated damage which in turn probably allow survival under natural conditions in spite of being continuously exposed to the UV-B component present in the solar radiation. Continued growth of many algae and cyanobacteria in the presence of intense solar UV-B radiation under natural conditions seems to be due to the active role of photoreactivation.

Differential effect of ultraviolet-B radiation on certain metabolic processes in a chromatically adapting Nostoc

The impact of UV-B radiation on growth, pigmentation and certain physiological processes has been studied in a N2-fixing chromatically adapting cyanobacterium, Nostoc spongiaeforme. A brownish form (phycoerythrin rich) was found to be more tolerant to UV-B than the blue-green (phycocyanin rich) form of N. spongiaeforme. Continuous exposure to UV-B (5.5 W m-2) for 90 min caused complete killing of the blue-green strain whereas the brown strain showed complete loss of survival after 180 min. Pigment content was more strongly inhibited in the blue-green strain than in the brown. Nitrogenase activity was completely abolished in both strains within 35 min of UV-B treatment. Restoration of nitrogenase occurred upon transfer to fluorescent or incandescent light after a lag of 5-6 h, suggesting fresh synthesis of nitrogenase. Unlike the above processes, in vivo nitrate reductase activity was stimulated by UV-B treatment, the degree of enhancement being significantly higher in the blue-green strain. Like the effect of UV-B on nitrogenase, 14CO2 uptake was also completely abolished by UV-B treatment in both strains. Our findings suggest that UV-B may produce a deleterious effect on several metabolic activities of cyanobacteria, especially in cells lacking phycoerythrin. Strains containing phycoerythrin appear to be more tolerant to UV-B, probably because of their inherent property of adapting to a variety of light qualities.

Alteration of the Fe protein of nitrogenase by oxygen in the cyanobacterium Anabaena sp. strain CA

Changes in protein composition were noted when heterocysts of Anabaena sp. strain CA were isolated from filaments grown in 1% CO2-99% N2 and subsequently exposed to oxygen. Immunospecific Western blot analysis showed that the Fe protein of nitrogenase is altered. In cells grown under microaerobic conditions, the Fe protein was found in a form with an apparent molecular weight of 30,000. Exposure to oxygen caused a shift in the migration of this polypeptide to a position corresponding to an apparent molecular weight of 31,500. This modification was reversible upon removal of oxygen from the culture. Chloramphenicol did not inhibit the alteration in either direction. Suppression by ammonium nitrate of the recovery of nitrogenase activity from the effects of oxygen did not prevent the alteration of the protein. Other inhibitors of nitrogenase activity, (metronidazole, carbonyl cyanide m-chlorophenylhydrazone, and phenazine methosulfate) were tested for their effect on Fe protein modification. Alteration of the Fe protein may relate to the protection of nitrogenase from the deleterious effects of oxygen.

Nitrogen starvation mediated by DL-7-azatryptophan in the cyanobacterium Anabaena sp. strain CA

The addition of DL-7-azatryptophan (AZAT), a tryptophan analog, to continuous cultures of Anabaena sp. strain CA grown with 10 mM nitrate as the nitrogen source resulted in the differentiation of heterocysts. Analysis of the intracellular amino acid pools of Anabaena sp. strain CA after the addition of AZAT showed a marked decline in the intracellular glutamate pool and a slight increase in the levels of glutamine. The in vitro activity of glutamate synthase, the second enzyme involved in primary ammonia assimilation in Anabaena spp., was partially inhibited by the presence of AZAT at concentrations which are effective in triggering heterocyst formation (15% inhibition at 10 microM AZAT and up to 85% inhibition at 1.0 mM AZAT). Azaserine, a glutamine analog and potent glutamate synthase inhibitor, had no effect on the triggering of heterocyst development from undifferentiated batch and continuous cultures of Anabaena sp. strain CA. However, the presence of 1.0 microM azaserine significantly decreased the intracellular glutamate pool and increased the glutamine pool. The addition of AZAT also caused a decrease in the C-phycocyanin content of Anabaena sp. strain CA as a result of its proteolytic degradation. AZAT also had an inhibitory effect on the nitrogenase activity of Anabaena sp. strain CA. All these results suggest that AZAT causes a general nitrogen starvation of Anabaena sp. strain CA filaments, triggering heterocyst synthesis.

Molecular aspects of nitrogen fixation by photosynthetic prokaryotes

The photosynthetic prokaryotes possess diverse metabolic capabilities, both in carrying out different types of photosynthesis and in their other growth modes. The nature of the coupling of these energy-generating processes with the basic metabolic demands of the cell, such as nitrogen fixation, has stimulated research for many years. In addition, nitrogen fixation by photosynthetic prokaryotes exhibits several unique features; the oxygen-evolving cyanobacteria have developed various strategies for protection of the oxygen-labile nitrogenase proteins, and some photosynthetic bacteria have been found to regulate their nitrogenase (N2ase) activity in a rapid response to fixed nitrogen, thus saving substantial amounts of energy. Recent advances in the biochemistry, physiology, and genetics of nitrogen fixation by cyanobacteria and photosynthetic bacteria are reviewed, with special emphasis on the unique features found in these organisms. Several major topics in cyanobacterial nitrogen fixation are reviewed. The isolation and characterization of N2ase and the isolation and sequence of N2ase structural genes have shown a great deal of similarity with other organisms. The possible pathways of electron flow to N2ase, the mechanisms of oxygen protection, and the control of nif expression and heterocyst differentiation will be discussed. Several recent advances in the physiology and biochemistry of nitrogen fixation by the photosynthetic bacteria are reviewed. Photosynthetic bacteria have been found to fix nitrogen microaerobically in darkness. The regulation of nif expression and possible pathways of electron flow to N2ase are discussed. The isolation of N2ase proteins, particularly the covalent modification of the Fe protein, the nature of the modifying group, properties of the activating enzyme, and regulating factors of the inactivation/activation process are reviewed.

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.

Isolation of cyanobacterial heterocysts with high and sustained dinitrogen-fixation capacity supported by endogenous reductants

A method is described for the preparation of cyanobacterial heterocysts with high nitrogen-fixation (acetylene-reduction) activity supported by endogenous reductants. The starting material was Anabaena variabilis ATCC 29413 grown in the light in the presence of fructose. Heterocysts produced from such cyanobacteria were more active than those from photoautotrophically-grown A. variabilis, presumably because higher reserves of carbohydrate were stored within the heterocysts. It proved important to avoid subjecting the cyanobacteria to low temperatures under aerobic conditions, as inhibition of respiration appeared to lead to inactivation of nitrogenase. Low temperatures were not harmful in the absence of O2. A number of potential osmoregulators at various concentrations were tested for use in heterocyst isolation. The optimal concentration (0.2 M sucrose) proved to be a compromise between adequate osmotic protection for isolated heterocysts and avoidance of inhibition of nitrogenase by high osmotic strength. Isolated heterocysts without added reductants such as H2 had about half the nitrogen-fixation activity expected on the basis of intact filaments. H2 did not increase the rate of acetylene reduction, suggesting that the supply of reductant from heterocyst metabolism did not limit nitrogen fixation under these conditions. Such heterocysts had linear rates of acetylene reduction for at least 2 h, and retained their full potential for at least 12 h when stored at 0 degree C under N2.

H2, N2, and O2 metabolism by isolated heterocysts from Anabaena sp. strain CA

Metabolically active heterocysts isolated from wild-type Anabaena sp. strain CA showed high rates of light-dependent acetylene reduction and hydrogen evolution. These rates were similar to those previously reported in heterocysts isolated from the mutant Anabaena sp. strain CA-V possessing fragile vegetative cell walls. Hydrogen production was observed with isolated heterocysts. The ratio of C2H4 to H2 produced ranged from 0.9 to 1.2, and H2 production exhibited unique biphasic kinetics consisting of a 1 to 2-min burst of hydrogen evolution followed by a lower, steady-state rate of hydrogen production. This burst was found to be dependent upon the length of the dark period immediately preceding illumination and may be related to dark-to-light ATP transients. The presence of 100 nM NiCl2 in the growth medium exerted an effect on both acetylene reduction and hydrogen evolution in the isolated heterocysts from strain CA. H2-stimulated acetylene reduction was increased from 2.0 to 3.2 mumol of C2H4 per mg (dry weight) per h, and net hydrogen production was abolished. A phenotypic Hup- mutant (N9AR) of Anabaena sp. strain CA was isolated which did not respond to nickel. In isolated heterocysts from N9AR, ethylene production rates were the same under both 10% C2H2-90% Ar and 10% C2H2-90% H2 with or without added nickel, and net hydrogen evolution was not affected by the presence of 100 nM Ni2+. Isolated heterocysts from strain CA were shown to have a persistent oxygen uptake of 0.7 mumol of O2 per mg (dry weight) per h, 35% of the rate of whole filaments, at air saturating O2 levels, indicating that O2 impermeability is not a requirement for active heterocysts.

High endogenous nitrogenase activity in isolated heterocysts of Anabaena sp. strain CA after nitrogen starvation

Metabolically active heterocysts were isolated from a mutant of Anabaena sp. strain CA with fragile vegetative cells. Heterocysts isolated from cultures grown in 1% CO2 in air reduced C2H2 at 57 and 10 nmol of C2H2 per mg (dry weight) per min under H2 and Ar, respectively. However, if whole filaments were sparged with 1% CO2 in 99% Ar for 12 h before heterocyst isolation, these heterocysts showed C2H2 reduction rates of 83 nmol of C2H4 per mg (dry weight) per min under either H2 or Ar, or 40% the activity of whole filaments grown in 1% CO2 in air. Heterocysts isolated from cultures sparged with 100% Ar or 1% CO2 in 99% N2 had the same C2H2 reduction pattern as heterocysts from cultures grown in 1% CO2 in air, i.e., low activity under Ar and high activity under H2. Labeling of whole filaments incubated with NaH14CO3 for 12 h under 1% CO2 in air or 1% CO2 in 99% Ar resulted in a twofold higher accumulation of 14C-labeled compounds in vegetative cells and heterocysts of Ar-incubated cells. Our results suggest that during incubation under 1% CO2 in 99% Ar, presumably a nitrogen starvation condition, continuing photosynthetic fixation of CO2 leads to accumulation of material(s) in the heterocysts that supports a high, persistent endogenous rate of C2H2 reduction. This material appears to be, in part, glycogen.