Nitrogen-fixing Cyanobacteria Research Articles

Two strictly polyphosphate-dependent gluco(manno)kinases from diazotrophic Cyanobacteria with potential to phosphorylate hexoses from polyphosphates.

The single-copy genes encoding putative polyphosphate-glucose phosphotransferases (PPGK, EC 2.7.1.63) from two nitrogen-fixing Cyanobacteria, Nostoc sp. PCC7120 and Nostoc punctiforme PCC73102, were cloned and functionally characterized. In contrast to their actinobacterial counterparts, the cyanobacterial PPGKs have shown the ability to phosphorylate glucose using strictly inorganic polyphosphates (polyP) as phosphoryl donors. This has proven to be an economically attractive reagent in contrast to the more costly ATP. Cyanobacterial PPGKs had a higher affinity for medium-long-sized polyP (greater than ten phosphoryl residues). Thus, longer polyP resulted in higher catalytic efficiency. Also in contrast to most their homologs in Actinobacteria, both cyanobacterial PPGKs exhibited a modest but significant polyP-mannokinase activity as well. Specific activities were in the range of 180-230 and 2-3 μmol min(-1) mg(-1) with glucose and mannose as substrates, respectively. No polyP-fructokinase activity was detected. Cyanobacterial PPGKs required a divalent metal cofactor and exhibited alkaline pH optima (approx. 9.0) and a remarkable thermostability (optimum temperature, 45 °C). The preference for Mg(2+) was noted with an affinity constant of 1.3 mM. Both recombinant PPGKs are homodimers with a subunit molecular mass of ca. 27 kDa. Based on database searches and experimental data from Southern blots and activity assays, closely related PPGK homologs appear to be widespread among unicellular and filamentous mostly nitrogen-fixing Cyanobacteria. Overall, these findings indicate that polyP may be metabolized in these photosynthetic prokaryotes to yield glucose (or mannose) 6-phosphate. They also provide evidence for a novel group-specific subfamily of strictly polyP-dependent gluco(manno)kinases with ancestral features and high biotechnological potential, capable of efficiently using polyP as an alternative and cheap source of energy-rich phosphate instead of costly ATP. Finally, these results could shed new light on the evolutionary origin of sugar kinases.

Insights into the relationship between the haem-binding pocket and the redox potential of c6 cytochromes: four atomic resolution structures of c6 and c6-like proteins from Synechococcus sp. PCC 7002.

The structure of cytochrome c6C from the mesophilic cyanobacterium Synechococcus sp. PCC 7002 has been determined at 1.03 Å resolution. This is the first structural report on the recently discovered cyanobacterial cytochrome c6-like proteins found in marine and nitrogen-fixing cyanobacteria. Despite high similarity in the overall three-dimensional fold between cytochromes c6 and c6C, the latter shows saliently different electrostatic properties in terms of surface charge distribution and dipole moments. Its midpoint redox potential is less than half of the value for typical c6 cytochromes and results mainly from the substitution of one residue in the haem pocket. Here, high-resolution crystal structures of mutants of both cytochromes c6 and c6C are presented, and the impact of the mutation of specific residues in the haem-binding pocket on the redox potential is discussed. These findings contribute to the elucidation of the structure-function relationship of c6-like cytochromes.

Response of nitrogen-fixing water fern Azolla biofertilization to rice crop.

The water fern Azolla harbors nitrogen-fixing cyanobacterium Anabaena azollae as symbiont in its dorsal leaves and is known as potent N2 fixer. Present investigation was carried out to study the influence of fresh Azolla when used as basal incorporation in soil and as dual cropped with rice variety Mahsoori separately and together with and without chemical nitrogen fertilizer in pots kept under net house conditions. Results showed that use of Azolla as basal or dual or basal plus dual influenced the rice crop positively where use of fern as basal plus dual was superior and served the nitrogen requirement of rice. There was marked increase in plant height, number of effective tillers, dry mass and nitrogen content of rice plants with the use of Azolla and N-fertilizers alone and other combinations. The use of Azolla also increased organic matter and potassium contents of the soil.

Nitrogen fixed by cyanobacteria is utilized by deposit-feeders.

Benthic communities below the photic zone depend for food on allochthonous organic matter derived from seasonal phytoplankton blooms. In the Baltic Sea, the spring diatom bloom is considered the most important input of organic matter, whereas the contribution of the summer bloom dominated by diazotrophic cyanobacteria is less understood. The possible increase in cyanobacteria blooms as a consequence of eutrophication and climate change calls for evaluation of cyanobacteria effects on benthic community functioning and productivity. Here, we examine utilization of cyanobacterial nitrogen by deposit-feeding benthic macrofauna following a cyanobacteria bloom at three stations during two consecutive years and link these changes to isotopic niche and variations in body condition (assayed as C:N ratio) of the animals. Since nitrogen-fixing cyanobacteria have δ15N close to -2‰, we expected the δ15N in the deposit-feeders to decrease after the bloom if their assimilation of cyanobacteria-derived nitrogen was substantial. We also expected the settled cyanobacteria with their associated microheterotrophic community and relatively high nitrogen content to increase the isotopic niche area, trophic diversity and dietary divergence between individuals (estimated as the nearest neighbour distance) in the benthic fauna after the bloom. The three surface-feeding species (Monoporeia affinis, Macoma balthica and Marenzelleria arctia) showed significantly lower δ15N values after the bloom, while the sub-surface feeder Pontoporeia femorata did not. The effect of the bloom on isotopic niche varied greatly between stations; populations which increased niche area after the bloom had better body condition than populations with reduced niche, regardless of species. Thus, cyanobacterial nitrogen is efficiently integrated into the benthic food webs in the Baltic, with likely consequences for their functioning, secondary production, transfer efficiency, trophic interactions, and intra- and interspecific competition.

Sucrose synthesis in the nitrogen-fixing Cyanobacterium Anabaena sp. strain PCC 7120 is controlled by the two-component response regulator OrrA.

The filamentous, nitrogen-fixing cyanobacterium Anabaena sp. strain PCC 7120 accumulates sucrose as a compatible solute against salt stress. Sucrose-phosphate synthase activity, which is responsible for the sucrose synthesis, is increased by salt stress, but the mechanism underlying the regulation of sucrose synthesis remains unknown. In the present study, a response regulator, OrrA, was shown to control sucrose synthesis. Expression of spsA, which encodes a sucrose-phosphate synthase, and susA and susB, which encode sucrose synthases, was induced by salt stress. In the orrA disruptant, salt induction of these genes was completely abolished. The cellular sucrose level of the orrA disruptant was reduced to 40% of that in the wild type under salt stress conditions. Moreover, overexpression of orrA resulted in enhanced expression of spsA, susA, and susB, followed by accumulation of sucrose, without the addition of NaCl. We also found that SigB2, a group 2 sigma factor of RNA polymerase, regulated the early response to salt stress under the control of OrrA. It is concluded that OrrA controls sucrose synthesis in collaboration with SigB2.

Effects of light and temperature on the photoautotrophic growth and photoinhibition of nitrogen-fixing cyanobacterium Cyanothece sp. ATCC 51142

The unicellular, nitrogen-fixing cyanobacterium Cyanothece sp. ATCC 51142 is a promising strain with a remarkable capability of producing large quantities of hydrogen, an energy carrier long being promoted as an ideal fuel. Under extreme environmental conditions, significant reduction of cellular photosynthetic capability is commonly observed in algae and cyanobacteria. Even less severe conditions can induce photo-inhibitive growth dynamics, which in turn result in a marked decrease in biomass and gas productivities. In this study a detailed analysis of the effect of two extrinsic parameters, namely light intensity and temperature, on the photoautotrophic growth of Cyanothece was performed in order to reveal critical conditions that would lead to undesired photoinhibition. A high degree of coherence between cyanobacterial growth and nutrient uptake kinetics was observed, as well as a strong dependence on the change of the two parameters. Nitrogen depletion was confirmed as a trigger, which transforms an exponential into a stationary growth phase. A non-linear relationship between the maximum specific growth rate and the irradiance up to 320μEm−2s−1 was identified and found to be dominated by light saturation rather than photoinhibition. The relationship between the specific growth rate and the temperature was found to be linear until a remarkable drop in the final biomass productivity and cyanobacterial photosynthetic capability was observed at 40°C. The cause of this is a high temperature-induced photoinhibition effect.

Evaluation of Antibacterial and Antioxidant Efficacy of the Fern Azolla caroliniana Symbiotic with the Cyanobacterium Anabaena azollae

Antibacterial efficacy of the water-fern, Azolla caroliniana Willd., symbiotic with the nitrogen fixing cyanobacterium, Anabaena azollae against seven multidrug resistant pathogenic bacteria were evaluated. Antioxidant and organoleptic properties of the fern were also assessed. Most Gram-negative and Gram-positive bacteria used in this study were highly resistant to 17 antibiotics of aminoglycoside, beta-lactam, cephalosporin and fluoroquinolone groups, as recorded from Kirby-Bauer method. The methanolic extract of A. caroliniana generated 20 mm as the maximum size of zone of inhibition against Staphylococcus aureus, as recorded from the agar-well diffusion method. The minimum inhibitory concentration value, 1.89 mg/ml was due to the methanolic extract against S. aureus and Proteus mirabilis, while the value, 4.27 mg/ml was against Enterococcus sp., Enterobacter aerogenes, Escherichia coli and Pseudomonas aeruginosa. The minimum bactericidal concentration value, 4.27 mg/ml was of the methanolic extract against S. aureus and P. mirabilis, while the value, 21.67 mg/ml was recorded against Citrobacter sp., E. aerogenes, E. coli and P. aeruginosa. Fifty percent inhibition concentration values of free radical scavenging activities of the fern with free radical generating chemicals, 1,1-diphenyl-2-picryl hydrazyl, as well as, 2, 2-azino-bis (ethylbenzthiazoline-6-sulfonic acid), hydrogen peroxide and nitric oxide of the methanolic extract were 8, 10, 10, and 78.8 µg/ml, respectively. In four solvent-extracts with methanol, ethanol, chloroform and n-hexane, several common phytochemicals and phenols were present, while alkaloids and saponins were absent. Processed Azolla was tasteless but not unpalatable. Azolla caroliniana could be used as an antibacterial as well as an antioxidant agent in complementary and alternate medicine.

The Hypothetical Protein ‘All4779’, and Not the Annotated ‘Alr0088’ and ‘Alr7579’ Proteins, Is the Major Typical Single-Stranded DNA Binding Protein of the Cyanobacterium, Anabaena sp. PCC7120

Single-stranded DNA binding (SSB) proteins are essential for all DNA-dependent cellular processes. Typical SSB proteins have an N-terminal Oligonucleotide-Binding (OB) fold, a Proline/Glycine rich region, followed by a C-terminal acidic tail. In the genome of the heterocystous nitrogen-fixing cyanobacterium, Anabaena sp. strain PCC7120, alr0088 and alr7579 are annotated as coding for SSB, but are truncated and have only the OB-fold. In silico analysis of whole genome of Anabaena sp. strain PCC7120 revealed the presence of another ORF ‘all4779’, annotated as a hypothetical protein, but having an N-terminal OB-fold, a P/G-rich region and a C-terminal acidic tail. Biochemical characterisation of all three purified recombinant proteins revealed that they exist either as monomer or dimer and bind ssDNA, but differently. The All4779 bound ssDNA in two binding modes i.e. (All4779)35 and (All4779)66 depending on salt concentration and with a binding affinity similar to that of Escherichia coli SSB. On the other hand, Alr0088 bound in a single binding mode of 50-mer and Alr7579 only to large stretches of ssDNA, suggesting that All4779, in all likelihood, is the major typical bacterial SSB in Anabaena. Overexpression of All4779 in Anabaena sp. strain PCC7120 led to enhancement of tolerance to DNA-damaging stresses, such as γ-rays, UV-irradiation, desiccation and mitomycinC exposure. The tolerance appears to be a consequence of reduced DNA damage or efficient DNA repair due to increased availability of All4779. The ORF all4779 is proposed to be re-annotated as Anabaena ssb gene.

Effects of Seasonal Changes on Pigment Composition ofAzolla filiculoidesLam.

Abstract Azolla is a floating fern, which contains the nitrogen-fixing cyanobacterium Anabaena azollae in the dorsal lobe cavity of the leaves. This study investigated the effects of seasonal changes on the pigment composition of Azolla filiculoides in a small pond located in Istanbul, Turkey. Sampling was conducted in March, April, May, October, November, December 2007. The average total chlorophyll and carotenoid content of the leaves was almost a half fold lower in March-April-May than in October-November-December. For late winter–spring, while the average chlorophyll a/b ratio of the leaves was 2.6, for autumn - early winter it was 5.5. In March, leaf dimensions reduced, whereas the anthocyanin accumulation in the leaves increased. In May, together with the increasing temperature values, while the leaf dimensions and biomass increased, the leaves returned to green. From October to November, both leaf dimensions and amount of anthocyanin in the leaves reduced. Results showed that the probable growth se...

Attachment of phycobilisomes in an antenna–photosystem I supercomplex of cyanobacteria

Oxygenic photosynthesis is driven by photosystems I and II (PSI and PSII, respectively). Both have specific antenna complexes and the phycobilisome (PBS) is the major antenna protein complex in cyanobacteria, typically consisting of a core from which several rod-like subcomplexes protrude. PBS preferentially transfers light energy to PSII, whereas a PSI-specific antenna has not been identified. The cyanobacterium Anabaena sp. PCC 7120 has rod-core linker genes (cpcG1-cpcG2-cpcG3-cpcG4). Their products, except CpcG3, have been detected in the conventional PBS. Here we report the isolation of a supercomplex that comprises a PSI tetramer and a second, unique type of a PBS, specific to PSI. This rod-shaped PBS includes phycocyanin (PC) and CpcG3 (hereafter renamed "CpcL"), but no allophycocyanin or CpcGs. Fluorescence excitation showed efficient energy transfer from PBS to PSI. The supercomplex was analyzed by electron microscopy and single-particle averaging. In the supercomplex, one to three rod-shaped CpcL-PBSs associate to a tetrameric PSI complex. They are mostly composed of two hexameric PC units and bind at the periphery of PSI, at the interfaces of two monomers. Structural modeling indicates, based on 2D projection maps, how the PsaI, PsaL, and PsaM subunits link PSI monomers into dimers and into a rhombically shaped tetramer or "pseudotetramer." The 3D model further shows where PBSs associate with the large subunits PsaA and PsaB of PSI. It is proposed that the alternative form of CpcL-PBS is functional in harvesting energy in a wide number of cyanobacteria, partially to facilitate the involvement of PSI in nitrogen fixation.

A high constitutive catalase activity confers resistance to methyl viologen-promoted oxidative stress in a mutant of the cyanobacterium Nostoc punctiforme ATCC 29133

A spontaneous methyl viologen (MV)-resistant mutant of the nitrogen-fixing cyanobacterium Nostoc punctiforme ATCC 29133 was isolated and the major enzymatic antioxidants involved in combating MV-induced oxidative stress were evaluated. The mutant displayed a high constitutive catalase activity as a consequence of which, the intracellular level of reactive oxygen species in the mutant was lower than the wild type (N. punctiforme) in the presence of MV. The superoxide dismutase (SOD) activity that consisted of a SodA (manganese-SOD) and a SodB (iron-SOD) was not suppressed in the mutant following MV treatment. The mutant was, however, characterised by a lower peroxidase activity compared with its wild type, and its improved tolerance to externally added H₂O₂ could only be attributed to enhanced catalase activity. Furthermore, MV-induced toxic effects on the wild type such as (1) loss of photosynthetic performance assessed as maximal quantum yield of photosystem II, (2) nitrogenase inactivation, and (3) filament fragmentation and cell lysis were not observed in the mutant. These findings highlight the importance of catalase in preventing MV-promoted oxidative damage and cell death in the cyanobacterium N. punctiforme. Such oxidative stress resistant mutants of cyanobacteria are likely to be a better source of biofertilisers, as they can grow and fix nitrogen in an unhindered manner in agricultural fields that are often contaminated with the herbicide MV, also commonly known as paraquat.

Content of Phycobilin Pigments in Two Strains of Cyanobacteria of the Atacama Desert, Chile

The aim of this study was to assess qualitative and quantitatively phycobilin pigments of two nitrogen-fixing Cyanobacteria with the purpose to optimize in the future, the culture conditions to high scale.

Phytoplankton composition changes during water level fluctuations in a high-altitude, tropical reservoir

To understand the responses of phytoplankton to water level fluctuations (WLFs) in tropical conditions at high altitude, limnological monitoring was performed monthly in Valle de Bravo (VB) reservoir, a waterbody affected by relatively strong WLFs due to water scarcity in central Mexico during 2008–2009. The thermal behaviour of the reservoir was warm, monomictic (Nov–Feb), and water levels fell to 12 m below capacity. Observed hypolimnetic temperature increases in VB are considered indirect evidence of boundary mixing events, previously attributed to internal waves driven by strong diurnal winds along a 7 km fetch. Unusually low biomasses were observed during circulation, probably due to low light availability (Zeu/Zmix < 1), grazing, and relatively low temperatures (<20 °C). In contrast, enhanced phytoplankton biomass was observed throughout stratification, which could be explained by nutrient inputs to the epilimnion through boundary mixing, as previously proposed for VB, and by higher (>20 °C) temperatures. Dominant species were Cyclotella ocellata, Fragilaria crotonensis, Woronichinia naegeliana, and Microcystis wesenbergii. Nitrogen-fixing cyanobacteria (Nostocales), a recurrent problem in this reservoir, were markedly less abundant than during high-water periods (2002–2005), probably due to dissolved inorganic nitrogen (DIN) inputs from the hypolimnion. A profuse formation of heterocysts was observed at the same time that DIN and the DIN to soluble reactive phosphorus ratio (DIN:SRP) were low (<2 μmol L−1 and ~1 respectively), suggesting heterocyst development could be a complementary proxy to assess DIN limitation. Intense water level decreases in this reservoir resulted in increasing biomasses of planktonic diatoms and a significant decrease in noxious algae such as Nostocales, thus ameliorating the water quality of the reservoir.

Effect of salinity on the growth rate and nutrient stoichiometry of two Baltic Sea filamentous cyanobacterial species

Summer blooms of nitrogen-fixing filamentous cyanobacteria are recurrent phenomena in the Baltic Sea. Salinity, varying from 0 to10 PSU in the surface layer of the Baltic Sea, is among the major factors affecting the basin-scale distribution of various bloom-forming cyanobacterial species. The effects of salinity on the growth rate and cellular carbon, nitrogen, and phosphorus ratios of two major cyanobacterial species that form dense blooms in the Baltic, Aphanizomenon sp. (strain KAC 15) and Nodularia spumigena (strain HEM), were studied. Cells were grown under N2-fixing conditions in a salinity gradient from 0 to 10 PSU. The growth rates of the species showed contrasting responses to salinity. For Aphanizomenon sp. the maximum growth rates (0.28-0.31 d -1 ) were observed at salinities of 0-2 PSU, while for N. spumigena the maximum growth rate occurred at 8-10 PSU (0.14-0.16 d -1 ). The latter species did not tolerate low salinities (< 2 PSU). The observed differences in salinity tolerances constrain the distribution patterns of these two species during cyanobacterial blooms, Aphanizomenon sp. being more abundant in the coastal and less saline areas. The variations in growth rates were largely reflected in cellular N : P and C : P ratios, which varied two-fold, and in C : Chla ratios with 5-fold variability. Cellular C : N ratios were rather constant at all salinities and close to the Redfield ratio for Aphanizomenon sp. (on average 5.9 g g -1 ) and above the Redfield ratio for N. spumigena (on average 8.0 g g -1 ). The relatively higher N : P and lower C : N ratio showed a higher need of N for Aphanizomenon sp. than for N. spumigena. This is partly explained by the higher abundance of N-rich phycobilin pigments in Aphanizomenon sp. as indicated by fluorescence measurements. The observed differences in pigmentation indicate species-specific strategies in light harvesting.

Discovery of an Endosymbiotic Nitrogen-Fixing Cyanobacterium UCYN-A in Braarudosphaera bigelowii (Prymnesiophyceae)

Braarudosphaera bigelowii (Prymnesiophyceae) is a coastal coccolithophore with a long fossil record, extending back to the late Cretaceous (ca. 100 Ma). A recent study revealed close phylogenetic relationships between B. bigelowii, Chrysochromulina parkeae (Prymnesiophyceae), and a prymnesiophyte that forms a symbiotic association with the nitrogen-fixing cyanobacterium UCYN-A. In order to further examine these relationships, we conducted transmission electron microscopic and molecular phylogenetic studies of B. bigelowii. TEM studies showed that, in addition to organelles, such as the nucleus, chloroplasts and mitochondria, B. bigelowii contains one or two spheroid bodies with internal lamellae. In the 18S rDNA tree of the Prymnesiophyceae, C. parkeae fell within the B. bigelowii clade, and was close to B. bigelowii Genotype III (99.89% similarity). Plastid 16S rDNA sequences obtained from B. bigelowii were close to the unidentified sequences from the oligotrophic SE Pacific Ocean (e.g. HM133411) (99.86% similarity). Bacterial16S rDNA sequences obtained from B. bigelowii were identical to the UCYN-A sequence AY621693 from Arabian Sea, and fell in the UCYN-A clade. From these results, we suggest that; 1) C. parkeae is the alternate life cycle stage of B. bigelowii sensu stricto or that of a sibling species of B. bigelowii, and 2) the spheroid body of B. bigelowii originated from endosymbiosis of the nitrogen-fixing cyanobacterium UCYN-A.

Cylindrospermopsis raciborskii dominates under very low and high nitrogen-to-phosphorus ratios

In freshwater ecosystems, a variety of factors mediate phytoplankton community structure, including herbivore community structure, light availability, temperature, mixing, and absolute and relative nutrient concentrations (total nitrogen (TN), total phosphorus (TP)). Ecological stoichiometry examines how the nutrient content of organisms and their environment may mediate population-, community-, and ecosystem-level processes. The manipulation of N:P ratios is a widely regarded tool for managing phytoplankton species composition given that nitrogen-fixing cyanobacteria should dominate algal communities under relatively low N:P (<64:1, by atoms) given their ability to convert dissolved dinitrogen gas into organic nitrogen. However, due to the physiological expense of nitrogen fixation, diazotrophs should be outcompeted by non-nitrogen fixing phytoplankton under higher N:P when other environmental factors are similar. We tested this hypothesis in a field experiment using 2500-L limnocorrals installed in a eutrophic lake (ambient N:P ∼40:1 (by atoms); TN ∼1360 μgL−1; TP ∼75 μgL−1). At the start of the experiment, we randomly assigned limnocorrals among the ambient (40:1) and low (7:1) or high (122:1) N:P treatments (n = 4 replicates/treatment), which were established by adding P or N at the start of the experiment, respectively. The phytoplankton community in the enclosures at the start of the experiment was diverse (i.e., 18 phytoplankton genera) and dominated by chlorophytes (including Coelastrum and Scenedesmus (30% and 13% of total biomass, respectively)) and cyanobacteria (including Anabaena and Cylindrospermopsis (23% and 17% of total biomass, respectively)). In contrast to predictions based on ecological stoichiometry, the phytoplankton community in all N:P treatments increased in abundance and was almost entirely composed of the nitrogen-fixing cyanobacterium, Cylindrospermopsis raciborskii, by the conclusion of the study. Moreover, concentrations of the cyanobacterial neurotoxin, saxitoxin, were enhanced under the two highest N:P conditions. The ability of C. raciborskii to dominate phytoplankton communities under such extreme N:P shows that short-term management of nutrient stoichiometry through fertilization is not likely to be effective for controlling blooms of this noxious cyanobacterium and may help to explain the rapid expansion of this invasive species to temperate latitudes.

Cellular interactions: lessons from the nitrogen-fixing cyanobacteria.

Marine nitrogen-fixing cyanobacteria play a central role in the open-ocean microbial community by providing fixed nitrogen (N) to the ocean from atmospheric dinitrogen (N2 ) gas. Once thought to be dominated by one genus of cyanobacteria, Trichodesmium, it is now clear that marine N2 -fixing cyanobacteria in the open ocean are more diverse, include several previously unknown symbionts, and are geographically more widespread than expected. The next challenge is to understand the ecological implications of this genetic and phenotypic diversity for global oceanic N cycling. One intriguing aspect of the cyanobacterial N2 fixers ecology is the range of cellular interactions they engage in, either with cells of their own species or with photosynthetic protists. From organelle-like integration with the host cell to a free-living existence, N2 -fixing cyanobacteria represent the range of types of interactions that occur among microbes in the open ocean. Here, we review what is known about the cellular interactions carried out by marine N2 -fixing cyanobacteria and where future work can help. Discoveries related to the functional roles of these specialized cells in food webs and the microbial community will improve how we interpret their distribution and abundance patterns and contributions to global N and carbon (C) cycles.

High radiation and desiccation tolerance of nitrogen-fixing cultures of the cyanobacterium Anabaena sp. strain PCC 7120 emanates from genome/proteome repair capabilities.

The filamentous nitrogen-fixing cyanobacterium, Anabaena sp. strain PCC 7120 was found to tolerate very high doses of 60Co-gamma radiation or prolonged desiccation. Post-stress, cells remained intact and revived all the vital functions. A remarkable capacity to repair highly disintegrated genome and recycle the damaged proteome appeared to underlie such high radioresistance and desiccation tolerance. The close similarity observed between the cellular response to irradiation or desiccation stress lends strong support to the notion that tolerance to these stresses may involve similar mechanisms.

Carbon dioxide regulation of autotrophy and diazotrophy in the nitrogen-fixing cyanobacterium Nostoc muscorum

To understand how carbon and nitrogen metabolism are regulated in diazotrophically and non-diazotrophically grown cultures of the cyanobacterium Nostoc muscorum, we investigated the role of bicarbonate (HCO3−) in regulating diazotrophy and autotrophy. Results showed that HCO3− concentration up to 12molm−3 enhanced growth, specific growth rate, photosynthetic pigments, photosynthetic O2 evolution and nitrogenase activity under diazotrophic growth conditions. The co-existence of different nitrogen sources in the growth medium further accelerate the examined parameters in the order of NO3−<NO2−<NH4+<proline. Further, we examined the Ca++-dependent ATPase activity and cytochrome c oxidase activity in the presence of graded concentration of HCO3− under diazotrophically grown and non-diazotrophically grown cultures. The activity of these enzymes was higher in the cells grown under elevated HCO3− concentration. The Ca++-dependent ATPase activity was higher than that of cytochrome c oxidase, when NAPH was supplied as the electron donor. This finding suggested that photosynthetic generation of ATP utilized NADPH as an electron donor and cytochrome c oxidase activity is independent of NADPH.

Model based optimization of high cell density cultivation of nitrogen-fixing cyanobacteria

In the present study, fed-batch cultivation of Cyanothece sp. ATCC 51142, a known hydrogen producer, was optimized for maximizing biomass production. Decline in growth of this organism in dense cultures was attributed to increased substrate consumption for maintenance and respiration, and photolimitation due to self shading. A model incorporating these aspects was developed, and by using control vector parameterization (CVP), substrate feeding recipe was optimized to achieve 12-fold higher biomass concentration. The optimization results were verified experimentally on shake flask and bioreactor. The latter resulted in greater exponential growth rate possibly by overcoming photolimitation by simulating flashing light effect. Such a strategy can be readily applied for mixotrophic cultivation of cyanobacterial cultures in the first stage followed by photoautotrophic growth at the production stage.