Regulation of Expression of Nitrate and Dinitrogen Assimilation by Anabaena Species

Dissolved nitrogen uptake versus nitrogen fixation: Mode of nitrogen acquisition affects stable isotope signatures of a diazotrophic cyanobacterium and its grazer

Field studies suggest that changes in the stable isotope ratios of phytoplankton communities can be used to track changes in the utilization of different nitrogen sources, i.e., to detect shifts from dissolved inorganic nitrogen (DIN) uptake to atmospheric nitrogen (N2) fixation by diazotrophic cyanobacteria as an indication of nitrogen limitation. We explored changes in the stable isotope signature of the diazotrophic cyanobacterium Trichormus variabilis in response to increasing nitrate (NO3-) concentrations (0 to 170 mg L-1) under controlled laboratory conditions. In addition, we explored the influence of nitrogen utilization at the primary producer level on trophic fractionation by studying potential changes in isotope ratios in the freshwater model Daphnia magna feeding on the differently grown cyanobacteria. We show that δ 15N values of the cyanobacterium increase asymptotically with DIN availability, from -0.7 ‰ in the absence of DIN (suggesting N2 fixation) to 2.9 ‰ at the highest DIN concentration (exclusive DIN uptake). In contrast, δ 13C values of the cyanobacterium did not show a clear relationship with DIN availability. The stable isotope ratios of the consumer reflected those of the differently grown cyanobacteria but also revealed significant trophic fractionation in response to nitrogen utilization at the primary producer level. Nitrogen isotope turnover rates of Daphnia were highest in the absence of DIN as a consequence of N2 fixation and resulting depletion in 15N at the primary producer level. Our results highlight the potential of stable isotopes to assess nitrogen limitation and to explore diazotrophy in aquatic food webs.

Growth and photosynthetic performance of Nostoc linckia (formerly N. calcicola) cells grown in BG11 and BG110 media

The biotechnological potential of Nostoc linckia as a biofertilizer and source of bioactive compounds makes it important to study its growth physiology and productivity. Since nitrogen is a fundamental component of N. linckia biomass, we compared the growth and biochemical composition of cultures grown in BG11 (i.e., in the presence of nitrate) and BG11₀ (in the absence of nitrate). Cultures grown in BG11 accumulated more cell biomass reaching a dry weight of 1.65 ± 0.06 g L^-1, compared to 0.92 ± 0.01 g L^-1 in BG11₀ after 240 h of culture. Biomass productivity was higher in culture grown in BG11 medium (average 317 ± 38 mg L^-1 day^-1) compared to that attained in BG11₀ (average 262 ± 37 mg L^-1 day^-1). The chlorophyll content of cells grown in BG11 increased continuously up to (39.0 ± 1.3 mg L^-1), while in BG11₀ it increased much more slowly (13.6 ± 0.8 mg L^-1). Biomass grown in BG11 had higher protein and phycobilin contents. However, despite the differences in biochemical composition and pigment concentration, between BG11 and BG11₀ cultures, both their net photosynthetic rates and maximum quantum yields of the photosystem II resulted in similar.

Elevated CO2 significantly increases N2 fixation, growth rates, and alters microcystin, anatoxin, and saxitoxin cell quotas in strains of the bloom-forming cyanobacteria, Dolichospermum

The effect of rising CO₂ levels on cyanobacterial harmful algal blooms (CHABs) is an emerging concern, particularly within eutrophic ecosystems. While elevated pCO₂ has been associated with enhanced growth rates of some cyanobacteria, few studies have explored the effect of CO₂ and nitrogen availability on diazotrophic (N₂-fixing) cyanobacteria that produce cyanotoxins. Here, the effects of elevated CO₂ and fixed nitrogen (NO₃^-) availability on the growth rates, toxin production, and N₂ fixation of microcystin, saxitoxin, and anatoxin-a - producing strains of the genus Dolichospermum were quantified. Growth rates of all Dolichospermum spp. were significantly increased by CO₂ or both CO₂ and NO₃^- with rates being highest in treatments with the highest levels of CO₂ and NO₃^-for all strains. While NO₃^- suppressed N₂ fixation, diazotrophy significantly increased when NO₃^--enriched Dolichospermum spp. were supplied with higher CO₂ compared to cultures grown under lower CO₂ levels. This suggests that diazotrophy will play an increasingly important role in N cycling in CO₂-enriched, eutrophic lentic systems. NO₃^- significantly increased quotas of the N-rich cyanotoxins, microcystin and saxitoxin, at ambient and enriched CO₂ levels, respectively. In contrast, elevated CO₂ significantly decreased cell quotas of microcystin and saxitoxin, but significantly increased cell quotas of the N-poor cyanotoxin, anatoxin. N₂ fixation was significantly negatively and positively correlated with quotas of N-rich and N-poor cyanotoxins, respectively. Findings suggest cellular quotas of N-rich toxins (microcystin and saxitoxin) may be significantly reduced, or cellular quotas of N-poor toxins (anatoxin) may be significantly enhanced, under elevated CO₂ conditions during diazotrophic cyanobacterial blooms. Finally, in the future, ecosystems that experience combinations of excessive N loading and CO₂ enrichment may become more prone to toxic blooms of Dolichospermum.

Nitrogen and phosphorus significantly alter growth, nitrogen fixation, anatoxin-a content, and the transcriptome of the bloom-forming cyanobacterium, Dolichospermum

While freshwater cyanobacteria are traditionally thought to be limited by the availability of phosphorus (P), fixed nitrogen (N) supply can promote the growth and/or toxin production of some genera. This study characterizes how growth on N2 (control), nitrate (NO3 -), ammonium (NH4 +), and urea as well as P limitation altered the growth, toxin production, N2 fixation, and gene expression of an anatoxin-a (ATX-A) - producing strain of Dolichospermum sp. 54. The transcriptomes of fixed N and P-limited cultures differed significantly from those of fixed N-deplete, P-replete (control) cultures, while the transcriptomes of P-replete cultures amended with either NH4 + or NO3 - were not significantly different relative to those of the control. Growth rates of Dolichospermum (sp. 54) were significantly higher when grown on fixed N relative to without fixed N; growth on NH4 + was also significantly greater than growth on NO3 -. NH4 + and urea significantly lowered N2 fixation and nifD gene transcript abundance relative to the control while cultures amended with NO3 - exhibited N2 fixation and nifD gene transcript abundance that was not different from the control. Cultures grown on NH4 + exhibited the lowest ATX-A content per cell and lower transcript abundance of genes associated ATX-A synthesis (ana), while the abundance of transcripts of several ana genes were highest under fixed N and P - limited conditions. The significant negative correlation between growth rate and cellular anatoxin quota as well as the significantly higher number of transcripts of ana genes in cultures deprived of fixed N and P relative to P-replete cultures amended with NH4 + suggests ATX-A was being actively synthesized under P limitation. Collectively, these findings indicate that management strategies that do not regulate fixed N loading will leave eutrophic water bodies vulnerable to more intense and toxic (due to increased biomass) blooms of Dolichospermum.

Heterocyte production, gene expression and phylogeography in Raphidiopsis ( = Cylindrospermopsis) Raciborskii

Raphidiopsis ( = Cylindrospermopsis) raciborskii was described as a subtropical-tropical cyanobacterium, later reported expanding into temperate regions. Heterocyte presence used to distinguish Cylindrospermopsis from the very similar Raphidiopsis, but recently the two genera were recognized as one and unified. This study aimed to investigate how heterocyte production is related to nitrogen (N) limitation in heterocytous and non-heterocytous strains of R.raciborskii. High N-concentrations did not inhibit heterocyte development in some strains, while prolonged N-starvation periods never stimulated production in others. RT-qPCR was used to examine the genetic background, through the expression patterns of nifH, ntcA and hetR. While gene expression increased under N-restriction, N-sufficiency did not suppress nifH transcripts as previously observed in other diazotrophyc cyanobacteria, suggesting that heterocyte production in R. raciborskii is not regulated by N-availability. Heterocytous and non-heterocytous strains were genotypically characterized to assess their phylogenetic relationships,. In the phylogenetic tree, clusters were intermixed and confirmed Raphidiopsis and Cylindrospermopsis as the same genus. The tree supported previous findings of earlier splitting of American strains, while contesting the African origin hypothesis. The existence of two lines of Chinese strains, with distinct evolutionary patterns, is a significant addition that could lead to new hypotheses of the species biogeography.

Nitrogen Sources and Iron Availability Affect Pigment Biosynthesis and Nutrient Consumption in Anabaena sp. UTEX 2576

Anabaena sp. UTEX 2576 metabolizes multiple nitrogen (N) sources and is deemed a biotechnological platform for chemical production. Cyanobacteria have been identified as prolific producers of biofertilizers, biopolymers, biofuels, and other bioactive compounds. Here, we analyze the effect of different N-sources and Fe availability on the bioproduction of phycobiliproteins and β-carotene. We characterize nutrient demand in modified BG11 media, including data on CO₂ fixation rates, N-source consumption, and mineral utilization (e.g., phosphorus (P), and 11 metallic elements). Results suggest that non-diazotrophic cultures grow up to 60% faster than diazotrophic cells, resulting in 20% higher CO₂-fixation rates. While the production of β-carotene was maximum in medium with NaNO₃, Fe starvation increased the cellular abundance of C-phycocyanin and allophycocyanin by at least 22%. Compared to cells metabolizing NaNO₃ and N₂, cultures adapted to urea media increased their P, calcium and manganese demands by at least 72%, 97% and 76%, respectively. Variations on pigmentation and nutrient uptake were attributed to changes in phycocyanobilin biosynthesis, light-induced oxidation of carotenoids, and urea-promoted peroxidation. This work presents insights into developing optimal Anabaena culture for efficient operations of bioproduction and wastewater bioremediation with cyanobacteria.

Taxonomic structure and potential nitrogen metabolism of microbial assemblage in a large hypereutrophic steppe lake

Recent studies have expanded the interests about microbial community and function following the rapid development of high-throughput sequencing techniques in the freshwater ecosystem. In this study, we aimed to attain a deep understanding of microbial community structure and potential nitrogen metabolism in Hulun Lake, a shallow hypereutrophic steppe lake in the Mongolian Plateau in China. The result demonstrated that cyanobacteria were the most dominant phylum. Network analysis showed both intra- and inter-phylum co-occurrence were pervasive, and there were modular structures in the microbial assemblages. The cluster dominated by proteobacteria was mainly negatively connected to the cluster dominated by both proteobacteria and actinobacteria. Cyanobacteria were tightly clustered together and positively connected to these two clusters. The major nitrogen metabolism pathways were glutamine synthetase-glutamate synthase and assimilatory nitrate reduction, indicating the nitrogen was mainly retained in the lake by microbial uptake. Cyanobacteria contributed 43.25% gene reads involved in the overall nitrogen metabolism but mainly contributed to assimilatory nitrate reduction and nitrogen fixation, aggravating the lake eutrophication. This study adds to our knowledge of microbial assemblages and nitrogen metabolism in the shallow hypereutrophic lake and provided an insight understanding for the purposes of lake ecosystem's protection and efficient management in the Mongolian Plateau.

Identification of surface polysaccharides in akinetes, heterocysts and vegetative cells of Anabaena cylindrica using fluorescein-labeled lectins

In response to environmental changes, Anabaena cylindrica differentiate three cell types: vegetative cells for photosynthesis, heterocysts for nitrogen fixation, and akinetes for stress survival. Cell-surface polysaccharides play important roles in cyanobacterial ecophysiology. In this study, specific cell-surface sugars were discovered in heterocysts, akinetes and vegetative cells of A. cylindrica using 20 fluorescein-labeled lectins. Both N-acetylglucosamine-binding lectins WGA and succinylated WGA bound specifically to the vegetative cells. Akinetes bound to three mannose-binding lectins (LCA, PSA, and ConA), and one of the galactose-binding lectins (GSL-I). Heterocyst also bound to ConA. However, the heterocysts in all4388 mutant of Anabaena sp. PCC 7120, in which the putative polysaccharide export protein gene all4388 was disrupted, exhibited diminished binding to ConA. Identification of distinct cell-surface sugar helped us to understand the role of polysaccharide for each cell type. Fluorescence-activated cell sorting may be applicable in isolating each cell type for comparative "omics" studies among the three cell types.

Influences of anthropogenic land use on microbial community structure and functional potentials of stream benthic biofilms

Stream ecosystems are the primary receivers of nutrient and organic carbon exported from terrestrial ecosystems and are profoundly influenced by the land use of the surrounding landscape. The aquatic impacts of anthropogenic land use are often first observed in stream benthic biofilms. We studied the benthic biofilms in streams flowing through forest (upstream) and anthropogenic land use (downstream) areas in southwestern China. The results showed that anthropogenic land use increased nutrient and organic carbon in both stream water and benthic biofilms, which are closely related to the differences in the microbial communities. The taxonomic dissimilarity of the communities was significantly correlated with the functional gene dissimilarity, and the upstream sites had more distinct functional genes. Network analysis showed that upstream sites had more highly connected microbial networks. Furthermore, downstream sites had higher relative abundances of anammox and denitrification suggesting stronger nitrogen removal than upstream sites. Increased nutrients in both the stream water and biofilms caused by anthropogenic land use had severe impacts on the nitrogen cycle in stream ecosystems. Downstream sites also had stronger carbon metabolism than upstream sites. This study provides insights into the influences of anthropogenic land use on microbial community structure and functions of stream benthic biofilms.

Microbial functional genes elucidate environmental drivers of biofilm metabolism in glacier-fed streams

Benthic biofilms in glacier-fed streams harbor diverse microorganisms driving biogeochemical cycles and, consequently, influencing ecosystem-level processes. Benthic biofilms are vulnerable to glacial retreat induced by climate change. To investigate microbial functions of benthic biofilms in glacier-fed streams, we predicted metagenomes from 16s rRNA gene sequence data using PICRUSt and identified functional genes associated with nitrogen and sulfur metabolisms based on KEGG database and explored the relationships between metabolic pathways and abiotic factors in glacier-fed streams in the Tianshan Mountains in Central Asia. Results showed that the distribution of functional genes was mainly associated with glacier area proportion, glacier source proportion, total nitrogen, dissolved organic carbon, and pH. For nitrogen metabolism, the relative abundance of functional genes associated with dissimilatory pathways was higher than those for assimilatory pathways. The relative abundance of functional genes associated with assimilatory sulfate reduction was higher than those involved with the sulfur oxidation system and dissimilatory sulfate reduction. Hydrological factors had more significant correlations with nitrogen metabolism than physicochemical factors and anammox was the most sensitive nitrogen cycling pathway responding to variation of the abiotic environment in these glacial-fed streams. In contrast, sulfur metabolism pathways were not sensitive to variations of abiotic factors in these systems.

Hopanoids play a role in stress tolerance and nutrient storage in the cyanobacterium Nostoc punctiforme

Hopanes are abundant in ancient sedimentary rocks at discrete intervals in Earth history, yet interpreting their significance in the geologic record is complicated by our incomplete knowledge of what their progenitors, hopanoids, do in modern cells. To date, few studies have addressed the breadth of diversity of physiological functions of these lipids and whether those functions are conserved across the hopanoid-producing bacterial phyla. Here, we generated mutants in the filamentous cyanobacterium, Nostoc punctiforme, that are unable to make all hopanoids (shc) or 2-methylhopanoids (hpnP). While the absence of hopanoids impedes growth of vegetative cells at high temperature, the shc mutant grows faster at low temperature. This finding is consistent with hopanoids acting as membrane rigidifiers, a function shared by other hopanoid-producing phyla. Apart from impacting fitness under temperature stress, hopanoids are dispensable for vegetative cells under other stress conditions. However, hopanoids are required for stress tolerance in akinetes, a resting survival cell type. While 2-methylated hopanoids do not appear to contribute to any stress phenotype, total hopanoids and to a lesser extent 2-methylhopanoids were found to promote the formation of cyanophycin granules in akinetes. Finally, although hopanoids support symbiotic interactions between Alphaproteobacteria and plants, they do not appear to facilitate symbiosis between N. punctiforme and the hornwort Anthoceros punctatus. Collectively, these findings support interpreting hopanes as general environmental stress biomarkers. If hopanoid-mediated enhancement of nitrogen-rich storage products turns out to be a conserved phenomenon in other organisms, a better understanding of this relationship may help us parse the enrichment of 2-methylhopanes in the rock record during episodes of disrupted nutrient cycling.

Physiological and gene expression responses to nitrogen regimes and temperatures in Mastigocladus sp. strain CHP1, a predominant thermotolerant cyanobacterium of hot springs

Cyanobacteria are widely distributed primary producers with significant implications for the global biogeochemical cycles of carbon and nitrogen. Diazotrophic cyanobacteria of subsection V (Order Stigonematales) are particularly ubiquitous in photoautotrophic microbial mats of hot springs. The Stigonematal cyanobacterium strain CHP1 isolated from the Porcelana hot spring (Chile) was one of the major contributors of the new nitrogen through nitrogen fixation. Further morphological and genetic characterization verified that the strain CHP1 belongs to Stigonematales, and it formed a separate clade together with other thermophiles of the genera Fischerella and Mastigocladus. Strain CHP1 fixed maximum N₂ in the light, independent of the temperature range. At 50°C nifH gene transcripts showed high expression during the light period, whereas the nifH gene expression at 45°C was arrhythmic. The strain displayed a high affinity for nitrate and a low tolerance for high ammonium concentrations, whereas the narB and glnA genes showed higher expression in light and at the beginning of the dark phase. It is proposed that Mastigocladus sp. strain CHP1 would represent a good model for the study of subsection V thermophilic cyanobacteria, and for understanding the adaptations of these photoautotrophic organisms inhabiting microbial mats in hot springs globally.

Variation of Spirulina maxima biomass production in different depths of urea-used culture medium

Fewer studies have assessed the outdoor cultivation of Spirulina maxima compared with S. platensis, although the protein content of S. maxima is higher than S. platensis. Spirulina growth medium requires an increased amount of NaHCO3, Na2CO3, and NaNO3, which increases the production cost. Therefore, the current study used a low-cost but high-efficiency biomass production medium (Medium M-19) after testing 33 different media. The medium depth of 25 cm (group A) was sub-divided into A1 (50% cover with a black curtain (PolyMax, 12 oz ultra-blackout), A2 (25% cover), and A3 (no cover). Similarly the medium depths of 30 and 35 cm were categorized as groups B (B1, B2, and B3) and C (C1, C2, and C3), respectively, and the effects of depth and surface light availability on growth and biomass production were assessed. The highest biomass production was 2.05 g L-1 in group A2, which was significantly higher (p < 0.05) than that in all other groups and sub-groups. Spirulina maxima died in B1 and C1 on the fifth day of culture. The biochemical composition of the biomass obtained from A2 cultures, including protein, carbohydrate, lipid, moisture, and ash, was 56.59%, 14.42%, 0.94%, 5.03%, and 23.02%, respectively. Therefore, S. maxima could be grown outdoors with the highest efficiency in urea-enriched medium at a 25-cm medium depth with 25% surface cover or uncovered.

Phylogenetic comparison among the heterocystous cyanobacteria based on a polyphasic approach

Phylogenetic comparison has been done among the selected heterocystous cyanobacteria belonging to the sections IV and V. The hierarchical cluster analysis based on antibiotics sensitivity showed a distant relationship between the members of Nostocales and Stigonematales. Thus, multiple antibiotic resistance pattern used as marker provide easy, fast, and reliable method for strain discrimination and genetic variability. However, morphological, physiological (both based on principal component analysis) and biochemical analysis grouped true branching cyanobacteria along with the members of section IV. Molecular analysis based on 16S rRNA gene sequences revealed that Hapalosiphon welwitschii and Westiellopsis sp. were grouped in cluster I whereas Scytonema bohnerii, a false branching genera showed a close proximity with Calothrix brevissima in cluster II. Cluster III of clade 2 included Nostoc calcicola and Anabaena oryzae which proved the heterogeneity at the generic level. Cluster IV the largest group of clade 2 based on 16S rRNA gene sequences includes six strains of the genera Nostoc, Anabaena, and Cylindrospermum showing ambiguous evolutionary relationship. In cluster IV, Anabaena sp. and Anabaena doliolum were phylogenetically linked by sharing 99% sequence similarity. Probably, they were of the same genetic makeup but appear differently under the diverse physiological conditions. Section IV showed polyphyletic origin whereas section V showed monophyletic origin. Results suggested that either morphological or physiological or biochemical or molecular attribute is not sufficient to provide true diversity and phylogeny of the cyanobacteria at the generic level and thus, a polyphasic approach would be more appropriate and reliable.

Characteristics of Hormogonia Formation by Symbiotic Nostoc spp. in Response to the Presence of Anthoceros punctatus or Its Extracellular Products

Nostocacean cyanobacteria typically produce gliding filaments termed hormogonia at a low frequency as part of their life cycle. We report here that all Nostoc spp. competent in establishing a symbiotic association with the hornwort Anthoceros punctatus formed hormogonial filaments at a high frequency in the presence of A. punctatus. The hormogonia-inducing activity was produced by A. punctatus under nitrogen-limited culture conditions. The hormogonia of the symbiotically competent Nostoc spp. were characterized as motile (gliding) filaments lacking heterocysts and with distinctly smaller cells than those of vegetative filaments; the small cells resulted from a continuation of cell division uncoupled from biomass increase. An essentially complete conversion of vegetative filaments to hormogonia occurred within 12 h of exposure of Nostoc sp. strain 7801 to A. punctatus growth-conditioned medium. Hormogonia formation was accompanied by loss of nitrogen fixation (acetylene reduction) and by decreases in photosynthetic CO(2) fixation and in vivo NH(4) assimilation of 30% and approximately 40%, respectively. The rates of acetylene reduction and CO(2) fixation returned to approximately the control rates within 72 to 96 h after hormogonia induction, as the cultures of Nostoc sp. strain 7801 differentiated heterocysts and reverted to the vegetative growth state. The relationship between hormogonia formation and symbiotic competence is discussed.

Characterization of nitrogen-fixing cyanobacteria in the Brazilian Amazon floodplain

The diversity of the free-living nitrogen-fixing cyanobacterial community in the floodplain sediments along the Solimões and Amazon Rivers and some of their tributaries (Japurá, Negro and Madeira) was investigated. Five cyanobacterial genera were morphologically identified, four of which (Nostoc, Calothrix, Cylindrospermum and Fischerella) have not previously been isolated from the Brazilian Amazon floodplain. Nostoc strains were the most commonly found heterocyst-forming cyanobacteria. Five strains (N. muscorum CENA18 and CENA61, N. piscinale CENA21, Cylindrospermum sp. CENA33 and Fischerella sp. CENA19) were selected for growth measurement, ability to fix N2 and phylogenetic analysis, based on their widespread distribution and morphological distinction. Molecular analyses employing 16S rRNA sequences indicated that some of the isolates may represent novel cyanobacterial species. Dinitrogen fixed by these strains was measured indirectly as acetylene reduction activity and ranged from 11.5 to 22.2 nmol C2H4 microg Chl a(-1) h(-1). These results provide evidence of widespread and importance of nitrogen-fixing cyanobacteria as a source of N inputs in the Amazonian ecosystem.

Morphological, physiochemical and molecular characterization of Anabaena strains

A set of 30 Anabaena strains, isolated from diverse geographical regions of India, were characterized using morphological and physiochemical attributes as well as molecular marker profiles. Significant differences were observed among the Anabaena strains with regard to the shape and size of trichomes and individual cells within a filament, besides qualitative and quantitative aspects of phycobiliprotein accumulation and activities of enzymes involved in nitrogen metabolism. Analyses of molecular polymorphisms in a selected set of 13 Anabaena strains, using primers based on repetitive sequences in the genome, led to unambiguous differentiation of the strains as well as understanding of their genetic relationships. Informative morphological, physio-chemical and molecular characters have been identified that could aid in differentiation and utilization of Anabaena strains as bioinoculants or as sources of pigments.

Characterization of a model system for the study of Nostoc punctiforme akinetes

Nostoc punctiforme is a filamentous cyanobacterium that is capable of dark heterotrophy and cellular differentiation into nitrogen-fixing heterocysts, motile hormogonia, or spore-like akinetes. The study of akinete differentiation at the molecular level has been limited by the asynchronous development and limited number of akinetes formed within a filament. A system in which to study the development and genetic regulation of akinetes was investigated using a zwf mutant lacking glucose-6-phosphate dehydrogenase, the initial enzyme of the oxidative pentose phosphate pathway. Upon dark incubation in the presence of fructose, the zwf(-) strain ceased growth and differentiated into akinete-like cells, whereas the wild-type strain exhibited heterotrophic growth. Dark-induced zwf akinetes exhibited periodic acid-Schiff staining characteristics identical to that observed for wild-type akinetes, and synchronous induction of akinetes occurred in treated cultures. Dark-induced zwf akinetes exhibited increased resistance to the environmental stresses of desiccation, cold, or treatment with lysozyme relative to vegetative cells of both strains. Transcription of the avaK akinete marker gene was strongly induced in developing zwf akinetes as shown by Northern blotting and green fluorescent protein transcriptional reporter fusions. ATP levels did not vary significantly between dark incubated strains, indicating that a signal other than energy level may trigger akinete formation. This phenotypic and genetic evidence showing near-synchronous induction of dark-induced zwf akinetes indicates that this system will provide a valuable tool for the molecular genetic study of akinete development in N. punctiforme.

Photosynthetic nitrate assimilation in cyanobacteria

Nitrate uptake and reduction to nitrite and ammonium are driven in cyanobacteria by photosynthetically generated assimilatory power, i.e., ATP and reduced ferredoxin. High-affinity nitrate and nitrite uptake takes place in different cyanobacteria through either an ABC-type transporter or a permease from the major facilitator superfamily (MFS). Nitrate reductase and nitrite reductase are ferredoxin-dependent metalloenzymes that carry as prosthetic groups a [4Fe-4S] center and Mo-bis-molybdopterin guanine dinucleotide (nitrate reductase) and [4Fe-4S] and siroheme centers (nitrite reductase). Nitrate assimilation genes are commonly found forming an operon with the structure: nir (nitrite reductase)-permease gene(s)-narB (nitrate reductase). When the cells perceive a high C to N ratio, this operon is transcribed from a complex promoter that includes binding sites for NtcA, a global nitrogen-control regulator that belongs to the CAP family of bacterial transcription factors, and NtcB, a pathway-specific regulator that belongs to the LysR family of bacterial transcription factors. Transcription is also affected by other factors such as CnaT, a putative glycosyl transferase, and the signal transduction protein P(II). The latter is also a key factor for regulation of the activity of the ABC-type nitrate/nitrite transporter, which is inhibited when the cells are incubated in the presence of ammonium or in the absence of CO(2). Notwithstanding significant advance in understanding the regulation of nitrate assimilation in cyanobacteria, further post-transcriptional regulatory mechanisms are likely to be discovered.

Morphological characteristics of Cylindrospermopsis raciborskii (Wołoszyńska) Seenayya et Subba Raju in laboratory cultures

The freshwater cyanoprokaryote Cylindrospermopsis raciborskii has become increasingly prevalent in tropical and temperate water bodies worldwide. The morphological characteristics of this species were investigated under different growth rates in continuous cultures (at steady state) and in batch (phosphorus starved) cultures with different mineral nitrogen forms. The species displays an enormous morphological variability under controlled condition. The occurrence of extreme long twisted filaments was found near the maximum growth rate and under high ammonium concentration. Rarely the heterocytes of Cylindrospermopsis raciborskii arise intercalarly between two neighbouring cells(i.e. intercalary heterocytes were found). The morphological features are highly effected by environmental conditions and nutrient availability. Under P-starvation extreme morphology appeared. The specifications of C. africana and C. cuspis overlap with that of C. raciborskii accordingly this is not clear characteristic feature to distinguish species. A pure culture of a pro- or eukaryote alga growing in continuous cultures is a good method for giving a suitable overview on all morphological possibilities of a tested organism.

Regulation of cellular differentiation in filamentous cyanobacteria in free-living and plant-associated symbiotic growth states

Certain filamentous nitrogen-fixing cyanobacteria generate signals that direct their own multicellular development. They also respond to signals from plants that initiate or modulate differentiation, leading to the establishment of a symbiotic association. An objective of this review is to describe the mechanisms by which free-living cyanobacteria regulate their development and then to consider how plants may exploit cyanobacterial physiology to achieve stable symbioses. Cyanobacteria that are capable of forming plant symbioses can differentiate into motile filaments called hormogonia and into specialized nitrogen-fixing cells called heterocysts. Plant signals exert both positive and negative regulatory control on hormogonium differentiation. Heterocyst differentiation is a highly regulated process, resulting in a regularly spaced pattern of heterocysts in the filament. The evidence is most consistent with the pattern arising in two stages. First, nitrogen limitation triggers a nonrandomly spaced cluster of cells (perhaps at a critical stage of their cell cycle) to initiate differentiation. Interactions between an inhibitory peptide exported by the differentiating cells and an activator protein within them causes one cell within each cluster to fully differentiate, yielding a single mature heterocyst. In symbiosis with plants, heterocyst frequencies are increased 3- to 10-fold because, we propose, either differentiation is initiated at an increased number of sites or resolution of differentiating clusters is incomplete. The physiology of symbiotically associated cyanobacteria raises the prospect that heterocyst differentiation proceeds independently of the nitrogen status of a cell and depends instead on signals produced by the plant partner.

Environmental factors influencing the nitrogen fixation activity of free-living terrestrial cyanobacteria from a high arctic area, Spitsbergen

The influence of environmental factors on the nitrogen fixation activity of free-living, terrestrial cyanobacteria from a high arctic area were investigated using experimental manipulations with two different types of field samples, including macroscopic sheets of Nostoc commune and soil samples with a cyanobacterial crust from a Puccinellia salt marsh. In addition, a cultured Anabaena sp. previously isolated from the salt marsh was examined. Nitrogen fixation activity was measured using the acetylene reduction method. The nitrogen fixation mainly took place in the light, but even after 12 h incubation in darkness, low activities were maintained. Phosphorus fertilization stimulated the nitrogen fixation activity, and the highest activities were obtained with about 300 μM phosphate, both in the field samples and the cultured Anabaena sp. Ammonium (28 mM) immediately inhibited the nitrogen fixation activity of the cultured Anabaena sp, whereas 14 mM urea and 540 μM glutamate led to a weaker and slower inhibition of the nitrogen fixation activity, showing that the cultured Anabaena sp. was able to assimilate these combined nitrogen sources. Nitrate did not have any inhibitory effect on nitrogen fixation activity, either in the field samples or in the cultured Anabaena sp. Both the field samples and the cultured Anabaena sp. showed tolerance against sodium chloride concentrations corresponding to the concentration in seawater. The temperature optimum of the nitrogen fixation activity of the cultured Anabaena sp. was about 20°C. Key words: nitrogen fixation, cyanobacteria, Nostoc commune, Anabaena sp., high arctic.

A hormogonium regulating locus, hrmUA, of the cyanobacterium Nostoc punctiforme strain ATCC 29133 and its response to an extract of a symbiotic plant partner Anthoceros punctatus

Transposon-generated mutant strain UCD 328 of Nostoc punctiforme strain ATCC 29133 has a phenotype of an increased sensitivity to a hormogonium-inducing factor exuded by a symbiotic plant partner, Anthoceros punctatus, and an initial increased hormogonium-dependent infection of the plant. Sequence analysis showed that the transposition site in strain UCD 328 lies within a 1,251-bp open reading frame (ORF), designated hrmA, that displays no significant similarity to known database sequences. A second, 837-bp ORF (hrmU) ends 2 bp 5' from the start of hrmA and has the signature sequences belonging to a family of NAD(P)H-dependent oxidoreductases. Strains having insertional mutations in hrmU or hrmA reproduce the strain UCD 328 phenotype. Transcriptional fusions of luxAB to hrmU or hrmA show an 8- to 10-fold peak increase in luciferase activity 13 to 20 h after the start of incubation in the presence of an aqueous extract of A. punctatus. A promoter induced by the extract was deduced to be between 2.0 to 3.4 kb from the translational start of hrmU. A multicopy plasmid that contains hrmUA within a 6.2-kb fragment conferred an increased infection phenotype on wild-type N. punctiforme 29133. This plasmid and another plasmid containing 4.4 kb of DNA 5' of the transposition site prevented extract-dependent induction of hrmA-luxAB transcription in strain UCD 328, implicating titration of some trans-activator(s) by the cloned fragments. We hypothesize a role for hrmUA in the inhibition of hormogonium formation by the metabolism of an unknown hormogonium-regulating metabolite.

Genetic evidence of a major role for glucose-6-phosphate dehydrogenase in nitrogen fixation and dark growth of the cyanobacterium Nostoc sp. strain ATCC 29133

Heterocysts, sites of nitrogen fixation in certain filamentous cyanobacteria, are limited to a heterotrophic metabolism, rather than the photoautotrophic metabolism characteristic of cyanobacterial vegetative cells. The metabolic route of carbon catabolism in the supply of reductant to nitrogenase and for respiratory electron transport in heterocysts is unresolved. The gene (zwf) encoding glucose-6-phosphate dehydrogenase (G6PD), the initial enzyme of the oxidative pentose phosphate pathway, was inactivated in the heterocyst-forming, facultatively heterotrophic cyanobacterium, Nostoc sp. strain ATCC 29133. The zwf mutant strain had less than 5% of the wild-type apparent G6PD activity, while retaining wild-type rates of photoautotrophic growth with NH4+ and of dark O2 uptake, but it failed to grow either under N2-fixing conditions or in the dark with organic carbon sources. A wild-type copy of zwf in trans in the zwf mutant strain restored only 25% of the G6PD specific activity, but the defective N2 fixation and dark growth phenotypes were nearly completely complemented. Transcript analysis established that zwf is in an operon also containing genes encoding two other enzymes of the oxidative pentose phosphate cycle, fructose-1,6-bisphosphatase and transaldolase, as well as a previously undescribed gene (designated opcA) that is cotranscribed with zwf. Inactivation of opcA yielded a growth phenotype identical to that of the zwf mutant, including a 98% decrease, relative to the wild type, in apparent G6PD specific activity. The growth phenotype and lesion of G6PD activity in the opcA mutant were complemented in trans with a wild-type copy of opcA. In addition, placement in trans of a multicopy plasmid containing the wild-type copies of both zwf and opcA in the zwf mutant resulted in an approximately 20-fold stimulation of G6PD activity, relative to the wild type, complete restoration of nitrogenase activity, and a slight stimulation of N2-dependent photoautotrophic growth and fructose-supported dark growth. These results unequivocally establish that G6PD, and most likely the oxidative pentose phosphate pathway, represents the essential catabolic route for providing reductant for nitrogen fixation and respiration in differentiated heterocysts and for dark growth of vegetative cells. Moreover, the opcA gene product is involved by an as yet unknown mechanism in G6PD synthesis or catalytic activity.

Photosynthetic Nitrite Reduction as Influenced by the Internal Inorganic Carbon Pool in Air-Grown Cells of Synechococcus UTEX 625

Photosynthetic reduction of NO2- was studied in air-grown cells of a cyanobacterium, Synechococcus UTEX 625. Addition of NO2- resulted in significant amounts of chlorophyll a fluorescence quenching both in the absence and presence of CO2, fixation inhibitors, glycolaldehyde or iodoacetamide. The degree of NO2- quenching was insensitive to the O2 concentration in the medium. Addition of 100 [mu]M inorganic carbon in the presence of glycolaldehyde and O2, leading to formation of the carbon pool within the cells, resulted in pronounced fluorescence quenching. Removal of O2 from the medium restored the fluorescence yield completely, and the subsequent addition of NO2- quenched 36% of the variable fluorescence. From the response to added 3-(3,4-dichlorophenyl)-1,1-dimethylurea, the quenching by NO2- appeared to be photochemical quenching, and nonphotochemical quenching did not seem to be present. The reduction of NO2- observed on its addition to inorganic carbon-depleted cells remained uninfluenced by O2 or glycolaldehyde. The internal inorganic carbon pool in the cells stimulated NO2- reduction, both in the presence and absence of O2, by 4.8-fold. An increase in NO2- reduction by 0.5-fold was also observed in the presence of O2 during simultaneous assimilation of carbon and nitrogen in inorganic carbon-depleted cells. Contrary to this, under anaerobiosis, NO2- reduction was suppressed when carbon and nitrogen assimilation occurred together.

Control of Nitrogenase mRNA Levels by Products of Nitrate Assimilation in the Cyanobacterium Anabaena sp. Strain PCC 7120

Nitrate inhibited nitrogenase synthesis and heterocyst development in the cyanobacterium Anabaena sp. strain PCC 7120. Inhibition of dinitrogen fixation by nitrate did not take place, however, in nitrate reductase-deficient derivatives of this strain. Hybridization of total RNA isolated from cells grown on different nitrogen sources with an internal fragment of the nifD gene showed that regulation of nitrogenase activity by nitrate is exerted through a negative control of the nitrogenase mRNA levels.

Regulated nitrate transport in the cyanobacterium Anacystis nidulans

Intracellular accumulation of nitrate, indicative of the operation of an active nitrate transport system, has been measured in intact cells of the cyanobacterium Anacystis nidulans. The ability of the cells to accumulate nitrate was effectively hindered by either ammonium addition or selective inhibition of CO2 fixation by DL-glyceraldehyde, with the effect of either compound being prevented by previously blocking ammonium assimilation. The results support the contention that nitrate utilization in cyanobacteria is regulated at the level of nitrate transport through the concerted action of ammonium assimilation and CO2 fixation.

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 N Source on the Steady State Growth and N Assimilation of P-limited Anabaena flos-aquae

Phosphate-limited chemostat cultures were used to study cell growth and N assimilation in Anabaena flos-aquae under various N sources to determine the relative energetic costs associated with the assimilation of NH(3), NO(3) (-), or N(2). Expressed as a function of relative growth rate, steady state cellular P contents and PO(4) assimilation rates did not vary with N-source. However, N-source did alter the maximal PO(4)-limited growth rate achieved by the cultures: the NO(3) (-) and N(2) cultures attained only 97 and 80%, respectively, of the maximal growth rate of the NH(3) grown cells. Cellular biomass and C contents did not vary with growth rate, but changed with N source. The NO(3) (-)-grown cells were the smallest (627 +/- 34 micromoles C . 10(-9) cells), while NH(3)-grown cells were largest (900 +/- 44 micromoles C . 10(-9) cells) and N(2)-fixing cells were intermediate (726 +/- 48 micromoles C . 10(-9) cells) in size. In the NO(3) (-)-and N(2)-grown cultures, N content per cell was only 57 and 63%, respectively, of that in the NH(3)-grown cells. Heterocysts were absent in NH(3)-grown cultures but were present in both the N(2) and NO(3) (-) cultures. In the NO(3) (-)-grown cultures C(2)H(2) reduction was detected only at high growth rates, where it was estimated to account for a maximum of 6% of the N assimilated. In the N(2)-fixing cultures the acetylene:N(2) ratio varied from 3.4:1 at lower growth rates to 3.0:1 at growth rates approaching maximal.Compared with NH(3), the assimilation of NO(3) (-) and N(2) resulted either in a decrease in cellular C (NO(3) (-) and N(2) cultures) or in a lower maximal growth rate (N(2) culture only). The observed changes in cell C content were used to calculate the net cost (in electron pair equivalents) associated with growth on NO(3) (-) or N(2) compared with NH(3).

Fixation of [(13)N]N 2 and transfer of fixed nitrogen in the Anthoceros-Nostoc symbiotic association

The initial product of fixation of [(13)N]N2 by pure cultures of the reconstituted symbiotic association between Anthoceros punctatus L. and Nostoc sp. strain ac 7801 was ammonium; it accounted for 75% of the total radioactivity recovered in methanolic extracts after 0.5 min and 14% after 10 min of incubation. Glutamine and glutamate were the primary organic products synthesized from [(13)N]N2 after incubation times of 0.5-10 min. The kinetics of labeling of these two amino acids were characteristic of a precursor (glutamine) and product (glutamate) relationship. Results of inhibition experiments with methionine sulfoximine (MSX) and diazo-oxonorleucine were also consistent with the assimilation of N2-derived NH 4 (+) by Anthoceros-Nostoc through the sequential activities of glutamine synthetase (EC 6.3.1.2) and glutamate synthase (EC 1.4.7.1), with little or no assimilation by glutamate dehydrogenase (EC 1.3.1.3). Isolated symbiotic Nostoc assimilated exogenous (13)NH 4 (+) into glutamine and glutamate and their formation was inhibited by MSX, indicating operation of the glutamine synthetase-glutamate synthase (GS-GOGAT) pathway: However, relative to free-living cultures, isolated symbiotic Nostoc assimilated 80% less exogenous ammonium into glutamine and glutamate, implying that symbiotic Nostoc could assimilate only a fraction of N2-derived NH 4 (+) . This implication was tested by using Anthoceros associations reconstituted with wild-type or MSX-resistant strains of Nostoc incubated with [(13)N]N2 in the presence of MSX. The results of these experiments indicated that, in situ, symbiotic Nostoc assimilated about 10% of the N2-derived NH 4 (+) and that NH 4 (+) was made available to Anthoceros tissue where it was apparently assimilated by the GS-GOGAT pathway. Since less than 1% of the fixed N2 was lost to the suspension medium, it appears that transfer of NH 4 (+) from symbiont to host tissue was very efficient in this extracellular symbiotic association.

Glutamine and glutamate transport by Anabaena variabilis

Anabaena variabilis, a dinitrogen-fixing cyanobacterium, has high- and low-affinity systems for the transport of glutamine and glutamate. The high-affinity systems have Km values of 13.8 and 100 microM and maximal rates of 13.2 and 14.4 nmol X min-1 X mg of chlorophyll a-1 for glutamine and glutamate, respectively. The low-affinity systems have Km values of 1.1 and 1.4 mM and maximal rates of 125 and 100 nmol X min-1 X mg of chlorophyll a-1 for glutamine and glutamate, respectively. Glutamine was unable to support growth of A. variabilis in the absence of any other nitrogen source, and glutamate alone at 500 microM was inhibitory to its growth. The analog L-methionine-DL-sulfoximine (MSX) was transported by a high-affinity system with a Km of 34 microM. Competition experiments and the transport characteristics of a specific class of MSX-resistant mutants imply that glutamine, glutamate, and MSX share a common component for transport. A second class of MSX-resistant mutants had a glutamine synthetase activity with altered affinity constants for glutamine and glutamate relative to the wild-type enzyme.