Heterocystous Cyanobacteria Research Articles

Response of <i>Nodularia spumigena</i> to <i>p</i>CO<sub>2</sub> – Part 1: Growth, production and nitrogen cycling

Abstract. Heterocystous cyanobacteria of the genus Nodularia form extensive blooms in the Baltic Sea and contribute substantially to the total annual primary production. Moreover, they dispense a large fraction of new nitrogen to the ecosystem when inorganic nitrogen concentration in summer is low. Thus, it is of ecological importance to know how Nodularia will react to future environmental changes, in particular to increasing carbon dioxide (CO2) concentrations and what consequences there might arise for cycling of organic matter in the Baltic Sea. Here, we determined carbon (C) and dinitrogen (N2) fixation rates, growth, elemental stoichiometry of particulate organic matter and nitrogen turnover in batch cultures of the heterocystous cyanobacterium Nodularia spumigena under low (median 315 μatm), mid (median 353 μatm), and high (median 548 μatm) CO2 concentrations. Our results demonstrate an overall stimulating effect of rising pCO2 on C and N2 fixation, as well as on cell growth. An increase in pCO2 during incubation days 0 to 9 resulted in an elevation in growth rate by 84 ± 38% (low vs. high pCO2) and 40 ± 25% (mid vs. high pCO2), as well as in N2 fixation by 93 ± 35% and 38 ± 1%, respectively. C uptake rates showed high standard deviations within treatments and in between sampling days. Nevertheless, C fixation in the high pCO2 treatment was elevated compared to the other two treatments by 97% (high vs. low) and 44% (high vs. mid) at day 0 and day 3, but this effect diminished afterwards. Additionally, elevation in carbon to nitrogen and nitrogen to phosphorus ratios of the particulate biomass formed (POC : POP and PON : POP) was observed at high pCO2. Our findings suggest that rising pCO2 stimulates the growth of heterocystous diazotrophic cyanobacteria, in a similar way as reported for the non-heterocystous diazotroph Trichodesmium. Implications for biogeochemical cycling and food web dynamics, as well as ecological and socio-economical aspects in the Baltic Sea are discussed.

The pknH gene restrictively expressed in heterocysts is required for diazotrophic growth in the cyanobacterium Anabaena sp. strain PCC 7120

Anabaena sp. strain PCC 7120 is a filamentous cyanobacterium in which certain vegetative cells differentiate into heterocysts, which are specialized cells for nitrogen fixation. Heterocysts are unable to carry out photosynthesis and are supplied with carbohydrate required for nitrogen fixation from neighbouring vegetative cells. Thus, filament integrity is very important for diazotrophic growth of the heterocystous cyanobacteria. The pknH gene (alr1336), encoding a putative Ser/Thr protein kinase, was upregulated in heterocysts after nitrogen deprivation. Its expression was developmentally regulated by the hetR gene. Expression levels of genes involved in heterocyst maturation, such as hepA, hglE and nifH, in the pknH disruptant were similar to those of the wild-type strain. The disruptant was able to form heterocysts with nitrogenase activity, but most heterocysts were detached from filaments. Hence, the pknH disruptant showed a growth defect in the medium without combined nitrogen. It is concluded that the pknH gene is not involved in the development of heterocyst function but is involved in maintaining connections between heterocysts and vegetative cells.

Response of biological soil crust diazotrophs to season, altered summer precipitation, and year-round increased temperature in an arid grassland of the colorado plateau, USA.

Biological soil crusts (biocrusts), which supply significant amounts of fixed nitrogen into terrestrial ecosystems worldwide (∼33 Tg y−1), are likely to respond to changes in temperature and precipitation associated with climate change. Using nifH gene-based surveys, we explored variation in the diazotrophic community of biocrusts of the Colorado Plateau, USA in response to season (autumn vs. spring), as well as field manipulations that increased the frequency of small volume precipitation events and year-round soil temperature. Abundance of nifH genes in biocrusts ranged from 3 × 106 to 1 × 108 g−1 soil, and nifH from heterocystous cyanobacteria closely related to Scytonema hyalinum, Spirirestis rafaelensis, and Nostoc commune comprised >98% of the total. Although there was no apparent seasonal effect on total nifH gene abundance in the biocrusts, T-RFLP analysis revealed a strong seasonal pattern in nifH composition. Spirirestis nifH abundance was estimated to oscillate 1 to >2 orders of magnitude between autumn (low) and spring (high). A year-round increase of soil temperature (2–3°C) had little effect on the diazotroph community structure over 2 years. Altered summer precipitation had little impact on diazotroph community structure over the first 1.5 years of the study, when natural background patterns across years and seasons superseded any treatment effects. However, after the second summer of treatments, nifH abundance was 2.6-fold lower in biocrusts receiving altered precipitation. Heterocystous cyanobacteria were apparently more resilient to altered precipitation than other cyanobacteria. The results demonstrate that diazotrophic community composition of biocrusts in this semi-arid grassland undergoes strong seasonal shifts and that the abundance of its dominant members decreased in response to more frequent, small volume precipitation events.

Light intensity determines the extent of mercury toxicity in the cyanobacterium Nostoc muscorum

The extent of mercury (Hg) toxicity in the heterocystous cyanobacterium Nostoc muscorum grown for 72 h in three different light intensities was tested for various physiological parameters viz. growth, pigment contents, photosynthesis, respiration, reactive oxygen species (ROS), malondialdehyde formation and antioxidants. A general reduction in growth and pigments, whole cell O2-evolution, photosynthetic electron transport activities and 14CO2-fixation was observed in a metal concentration–dependent manner, and this effect was more pronounced in high light (130 μmol photon m−2 s−1)–exposed cells as compared to low (10 μmol photon m−2 s−1) and normal (70 μmol photon m−2 s−1) light intensity–exposed cells; however, carotenoids and respiration showed reverse trend. Among photosynthetic electron transport activities, whole chain activity was found to be most sensitive in comparison with photosystem II (PS II) and photosystem I (PS I). Comparing the different photosynthetic processes, 14CO2-fixation was most affected in cyanobacterial cells when exposed to Hg and different light intensities. After application of various exogenous electron donors, diphenyl carbazide was found to be more effective to restore PS II activity, suggesting that site of damage lies in between oxygen evolving complex and PS II. Level of oxidative stress (superoxide radical and lipid peroxidation) was maximum at 3.0 μM of Hg when coupled with high light intensity (except hydrogen peroxide). A dose-dependent increase in enzymatic antioxidants such as superoxide dismutase, peroxidase and catalase as well as non-enzymatic antioxidants such as proline, ascorbate, cysteine (except under high light intensity) and non-protein thiols [NP-SH] was observed, which further increased with the increase in light intensity. It was noticed that Hg intoxicates N. muscorum through ROS production, which is aggravated along with the increase in light intensity. Overall results suggest that the severity of the metal stress does increase with Hg concentrations but when coupled with light, it was the light intensity that determines the extent of Hg toxicity.

Composition and diversity of nifH genes of nitrogen-fixing cyanobacteria associated with boreal forest feather mosses

Recent studies have revealed that nitrogen fixation by cyanobacteria living in association with feather mosses is a major input of nitrogen to boreal forests. We characterized the community composition and diversity of cyanobacterial nifH phylotypes associated with each of two feather moss species (Pleurozium schreberi and Hylocomium splendens) on each of 30 lake islands varying in ecosystem properties in northern Sweden. Nitrogen fixation was measured using acetylene reduction, and nifH sequences were amplified using general and cyanobacterial selective primers, separated and analyzed using density gradient gel electrophoresis (DGGE) or cloning, and further sequenced for phylogenetic analyses. Analyses of DGGE fingerprinting patterns revealed two host-specific clusters (one for each moss species), and sequence analysis showed five clusters of nifH phylotypes originating from heterocystous cyanobacteria. For H. splendens only, N(2) fixation was related to both nifH composition and diversity among islands. We demonstrated that the cyanobacterial communities associated with feather mosses show a high degree of host specificity. However, phylotype composition and diversity, and nitrogen fixation, did not differ among groups of islands that varied greatly in their availability of resources. These results suggest that moss species identity, but not extrinsic environmental conditions, serves as the primary determinant of nitrogen-fixing cyanobacterial communities that inhabit mosses.

Characterization of nifH gene expression, modification and rearrangement in Nodularia spumigena strain AV1

The annually reoccurring blooms that characterize the surface waters of the Baltic Sea are dominated by filamentous, heterocystous cyanobacteria such as Nodularia spumigena. In a previous study, we have demonstrated that N. spumigena strain AV1 differentiates heterocysts in the absence of detectable nitrogen fixation activity, an unusual physiological trait that is clearly distinct from other well-studied cyanobacteria. To further analyze the uncoupling between these two processes, we analyzed the gene expression and modification of the nitrogenase enzyme (the enzyme responsible for nitrogen fixation) in N. spumigena AV1, as well as in several other N. spumigena strains. Here, we demonstrate the occurrence of two nifH gene copies in N. spumigena strain AV1, only one of which is located in a complete nifHDK cluster and several NifH protein forms. Furthermore, we demonstrate the occurrence of a DNA rearrangement mechanism acting within the nifH gene copy located in the nifHDK cluster and present only in the strains exhibiting the previously reported uncoupling between heterocyst differentiation and nitrogen fixation processes. These data stress the existence of a distinct and complex regulatory circuit related to nitrogen fixation in this ecologically significant bloom-forming cyanobacterium.

Nutrient stoichiometry of linked catchment-lake systems along a gradient of land use

Summary 1. Catchments export nutrients to aquatic ecosystems at rates and ratios that are strongly influenced by land use practices, and within aquatic ecosystems nutrients can be processed, retained, lost to the atmosphere, or exported downstream. The stoichiometry of carbon and nutrients can influence ecosystem services such as water quality, nutrient limitation, biodiversity, eutrophication and the sequestration of nutrients and carbon in sediments. However, we know little about how nutrient stoichiometry varies along the pathway from terrestrial landscapes through aquatic systems. 2. We studied the stoichiometry of nitrogen and phosphorus exported by three catchments of contrasting land use (forest versus agriculture) and in the water column and sediments of downstream reservoirs. We also related stoichiometry to phytoplankton nutrient limitation and the abundance of heterocystous cyanobacteria. 3. The total N : P of stream exports varied greatly among catchments and was 18, 54 and 140 (molar) in the forested, mixed-use and agricultural catchment, respectively. Total N : P in the mixed layers of the lakes was less variable but ordered similarly: 35, 52 132 in the forested, mixed-use and agricultural lake, respectively. In contrast, there was little variation among systems in the C : N and C : P ratios of catchment exports or in reservoir seston. 4. Phytoplankton in the forested lake were consistently N limited, those in the agricultural lake were consistently P limited, and those in the mixed-use lake shifted seasonally from P- to N limitation, reflecting N : P supply ratios. Total phytoplankton and cyanobacteria biomass were highest in the agricultural lake, but heterocystous (potentially N fixing) cyanobacteria were most abundant in the forested lake, corresponding to low N : P ratios. 5. Despite large differences in catchment export and water column N : P ratios, the N : P of sediment burial (integrated over several decades) was very low and remarkably similar (4.3–7.3) across reservoirs. N and P budgets constructed for the agricultural reservoir suggested that denitrification could be a major loss of N, and may help explain the relatively low N : P of buried sediment. 6. Our results show congruence between the catchment export N : P, reservoir N : P, phytoplankton N versus P limitation and the dominance of heterocystous cyanobacteria. However, the N : P stoichiometry of sediments retained in the lakes was relatively insensitive to catchment stoichiometry, suggesting that a common set of biogeochemical processes constrains sediment N : P across lakes of contrasting catchment land use.

Climate change modulates structural and functional lake ecosystem responses to introduced anadromous salmon

We integrated limnological, paleolimnological, and fisheries analyses in Tuya Lake, British Columbia, Canada, to explore the effects and interactions of climate warming and sockeye salmon introductions on northern lake ecology. We tracked millennially unprecedented, climate-correlated changes in inferred lake production, stratification, and trophic structure since the mid-1800s, most likely resulting from declining ice cover and enhanced stratification. Post-1970s algal (diatom) species turnover, coeval across several remote northern Cordilleran lakes, marked an apparent increase in warming and the induction of inferred nitrogen deficiencies in Tuya Lake. Lower post-stocking phosphorus (P) and nitrogen (N) concentrations and a strong P-modeled salmon biomass correlation (r2 = 0.87) indicated salmon production reduced epilimnetic nutrient availability. Post-stocking chlorophyll reductions, late-summer algal dominance by heterocystous cyanobacteria and low-N-tolerant diatoms, and a strong chlorophyll-modeled salmon biomass correlation (r2 = 0.87) indicated that salmon influenced lake productivity, most likely by enhancing climate-induced N-deficiencies. Predicted smolt-biomass nutrient exports were minimal, with post-introduction nutrient reductions likely related to planktivory and enhanced sedimentation losses. Our study highlights how climate warming changes northern lake ecosystem structure and functioning, influencing responses to subsequent stresses.

Growth and Physiological Features of Cyanobacterium Anabaena sp. Strain PCC 7120 in a Glucose-Mixotrophic Culture

Mixotrophic growth is one potential mode for mass culture of microalgae and cyanobacteria particularly suitable for the production of high value bioactive compounds and fine chemicals. The typical heterocystous cyanobacterium Anabaena sp. PCC 7120 was grown in the presence of exogenous glucose in light. Glucose improved the cell growth evidently, the maximal specific growth rate under mixotrophic condition (0.38 d( 1) being 1.6-fold of that of photoautotrophic growth. Mixotrophy caused a variation in cellular pigment composition, increasing the content of chlorophyll a and decreasing the contents of carotenoid and phycobiliprotein relative to chlorophyll a. Fluorescence emission from photosystem II (PSII) relative to photosystem I was enhanced in mixotrophic cells, implying an increased energy distribution in PSII. Glucokinase (EC 2.7.1.2) activity was further induced in the presence of glucose. The mixotrophic culture was scaled up in a 15 L airlift photobioreactor equipped with an inner and an outer light source. A modified Monod model incorporating the specific growth rate and the average light intensity in the reactor was developed to describe cell growth appropriately. The understanding of mixotrophic growth and relevant physiological features of Anabaena sp. PCC 7120 would be meaningful for cultivation and exploitation of this important cyanobacterial strain.

Evolution and variation of the nifD and hupL elements in the heterocystous cyanobacteria

In heterocystous cyanobacteria, heterocyst differentiation is accompanied by developmentally regulated DNA rearrangements that occur within the nifD and hupL genes, referred to as the nifD and hupL elements. These elements are segments of DNA that are embedded within the coding region of each gene and range from 4 to 24 kb in length. The nifD and hupL elements are independently excised from the genome during the later stages of differentiation by the site-specific recombinases, XisA and XisC, respectively, which are encoded within the elements themselves. Here we examine the variation and evolution of the nifD and hupL elements by comparing full-length nifD and hupL element sequences and by phylogenetic analysis of xisA and xisC gene sequences. There is considerable variation in the size and composition of the nifD and hupL elements, however, conserved regions are also present within representatives of each element. The data suggest that the nifD and hupL elements have undergone a complex pattern of insertions, deletions, translocations and sequence divergence over the course of evolution, but that conserved regions remain.

EFFECT OF TEMPERATURE ON THE SENSITIVITY OF NITROGENASE TO OXYGEN IN TWO HETEROCYSTOUS CYANOBACTERIA1

The effect of temperature and oxygen on nitrogenase activity in two heterocystous cyanobacteria, Anabaena variabilis Kütz. ATCC29413 and Nostoc sp. PCC7120, was investigated. The cyanobacteria were grown under a 12:12 light:dark (L:D) cycle at 27°C and were subsequently exposed to different temperatures (27, 36, 39, and 42°C) at different steady‐state O2 concentrations (20, 10, 5, 0%). Light response curves of nitrogenase activity were recorded under each of these conditions using an online acetylene reduction assay combined with a sensitive laser photoacoustic ethylene detection method. The light response curves were fitted with the rectangular hyperbola model from which the model parameters Nm, Nd, and α were derived. In both strains, nitrogenase activity (Ntot = Nm + Nd) was the highest at 39°C–42°C and at 0% O2. The ratio Ntot/Nd was 4.1 and 3.1 for Anabaena and Nostoc, respectively, indicating that respectively 25% and 33% of nitrogenase activity was supported by respiration (Nd). Ntot/Nd increased with decreasing O2 concentration and with increasing temperature. Hence, each of these factors caused a relative increase in the light‐driven nitrogenase activity (Nm). These results demonstrate that photosynthesis and respiration both contribute to nitrogenase activity in Anabaena and Nostoc and that their individual contributions depend on both O2 concentration and temperature as the latter may dynamically alter the flux of O2 into the heterocyst.

ISOLATION OF CALOTHRIX RHIZOSOLENIAE (CYANOBACTERIA) STRAIN SC01 FROM CHAETOCEROS (BACILLARIOPHYTA) SPP. DIATOMS OF THE SUBTROPICAL NORTH PACIFIC OCEAN1

In November 2004, Chaetoceros spp. (diatom) cells were collected from 5 m at Station ALOHA (22º45′ N, 158º0′ W) in the subtropical North Pacific Ocean. Attached to the spines of several Chaetoceros spp. were symbiotic heterocystous cyanobacterial cells, identified as Calothrix rhizosoleniae Lemmerm. The symbiotic diatom cells were handpicked and placed in N-deplete media. The host cells have been absent since isolation, and the cyanobacterial symbiont has persisted in a free-living state in two different liquid and solid media: one made from artificial seawater, and another with natural seawater. All media are devoid of fixed inorganic nitrogen. Trichome length varied from three to four vegetative cells when growing colonially, to >10 vegetative cells when growing as single trichomes. The DNA and amino acid sequences of the narB, rbcL, and rnpB genes are most similar to those from other heterocystous cyanobacteria (Anabaena and Fischerella). Acetylene reduction (AR) assays were run in conjunction with multiplexing quantitative reverse transcription–PCR (qRT–PCR) assays. Gene transcription for rbcL and nifH was high, coincident with maximum AR, and occurred in the middle of the photoperiod. Eight irradiance curves of nitrogenase activity at varying biomass concentrations showed evidence of photoinhibition at high light intensities. Here, we report on the genetic identification and photophysiology for the first symbiotic isolate, Ca. rhizosoleniae SC01, of an open-ocean diatom (Chaetoceros).

Biological dinitrogen fixation by selected soil cyanobacteria as affected by strain origin, morphotype, and light conditions

The potential for N(2) fixation by heterocystous cyanobacteria isolated from soils of different geographical areas was determined as nitrogenase activity (NA) using the acetylene reduction assay. Morphology of cyanobacteria had the largest influence on NA determined under light conditions. NA was generally higher in species lacking thick slime sheaths. The highest value (1446 nmol/h C(2)H(4) per g fresh biomass) was found in the strain of branched cyanobacterium Hassalia (A Has1) from the polar region. A quadratic relationship between NA and biomass was detected in the Tolypothrix group under light conditions. The decline of NA in dark relative to light conditions ranged from 37 to 100 % and differed among strains from distinct geographical areas. Unlike the NA of temperate and tropical strains, whose decline in dark relative to light was 24 and 17 %, respectively, the NA of polar strains declined to 1 % in the dark. This difference was explained by adaptation to different light conditions in temperate, tropical, and polar habitats. NA was not related to the frequency of heterocysts in strains of the colony-forming cyanobacterium Nostoc. Colony morphology and life cycle are therefore more important for NA then heterocyst frequency. NA values probably reflect the environmental conditions where the cyanobacterium was isolated and the physiological and morphological state of the strain.

Unusual radioresistance of nitrogen-fixing cultures of Anabaena strains

Nitrogen-fixing cultures of two species of the filamentous, heterocystous cyanobacterium Anabaena, namely Anabaena sp. strain L-31 and Anabaena torulosa were found to be highly tolerant to 60Co gamma radiation. No adverse effect on diazotrophic growth and metabolism were observed up to a dose of 5 kGy. At higher doses, radiation tolerance showed a correspondence with the inherent osmotolerance, with Anabaena L-31 being the more radiation tolerant as well as osmotolerant strain. In Anabaena L-31, exposure to 6 kGy of gamma rays resulted in genome disintegration, but did not reduce viability. Irradiation delayed heterocyst differentiation and nitrogen fixation, and marginally affected diazotrophic growth. All the affected parameters recovered after a short lag, without any discernible postirradiation phenotype. The radiation tolerance of these Gram-negative photoautodiazotrophs is comparable with that of the adiazotrophic photoautotrophic cyanobacterium Chroococcidiopsis or adiazotrophic heterotroph Deinococcus radiodurans. This is the first report of extreme radioresistance in nitrogen-fixing Anabaena cultures.

Inferring the Evolutionary History of Mo-Dependent Nitrogen Fixation from Phylogenetic Studies of nifK and nifDK

The ability to fix nitrogen is widely, but sporadically distributed among the Bacteria and Archaea suggesting either a vertically inherited, ancient function with widespread loss across genera or an adaptive feature transferred laterally between co-inhabitants of nitrogen-poor environments. As previous phylogenetic studies of nifH and nifD have not completely resolved the evolutionary history of nitrogenase, sixty nifD, nifK, and combined nifDK genes were analyzed using Bayesian, maximum likelihood, and parsimony algorithms to determine whether the individual and combined datasets could provide additional information. The results show congruence between the 16S and nifDK phylogenies at the phyla level and generally support vertical descent with loss. However, statistically significant differences between tree topographies suggest a complex evolutionary history with the underlying pattern of vertical descent obscured by recurring lateral transfer events and different patterns of evolution between the genes. Results support inheritance from the Last Common ancestor or an ancient lateral transfer of the nif genes between Bacteria and Archaea, ongoing gene transfer between cohabitants of similar biogeographic regions, acquisition of nitrogen-fixing capability via symbiosis islands, possible xenologous displacement of one gene in the operon, and possible retention of ancestral genes in heterocystous cyanobacteria. Analyses support the monophyly of the Cyanobacteria, alphabetagamma-Proteobacteria, and Actinobacteria (Frankia) and provide strong support for the placement of Frankia nif genes at the base of combined the Cyanobacteria/Proteobacteria clades.

Temporal and spatial variability of nifH expression in three filamentous Cyanobacteria in coastal microbial mats

Cyanobacteria are often the most conspicuous structural part of microbial mats. They are also the only oxygenic phototrophs capable of N2 fixation (diazotrophy). This represents an important advantage for persistence in the often N-depleted marine intertidal microbial mats. In this study the daily pattern of expression of the structural gene for dinitrogenase reductase, nifH and the 16S rRNA gene of 3 benthic filamentous diazotrophic Cyanobacteria (Lyngbya sp., Nodularia sp. and Anabaena sp.) was measured by using quantitative RT-PCR. Gene expression patterns were compared and related to the daily pattern of nitrogenase activity. Microscopic observations revealed that the non-heterocystous Lyngbya sp. was the major cyanobacterial morphotype in all the microbial mats studied. When normalized to copy number, nifH as well as 16S rRNA gene expression by Lyngbya sp. was higher than by the heterocystous Cyanobacteria in all but one mat type. Related to the total amount of extracted RNA, Lyngbya sp. dominated 16S rRNA gene expression as well but was outcompeted by the heterocystous Cyanobacteria with respect to total nifH expression. This was due to the low DNA copy number of Lyngbya sp. nifH. The results revealed different cell-specific expression levels and varying contribution to the overall nifH and 16S rRNA gene expression by the 3 Cyanobacteria over the course of a daily cycle as well as in the different mat types. With respect to the overall nifH expression, the structurally dominant diazotroph was not the most active. Moreover, nifH expression pattern did not follow nitrogenase activity.

Bacterial diversity, pigments and nitrogen fixation of biological desert crusts from the Sultanate of Oman

Biological desert crusts are relatively common in the arid deserts of the Sultanate of Oman; however, little is known about their microbial community composition and role in soil fertilization. We compared three crusts from geographically different locations for their soil texture, bacterial community structure, pigment composition and nitrogenase activity. The crusts were growing on alkaline (pH 7.6-8.7) loamy sand and silty loam soils. Microscopically, Microcoleus vaginatus was the most abundant cyanobacterium, but Nostoc and Scytonema types dominated in cultures. The 16S rRNA gene sequences showed close similarities in the crusts' bacterial composition, with 77-81% of the total clones belonging to cyanobacteria and the rest distributed among Alpha- and Deltaproteobacteria, Bacteriodetes, Gemmatimonas and Planctomycetes. Thirty-seven percent of the cyanobacterial clones were affiliated with heterocystous types such as Nostoc, Scytonema, Brasilonema and Petalonema. Chlorophyll a concentrations suggest a similar abundance of phototrophs in all crusts. High levels of the UVA sunscreen scytonemin were detected in the exposed crusts. The three crusts exhibited comparable acetylene reduction rates in the light and in the dark, with a maximum rate of 58.5+/-2.6 micromol C(2)H(2) reduced m(-2) h(-1). We conclude that the crusts, regardless of their geographical location, were rich in heterocystous cyanobacteria that can fix nitrogen and could possibly improve soil stability and productivity.

Molybdenum—nitrogen co‐limitation in freshwater and coastal heterocystous cyanobacteria

Molybdenum (Mo) is essential for the biological assimilation of inorganic nitrogen (N). We compared Mo requirements for N2‐fixation in two species of filamentous heterocystous cyanobacteria (HC) to test the hypothesis that coastal HC require higher Mo concentrations than freshwater HC. This expectation follows from the fact that Mo is more concentrated in seawater (~ 105 nmol L−1) than in most freshwaters (< 20 nmol L−1). Contrary to this hypothesis, we found that both strains maintained N2‐fixation for 30 d at 10 nmol L−1. Mo concentrations < 1 nmol L−1 induced N‐limitation in both species, as indicated by increased C:N ratios and decreased nitrogenase expression and activity. This response took time to induce, likely due to high‐affinity molybdate uptake by both species. Measurable N2‐fixation persisted in the coastal strain (Nostoc sp. CCMP 2511) for at most 12 d; 3 d were required for chlorophyll a concentrations to fall below those of Mo‐replete cultures. An additional 7 d and 11 d, respectively, were required for N2‐fixation rates and chlorophyll levels to decline in Mo‐limited freshwater cultures (Nostoc sp. PCC 7120). When Mo was high (> 1 µmol L−1), the freshwater strain exhibited considerable Mo storage (> 100 µmol mol−1 Mo: C) whereas cellular Mo remained < 10 µmol mol−1 Mo:C in the coastal strain. The high Mo content and extended time required for N2‐fixation to decrease in the freshwater strain could be due to expression of the gene mop, which encodes a putative molybdate‐storage protein. This study suggests the importance of Mo storage in freshwater HC.

Molybdenum-nitrogen co-limitation in freshwater and coastal heterocystous cyanobacteria

Molybdenum-nitrogen co-limitation in freshwater and coastal heterocystous cyanobacteria

Fluorescence microscopy reveals variations in cellular composition during formation of akinetes in the cyanobacterium Aphanizomenon ovalisporum

Akinetes are differentiated, thick-walled, resting cells produced by many species of the filamentous heterocystous cyanobacteria belonging to the Nostocales and Stigonematales. They are commonly developed after the end of exponential growth and are normally resistant to low temperatures and to desiccation for extended periods. Akinetes maintain a low level of metabolic activities such as photosynthesis, respiration and de novo synthesis of proteins and lipids. In a previous study we reported that mature akinetes of Aphanizomenon ovalisporum maintained only residual photosynthetic activity. However the cellular abundance of PSI and PSII reaction centres increased in akinetes relative to exponentially grown vegetative cells while the ratio of PSI to PSII remained relatively unchanged. In the present study we provide spectral and biochemical evidence that akinetes lose their phycobilisomes during their development. DAPI stained trichomes of exponentially grown cultures emitted fluorescence signals attributed to nucleic acids and polyphosphate granules. Akinetes that developed in akinete-induced cultures were preferentially enriched with nucleic acids and deprived of polyphosphate bodies, whereas adjacent vegetative cells were loaded with polyphosphate bodies. Examination of akinetes from akinete-induced cultures of A. ovalisporum by spectral confocal laser scanning microscopy indicated reduction of the phycobilisome fluorescence signal. Mature isolated akinetes could be divided into two distinct populations: (i) a majority of akinetes that lost their phycobilisome fluorescence signal, but maintain chlorophyll fluorescence attributed to the reaction centres, and (ii) a minor group of mature akinetes that maintained a high phycobilisome fluorescence signal.