Heterocystous Cyanobacteria Research Articles

Diurnal variation of rates of denitrification and nitrogen fixation of periphyton associated with Oryza glumaepatula Steud in an Amazonian Lake

The rates of denitrification and biological nitrogen fixation of the periphyton communities associated with the culms and adventitious root of Oryza glumaepatula in Lake Batata, State of Para, Brazilian Amazonia, were investigated over the course of 24 h. N2O was not detected during the period, demonstrating that denitrification was not occurring. The low concentration of nitrate in the water column was indicated as the main factor limiting the denitrification process. The rates of biological nitrogen fixation (BNF) in culms were lowest at night and early morning, while the highest values were measured at 1200 h. BNF rates in periphyton associated with adventitious roots were lowest during the morning, while highest values were measured at 1600 h. The difference in the daily patterns of BNF rates in the periphyton of culms and roots of O. glumaepatula in the euphotic zone was attributed to defferences in the availability of organic carbon which supports this process. In the periphyton of the culms, the BNF process was carried out by heterocystous cyanobacteria, which was photoautotrophs. In the periphyton of the adventitious roots, BNF was carried out by periphytic bacteria, and is heterotrophically mediated.

On-line monitoring of nitrogenase activity in cyanobacteria by sensitive laser photoacoustic detection of ethylene

A new and extremely sensitive method for measuring nitrogenase activity through acetylene reduction is presented. Ethylene produced by nitrogenase-mediated reduction of acetylene is detected by using laser photoacoustics (LPA). This method possesses a detection limit making it 3 orders of magnitude more sensitive than traditional gas chromatographic analysis. Photoacoustic detection is based on the strong and unique absorption pattern of ethylene in the CO(inf2) laser wavelength region (9 to 11 (mu)m). The high sensitivity allowed on-line monitoring of nitrogenase activity in a culture of the heterocystous cyanobacterium Nodularia spumigena, which was isolated from a water bloom in the Baltic Sea. This setup makes it unnecessary to take subsamples from the culture and avoids long incubations in sealed vials. The fast response of the LPA technique allows measurement of real-time dynamic changes of nitrogenase activity. The method was used to analyze in vivo saturation of nitrogenase by acetylene in N. spumigena. It is demonstrated that 20% acetylene does not saturate nitrogenase and that the degree of saturation depends on light intensity. With concentrations of acetylene as low as 2.5% it is possible to assess the degree of saturation and to extrapolate to total nitrogenase activity. In N. spumigena nitrogenase activity becomes independent of light intensity above 20 to 80 (mu)mol of photons m(sup-2) s(sup-1) at 20% O(inf2).

The RNase P RNA from cyanobacteria: short tandemly repeated repetitive (STRR) sequences are present within the RNase P RNA gene in heterocyst-forming cyanobacteria.

The RNase P RNA gene (rnpB) from 10 cyanobacteria has been characterized. These new RNAs, together with the previously available ones, provide a comprehensive data set of RNase P RNA from diverse cyanobacterial lineages. All heterocystous cyanobacteria, but none of the non-heterocystous strains analyzed, contain short tandemly repeated repetitive (STRR) sequences that increase the length of helix P12. Site-directed mutagenesis experiments indicate that the STRR sequences are not required for catalytic activity in vitro. STRR sequences seem to have recently and independently invaded the RNase P RNA genes in heterocyst-forming cyanobacteria because closely related strains contain unrelated STRR sequences. Most cyanobacteria RNase P RNAs lack the sequence GGU in the loop connecting helices P15 and P16 that has been established to interact with the 3'-end CCA in precursor tRNA substrates in other bacteria. This character is shared with plastid RNase P RNA. Helix P6 is longer than usual in most cyanobacteria as well as in plastid RNase P RNA.

Characterization of genes for a second Mo-dependent nitrogenase in the cyanobacterium Anabaena variabilis.

Anabaena variabilis ATCC 29413 is a filamentous heterocystous cyanobacterium that fixes nitrogen under a variety of environmental conditions. Under aerobic growth conditions, nitrogen fixation depends upon differentiation of heterocysts and expression of either a Mo-dependent nitrogenase or a V-dependent nitrogenase in those specialized cells. Under anaerobic conditions, a second Mo-dependent nitrogenase gene cluster, nifII, was expressed in vegetative cells long before heterocysts formed. A strain carrying a mutant gene in the nifII cluster did not fix nitrogen under anaerobic conditions until after heterocysts differentiated. The nifII cluster was similar in organization to the nifI cluster that is expressed in heterocysts and that includes nifBSUHDKENXW as well as three open reading frames that are conserved in both cyanobacterial nif clusters.

Ornithine cycle in Nostoc PCC 73102: presence of an in vitro functional argininosuccinate lyase

The presence of argininosuccinate lyase (ASL), an enzyme catalyzing the final step of arginine biosynthesis, was demonstrated in the heterocystous cyanobacterium Nostoc PCC 73102 by measuring in vitro enzyme activity and by visualization of ASL in native protein gels. Activity staining of a native PAGE gel revealed one ASL-dependent band with a molecular mass of about 240 kDa. A colorimetric assay for ASL based on the measurement of urea produced from arginine in the presence of an excess of arginase was further used to analyze the cyanobacterial ASL. The in vitro ASL activity was highest in cells in exponential growth phase and decreased significantly during the stationary phase of growth, ranging from 4.4 to 0.8 nmol of product formed (mg protein) −1 min −1. Including arginine, citrulline or ornithine in the growth medium resulted in no significant change in the ASL activities, indicating that Nostoc PCC 73102 ASL is not regulated by metabolites of the ornithine cycle.

Modification of the pIIprotein in response to carbon and nitrogen availability in filamentous heterocystous cyanobacteria

In the filamentous cyanobacterium Calothrix PCC 7504, which fixes N2 aerobically, the modification state of the regulatory PII protein (GlnB) was shown to depend on nitrogen and carbon availability, as observed in the unicellular non-fixing strain Synechococcus PCC 7942. However, the conditions for modifications, the time dependence of the process and the electrophoretic behavior of the native PII isoforms differed somewhat between the two strains. In another strain, Calothrix PCC 7601, which has lost the capability to fix N2, PII was modified only if ammonia plus an inhibitor of glutamine synthetase were present. It is proposed that: (i) the behavior of the PII proteins depends upon the physiological properties of the strains; and (ii) the modification system of PII per se may differ between the two cyanobacterial genera.

Glucose-6-phosphate Dehydrogenase from the Cyanobacterium,Anabaenasp. PCC 7120: Purification and Kinetics of Redox Modulation

Glucose-6-phosphate dehydrogenase is a particularly important enzyme in carbon catabolism in the chloroplasts of higher plants and in cyanobacteria. It catalyzes the first reaction in the oxidative pentose phosphate pathway which supplies reduced NADP for a variety of biosynthetic processes. The enzyme is known to be regulated by light. However, the dehydrogenase from plants has been difficult to purify and there is little information on kinetics and mechanism of deactivation. The glucose-6-phosphate dehydrogenase from the heterocystous cyanobacterium,Anabaenasp. PCC 7120, was purified to near homogeneity by chromatography on 2′,5′-ADP Sepharose chromatography. The cyanobacterial enzyme apparently has different aggregation states or conformations depending on its concentration in solution and the pH. At a pH of 8.0 and low ionic strength, the enzyme has relatively low activity and exhibits sigmoidal kinetics on binding substrate and cofactor. Activity increases and the enzyme exhibits the more classical hyperbolic kinetics at pH 7.0. At the lower pH, glucose-6-phosphate dehydrogenase is inhibited by catalytic amounts of reduced thioredoxin-1 fromAnabaenasp. The second thioredoxin from the cyanobacterium is much less effective, although its inhibitory effect is still greater than that of small molecule reducing agents such as glutathione. Glutamine was reported to stabilize the isolated enzyme, but actually is an activator at pH 8.0. The results suggest that cellular demand for reduced cofactor under nitrogen-fixing conditions overrides the pH-induced deactivation.

The biogeochemistry of two eutrophic marine lagoons and its effect on microphytobenthic communities

This paper summarizes the results of a study on the biogeochemistry of two eutrophic marine lagoons. The lagoons investigated were the Bassin d'Arcachon situated on the Atlantic coast, and Etang du Prevost on the Mediterranean coast. The sites chosen for this study were characterized by the presence of dense communities of microphytobenthos. Both lagoons receive a large input of nutrients but they differ in several aspects. The Bassin d'Arcachon receives a large amount of iron. Iron is of great importance in reducing the effects of eutrophication. Ferric iron is an efficient scavenger of phosphate and it has been proposed that this is one of the mechanisms that controls primary productivity and algal growth in this lagoon. The mechanisms of phosphate mobilization were studied by using sediment slurries. These experiments demonstrated that not only ferric iron but presumably also calcium was responsible for phosphate binding. Another effect of the high iron content in the Bassin d'Arcachon was the precipitation of sulfide as iron sulfide or pyrite. In the Etang du Prevost sulfate reduction resulted in the accumulation of free sulfide. The relative low content of iron in Etang du Prevost not only allowed the formation of free sulfide but may also have limited the binding capacity of phosphate in the sediment. On the other hand sulfate reduction was not important for the release of phosphate from the sediment. In Etang du Prevost primary productivity is nitrogen rather than phosphorus limited. In contrast in the Bassin d'Arcachon primary productivity was presumably mostly phosphate limited. In Etang du Prevost the non-heterocystous cyanobacterium Oscillatoria sp. was the dominant nitrogen-fixing species. Heterocystous species were excluded from this lagoon as a result of the presence of free sulfide. It was demonstrated that heterocystous cyanobacteria are more sensitive towards sulfide than non-heterocystous species. The absence of free sulfide explained the presence of the heterocystous cyanobacterium Anabaena sp. in Bassin d'Arcachon.

Response of an exopolysaccharide-producing heterocystous cyanobacterium to changes in metabolic carbon flux

The response of the exopolysaccharide-producing heterocystous cyanobacteriumCyanospira capsulata to changes in metabolic carbon flux was investigated to estimate the potential for improvement of the exopolysaccharide (EPS) yield. Carbon flux was altered by transferring the organism either to an argon atmosphere or to medium containing the nitrogen assimilation inhibitors L-methionine-D,L-sulfoximine (MSX), O-diazoacetyl-L-serine (AZAS) or D,L-7-azatryptophan (AZAT), or by adding glyoxylate, known to stimulate carbon metabolism. When carbon flux was modified by interfering with nitrogen metabolism, the concentration of total carbohydrates exceeded that of the control culture only in Ar- or AZAS-treated cell suspensions, but this difference was mainly due to enhancement of the quantity of bound carbohydrates. On the other hand, when carbon flux was modified by a single addition of glyoxylate (30 mM) or by daily additions of 10 mM glyoxylate without interfering with nitrogen metabolism, carbohydrate release into the medium was stimulated markedly; after 5 days of growth in the presence of the organic compound, the concentration of EPS was 43 % higher than in the control culture. The results demonstrate that, with enhanced carbon flux, the excess carbon is preferentially channeled byC. capsulata cells into the synthesis of an overflow product like EPS, whereas, with mere diversion of carbon flux from the process of nitrogen assimilation, the synthesis of carbon reserves is more heavily favoured.

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

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

First data on epilithic heterocystous cyanobacteria from Roman hypogea

Abstract Epilithic species of heterocystous cyanobacteria were isolated from Roman hypogea, grown at low photon flux densities and morphologically characterized. Determination of photosynthesis versus irradiance curves and nitrogen fixation activities on three of these strains, showed that their photosynthetic efficiency was low but sufficient to sustain nitrogen fixation.

A comparison of gene organization in thezwfregion of the genomes of the cyanobacteriaSynechococcussp. PCC 7942 andAnabaenasp. PCC 7120

The region of the genome encoding the glucose-6-phosphate dehydrogenase gene zwf was analysed in a unicellular cyanobacterium, Synechococcus sp. PCC 7942, and a filamentous, heterocystous cyanobacterium, Anabaena sp. PCC 7120. Comparison of cyanobacterial zwf sequences revealed the presence of two absolutely conserved cysteine residues which may be implicated in the light/dark control of enzyme activity. The presence in both strains of a gene fbp, encoding fructose-1,6-bisphosphatase, upstream from zwf strongly suggests that the oxidative pentose phosphate pathway in these organisms may function to completely oxidize glucose 6-phosphate to CO2. The amino acid sequence of fructose-1,6-bisphosphatase does not support the idea of its light activation by a thiol/disulfide exchange mechanism. In the case of Anabaena sp. PCC 7120, the tal gene, encoding transaldolase, lies between zwf and fbp.

A comparison of gene organization in the zwf region of the genomes of the cyanobacteria Synechococcus sp. PCC 7942 and Anabaena sp. PCC 7120

The region of the genome encoding the glucose-6-phosphate dehydrogenase gene zwf was analysed in a unicellular cyanobacterium, Synechococcus sp. PCC 7942, and a filamentous, heterocystous cyanobacterium, Anabaena sp. PCC 7120. Comparison of cyanobacterial zwf sequences revealed the presence of two absolutely conserved cysteine residues which may be implicated in the light/dark control of enzyme activity. The presence in both strains of a gene fbp, encoding fructose-1,6-bisphosphatase, upstream from zwf strongly suggests that the oxidative pentose phosphate pathway in these organisms may function to completely oxidize glucose 6-phosphate to CO 2. The amino acid sequence of fructose-1,6-bisphosphatase does not support the idea of its light activation by a thiol/disulfide exchange mechanism. In the case of Anabaena sp. PCC 7120, the tal gene, encoding transaldolase, lies between zwf and fbp.

Physiological ecology of cyanobacteria in microbial mats and other communities.

In this review some aspects of the physiological ecology of cyanobacteria are discussed by taking a microbial mat as an example. The majority of microbial mats are built and dominated by cyarsobacteria which are primary producers at the basis of the microbial foodweb in microbial mats. These micro-scale ecosystems are characterized by steep and fluctuating physico-chemical gradients of which those of light, oxygen and sulphide are the most conspicuous. Light is strongly attenuated in the sediment, and owing to constant sedimentation, the mat-forming cyanobacteria have to move upwards towards the light. However, at the sediment surface, light intensity, particularly in the u.v. part of the spectrum, is often deleterious. The gliding movement of the cyanobacteria, with photo- and chemotaxis, allows the organism to position itself in a thin layer at optimal conditions. The organic matter produced by cyanobacterial photosynthesis is decomposed by the ruicrobial community. Sulphate-reducing bacteria are important in the end-oxidation of the organic matter. These organisms are obligate anaerobes and produce sulphide. Gradients of sulphide and oxygen move up and down in the sediment as a response to diurnal variations of light intensity. Cyanobacteria, therefore, are sometimes exposed to large concentrations of the extremely toxic sulphide. Some species are capable of sulphide-dependent anoxygenic photosynthesis. Other cyanobacteria show increased rates of oxygenic photosynthesis in the presence of sulphide and have mechanisms to oxidize sulphide while avoiding sulphide toxicity. Iron might play an important role in this process. Under anoxic conditions in the dark, mat-forming cyanobacteria switch to fermentative metabolism. Many species are also capable of fermentative reduction of elemental sulphur to sulphide. The gradients of sulphide and oxygen are of particular importance for nitrogen fixation. Very few microbial mats are formed by heterocystous cyanobacteria, which are best adapted to diazntrophic growth. However, these organisms probably cannot tolerate greater concentrations of sulphide or anoxic conditions or both. Under such conditions non-heterocystous cyanobacteria become dominant as diazotrophs. These organisms avoid conditions of oxygen supersaturation. In the ecosystem, nitrogen fixation and photosynthesis might be separated temporally as well as spatially. In addition, non-heterocystous diazotrophic cyanobacteria have mechanisms at the subcellular level to protect the oxygen-sensitive nitrogenase from inaction. CONTENTS Summary 1 I. Introduction 2 II. Microbial mats 3 III. Cyanobacteria in light gradients 7 IV. Dark metabolism 10 V. Interactions with sulphide 13 VI. Nitrogen fixation 16 VII. References 28.

Effects of Light and CO 2 on Nitrite Liberation by the Heterocystous Cyanobacterium Anabaena variabilis

Summary Air-grown cultures of the heterocystous cyanobacterium Anabaena variabilis ATCC 29413, when incubated under continuous illumination in the presence of nitrate, liberated nitrite at a low rates during growth. Ammonium liberation was also observed but at a very low level. Cultures that were shifted to a higher light intensity showed increased rates of nitrite liberation. Increasing the CO 2 supply (from 0.035 % to 5 %, v/v) induced rapid nitrite uptake, irrespective of whether cultures were incubated in the light or in the dark, and stimulated photosynthetic O 2 evolution, glutamine synthetase activity, and nitrogenase activity. Results indicate that CO 2 availability is an important factor to the control of nitrite asimilation and are consistent with regulatory interactions involving products of CO 2 and nitrogen asimilation. Although nitrite accumlated up to 0.25 mM in A. variabilis cultures tested, nitrite toxicity was detected at concentrations higher than 0.6 mM. These results may be relevant to environments where cyanobacteria are subjected to diurnal, seasonal, and geographical variations of CO 2 levels and illumination rates.

Rearrangements of nitrogen fixation (nif) genes in the heterocystous cyanobacteria

In the vegetative cells of heterocystous cyanobacteria, such asAnabaena, two Operons harbouring the nitrogen fixaton (nif) genes contain two separate intervening DNA elements resulting in the dispersion of genes and impaired gene expression. A 11 kb element disrupts thenifD gene in thenifH, D-K operon. It contains a 11 bp sequence (GGATTACTCCG) directly repeated at its ends and harbours a gene,xisA, which encodes a site-specific recombinase. A large 55 kb element interrupts thefdxN gene in thenifB fdxN-nifS-nifU operon. It contains two 5 bp direct repeats (TATTC) at its ends and accommodates at least one gene,xisF, which encodes another site-specific recombinase. During heterocyst differentiation both the discontinuities are precisely excised by two distinct site-specific recombination events. One of them is brought about by the XisA protein between the 11 bp direct repeats. The second one is caused by the XisF protein and occurs between the 5 bp direct repeats. As a consequence the 11kb and 55 kb elements are removed from the chromosome as circles and functionalnif Operons are created. Nitrogenase proteins are then expressed from the rearranged genes in heterocysts and aerobic nitrogen fixation ensues. How these elements intruded thenif genes and how and why are they maintained in heterocystous cyanobacteria are exciting puzzles engaging considerable research effort currently. The unique developmental regulation of these gene rearrangements in heterocystous cyanobacteria is discussed.

Impact of Nutrients and Physical Factors on the Shift from Cyanobacterial to Chlorophyte Dominance in Shallow Danish Lakes

Phytoplankton dominance (as biomass) by heterocystous cyanobacteria, nonheterocystous cyanobacteria, and chlorophytes was studied along a trophic gradient (0.011–2.2 mg P∙L−1) by analyzing regularly collected semiquantitative data from 178 shallow Danish lakes (mean depth < 3 m) and quantitative data from 32 lakes. Heterocystous cyanobacteria were dominant at low total P (TP) (< 0.25 mg P∙L−1) and nonheterocystous cyanobacteria at intermediate TP (0.25–0.8 mg P∙L−1), while chlorophytes often were dominant at high TP (> 1 mg P∙L−1). In contrast with many earlier findings, heterocystous cyanobacteria were not dominant at low total N (TN):TP or low inorganic N concentrations; chlorophytes were dominant at extremely high pH, and the shift from cyanobacterial to chlorophyte dominance could not be explained by a change in the photic zone to mixing zone ratio. We suggest that chlorophyte dominance in hypertrophic shallow lakes is attributable to continuous input of nutrients and carbon from the sediment and external sources. This renders the fast-growing chlorophytes a superior competitor compared with the relatively slow-growing cyanobacteria, even when inorganic nutrient concentration is low and pH high. New predictive models relating phytoplankton dominance to TP in shallow lakes were developed, as former models failed to predict our observations satisfactorily.

Photosystem II genes isiA, psbDI and psbC in Anabaena sp. PCC 7120: cloning, sequencing and the transcriptional regulation in iron-stressed and iron-repleted cells

Under conditions of iron deprivation cyanobacteria produce flavodoxin to replace ferredoxin as the terminal electron acceptor of photosynthesis. In unicellular cyanobacteria, the gene for flavodoxin is the second open reading frame in a dicistronic operon whose transcription is tightly regulated by iron. The first gene, isiA, produces a protein that is very similar to CP43, a chlorophyll-binding, antenna protein of the photosystem II reaction center. In the filamentous, heterocystous cyanobacterium Anabaena sp. PCC 7120, isiA and the gene for flavodoxin are located in separate operons with independent promoters. In this paper, we report on the sequence of isiA and show that it is found in a monocistronic operon that is transcriptionally regulated to be expressed under iron stress but does not produce detectable transcripts under conditions of iron repletion. We also report on the sequence, organization and expression of the gene that codes for CP43, psbC. In Anabaena sp. PCC 7120, psbC has a genetic organization similar to that of other cyanobacteria and higher plants; the 5' end of psbC overlaps the 3' end of psbDI. Transcriptional analysis of the psbDC operon showed that it is constitutively expressed in both iron-repleted and iron-stressed conditions; however, a new monocistronic transcript was detected that contains psbC and is preferentially expressed under iron stress conditions.

Turbulence and planktonic nitrogen fixation: A mesocosm experiment

Heterocystic species of N‐fixing cyanobacteria are rare in the plankton of most estuaries and coastal seas, and rates of N fixation tend to be concomitantly low in the water columns of these systems. High turbulence in estuaries and coastal seas is commonly cited as a factor excluding planktonic, heterocystic cyanobacteria. We investigated the effects of turbulence in a 7‐week‐long mesocosm experiment. Reciprocating grids generated turbulence in 3.0‐m3 tanks containing water and plankton from Oneida Lake. Four mesocosms received a high level of turbulence (mean values of 0.2 and 0.7 mW kg−1 as determined by two different methods); such turbulence is much higher than has been observed in lakes and is at the upper end of that found in estuaries strongly influenced by tides. Four other tanks received levels of turbulence that were higher yet (mean estimates of 1.8 and 3.8 mW kg−1 from the two methods); this higher level of turbulence probably exceeds that found in coastal waters except in areas influenced by surf or in shallow estuaries with very significant tides. Heterocystic, planktonic cyanobacteria bloomed and fixed N in all eight mesocosms, and turbulence had no significant effect on N fixation. Rates were quite high, indicating no major deleterious effect of even extremely high turbulence on this process. The turbulence in our mesocosms was great enough to have prevented the formation of anoxic microzones on the surface of cyanobacterial cells. Thus, our results indicate that such microzones are not essential for high rates of planktonic N fixation by heterocystic cyanobacteria and that the relative absence of N fixation by these organisms in coastal seas is not due to a lower probability of such microzones developing in marine systems than in freshwaters.

Carbon and nitrogen dynamics in a maritime Antarctic stream

SUMMARY The carbon and nitrogen dynamics in a maritime Antarctic lake outflow stream were investigated. The stream and the algal communities could be split into two zones: a semi‐aquatic margin consisting of a perennial cyanobacteria/diatom mat and a flowing channel with a similar perennial mat that was overgrown by annual filamentous chlorophytes during the course of the summer. Neither algal community was limited by nutrient availability. Major nutrients were always available in the stream water. There were slight differences in the atomic ratios of the mats, the N:P ratios in the channel mat being lower than those in the marginal mat. However, both these and the total dissolved N:P ratio in the stream water were all close to those that indicate a balanced supply. There was no net carbon or nitrogen accumulation by the marginal mat suggesting that uptake processes were balanced by loss processes. Maximum rates of carbon fixation (0.1–0.5mgCg−1 dry weight h−1) were similar to those of other perennial Antarctic algal mats. Productivity appeared to be limited by physical factors, but the effects of irradiance and temperature could not be separated. There were no heterocystous cyanobacteria in the mat communities and rates of atmospheric nitrogen fixation were very low (0–10ngNmg−1 mat Nh−1). Fixation accounted for only 0.3% of the nitrogen accumulation of the channel mats, but was higher in the marginal mat where uptake of other sources of nitrogen was also low. Nitrogen accumulation by the channel mat averaged 0.34gNm−2 day−1. Only 0.05gNm−2 day−1 was accounted for by the uptake of dissolved inorganic nitrogen (nitrate plus ammonium). The major (80%) source of nitrogen appeared to be dissolved organic nitrogen. Recycling of nitrogen within the stream ecosystem may also be important.