Nitrogen fixation in subarctic streams influenced by beaver (Castor canadensis)

Nitrogen fixation was measured in four subarctic streams substantially modified by beaver (Castor canadensis) in Quebec. Acetylene-ethylene (C₂H₂ C₂H₄) reduction techniques were used during the 1982 ice-free period (May–October) to estimate nitrogen fixation by microorganisms colonizing wood and sediment. Mean seasonal fixation rates were low and patchy, ranging from zero to 2.3 × 10^–3 µmol C₂H₄ · cm^–2 · h^–1 for wood, and from zero to 7.0 × 10^–3 µmol C₂H₄ · g AFDM^–1 · h^–1 for sediment; 77% of all wood and 63% of all sediment measurements showed no C₂H₂ reduction. Nonparametric statistical tests were unable to show a significant difference (p > 0.05) in C₂H₂ reduction rates between or within sites for wood species or by sediment depth.Nitrogen contributed by microorganisms colonizing wood in riffles of beaver influenced watersheds was small (e.g., 0.207 g N · m^–2 · y^–1) but greater than that for wood in beaver ponds (e.g., 0.008 g N · m^–2 · y^–1) or for streams without beaver (e.g., 0.003 g N · m^–2 · y^–1). Although mass specific nitrogen fixation rates did not change significantly as beaver transform riffles into ponds, the nitrogen fixed by organisms colonizing sediment in pond areas (e.g., 5.1 g N · m^–2 · y^–1) was greater than that in riffles (e.g., 0.42 g N · m^–2 · y^–1). The annual nitrogen contribution is proportional to the amount of sediment available for microbial colonization. We estimate that total nitrogen accumulation in sediment, per unit area, is enhanced 9 to 44 fold by beaver damming a section of stream.     

Nitrogen fixation by Nodularia spumigena Mertens (Cyanobacteriaceae). 2: Laboratory studies

The effects of changes in diurnal light patterns, salinity, and phosphorus on nitrogen fixation (as measured by acetylene reduction) by Nodularia spumigena Mertens were examined. As well, the effects of added inorganic nitrogen on growth, nitrogen fixation and heterocyt frequencies, and changes in nitrogen fixation and heterocyst frequencies during the growth cycle of Nodularia in cultures were determined.The diurnal pattern of nitrogenase activity in Nodularia was primarily light-induced, though dark activity did occur. Nitrogenase activity following a period of darkness exceeded the normal light rate (> 90 compared to 50 nmol · C₂H₂ reduced · ml^–1 · h^–1). Nitrogen fixation was reduced by high and very low salinities (5 to 10 was the optimum range), and added phosphorus stimulated nitrogenase in P-starved cells. Added nitrogen (ammonium or nitrate) had no effect on the growth of Nodularia, but in short term studies, ammonium completely inhibited nitrogenase activity. Heterocyst frequencies were greatest in the log phase of growth (to 40 per mm). During stationary phase, nitrogenase activity was negligable.     

Biphasic nitrogenase activity in Azospirillum brasilense in long lasting batch cultures

Long lasting batch cultures of Azospirillum brasilense SP 7 ATCC 29145 grown in liquid malate medium for 8–14 days without any fixed nitrogen source exhibited a biphasic nitrogenase activity, when incubated under gas atmospheres of 99.0% N₂ and 1.0% O₂ or 99.5% N₂ and 0.5% O₂ respectively. Maximum specific nitrogenase activity was 1100 nmol C₂H₄·mg protein^-1·h^-1. Poly-3-hydroxybutanoic acid (PHBA) synthesis and growth of the cells also showed two phases. Maxima and minima of glutamine synthetase activity developed synchronously with nitrogenase activity, whereas those of glutamate dehydrogenase and alanine aminotransferase were reverse. During a 192 h period of growth protein increased 3–4-fold and PHBA 25 fold. At maximum accumulation of the polymer the PHBA-nitrogen ratio was 6:1 or 8:1. Azospirillum brasilense was also able to fix nitrogen on agar surfaces exposed to air, but nitrogen fixation was monophasic under these conditions during a 14 d period. Specific nitrogenase activity was dependent on the type and concentration of the source of fixed nitrogen (leucine, ammonia) in solidified media. With 1 mM leucine maximum specific nitrogenase activity was 110 nmol C₂H₄·mg protein^-1·h^-1.Non-Standard Abbreviations PHBA poly-3-hydroxybutanoic acid - TAPS tris(hydroxymethyl)methylaminopropane sulfonic acid - TES N-tris(hydroxymethyl)methyl-2-aminoethane sulfonic acid - TRICINE N-tris(hydroxymethyl)methylglycine - TRIS tris(hydroxymethyl)aminomethane     

Nitrogen fixation by periphyton and plankton on the Amazon floodplain at Lake Calado

Nitrogen fixation by periphyton and plankton was measured on the Amazon flood-plain using the acetylene reduction method calibrated with¹⁵N-N₂. The average ratio (± SD) of moles C₂H₄ reduced per mole N₂-N fixed was 3.4 ± 0.7, similar to other studies. Periphyton and plankton had high rates of light-dependent nitrogen fixation, with dark nitrogen fixation averaging 26% of the average rates in the light. The average daily (24 h) rates for periphyton nitrogen fixation in 1989 and 1990 were 1.79 and 0.51 mmol N₂-N·m^–2·d^–1 respectively, which are comparable to summer rates in many temperate cyanobacterial assemblages. Nitrogen fixation was depressed at N0₃ ^– concentrations as low as 0.5 M, and was below detection limits at concentrations of 4 M, which occurred during periods of river flooding. Planktonic nitrogen fixation rates were high (0.5–0.8 mmol N₂-N·m^–2·d^–1) during the high-water and drainage phases of the annual hydrograph when the floodplain waters were draining towards the river (low NO₃ ^–), but rates were undetectable (< 0.05 mmol N₂-N·m^–2·d^–1) when there was river flooding (high NO₃ ^–). Nitrogen fixation by periphyton and plankton in 1989–1990 accounted for approximately 8% of previously reported total annual nitrogen inputs to the floodplain at Lake Calado.     

The Utricularia-Cyanophyta association and its nitrogen-fixing capacity

Utricularia inflexa Forsk. growing in a shallow lake near Dar es Salaam, Tanzania was found to be abundantly associated with blue-green algae and other microorganisms, though blue-green algae were not found in the lake water. The epiphytes occurred both on the outer surfaces of the macrophyte and inside its traps. Seven genera of the Cyanophyta were observed, amongst which Anabaena was the most abundantly and consistently associated.Both epiphytes on the outer surfaces of the macrophyte and those inside mature traps fixed nitrogen in situ. Mature parts of the macrophyte fixed nitrogen at higher rates than immature parts, indicating the former was associated with a greater abundance of nitrogen fixers. The Utricularia-Cyanophyta association showed a nitrogen fixation (acetylene reduction) rate of 2012 nmol C₂H₄ g^–1 dry weight h^–1 or, in terms of lake area, of 4 500 nmol C₂H₄ m^–2 h^–1. The relative contributions of blue-green algae and bacteria were not determined.It is proposed that the association may involve a degree of physiological interdependence and that the association has potential as a biofertilizer for rice.Based on an M.Sc. thesis by G. M. Wagner submitted to the Department of Botany, University of Dar es Salaam, 1983.Based on an M.Sc. thesis by G. M. Wagner submitted to the Department of Botany, University of Dar es Salaam, 1983.     

Nitrogen fixation by the woody legumeLeucaena leucocephala in Tanzania

Summary The nitrogen fixation rate in a 4-year-old stand of the woody legumeLeucaena leucocephala (Lam.) de Wit. was estimated in the field at a rather dry site in Tanzania by use of an acetylene reduction technique. The diurnal mean value during April–May was 35 nmol C₂H₄ mg^–1 (dry weight) nodules h^–1, with a variation between 22±8 and 48±12 nmol C₂H₄ mg^–1 (dry weight) nodules h^–1 in early morning and at midday, respectively. The nodule biomass was determined by auger sampling to be 51±16 kg (dry weight) ha^–1. Most of the nodules were found at the 10–30 cm soil depth level. A rough calculation of the amount of nitrogen fixed annually arrived at 110±30 kg ha^–1. The results give strong support for the use ofL. leucocephala for soil enrichment in less humid areas of tropical Africa.     

Photosynthetic production of hydrogen peroxide by Ulva rigida C. Ag. (Chlorophyta)

Production of hydrogen peroxide has been found in Ulva rigida (Chlorophyta). The formation of H₂O₂ was light dependent with a production of 1.2 mol·g FW^–1·h^–1 in sea water (pH 8.2) at an irradiance of 700 mol photons m^–2·s^–1. The excretion was also pH dependent: in pH 6.5 the production was not detectable (< 5 nmol·g FW^–1·h^–1) but at pH 9.0 the production was 5.0 mol·g FW^–1·h^–1. The production of H₂O₂ was totally inhibited by 3-(3,4-dichlorophenyl)-1,1 dimethylurea (DCMU). The ability of U. rigida growing in tanks (7501) under a natural light regime to excrete H₂O₂ was checked and found to be seven times higher at 08.00 hours than other times of the day. The H₂O₂ concentration in the cultivation tank (density: 2 g FW·l^–1) reached the highest value (3 M) at 11.00 hours. Photosynthesis was not influenced by H₂O₂ formation. The H₂O₂ is suggested to come from the Mehler reaction (pseudocyclic photophosphorylation). With an oxygen evolution of 120 mmol·g FW^–1·h^–1 at pH 8.2 and 90 mmol·g FW^–1·h^–1 at pH 9.0, 0.5% and 2.7% of the electrons were used for extracellular H₂O₂ production. The H₂O₂ production is sufficiently high to be of physiological and ecological significance, and is suggested to be a part of the defence against epi and endophytes.Abbreviations ACL artificial, continuous light - DCMU 3-(3,4-dichlorophenyl)-1,1-dimethylurea - GNL greenhouse - LDC Luminol-dependent chemiluminescence - SOD Superoxide dismutase This investigation was supported by SAREC (Swedish Agency for Research Cooperation with Developing Countries), Hierta-Retzius Foundation, Marianne and Marcus Wallenberg Foundation, the Swedish Environmental Protection Board, and CICYT Spain.     

A function of cyanobacterial mats in phosphorus-limited tropical wetlands

Cyanobacterial mats are important components of oligotrophic wetland ecosystems in the limestone-based regions of the Caribbean. Our goals were to: (1) Estimate the biomass and primary production of cyanobacterial mats, quantify the extent of nitrogen fixation and measure the activity of alkaline phosphatase (APA) in representative marshes of northern Belize; (2) Record changes in these variables following nutrient additions. The mat biomass ranged from 200 to 700 g m^–2 AFDM, with the epipelon contributing up to 87% of the total. Tissue nitrogen was similar in all marshes (1.1–1.5%), while tissue phosphorus was extremely low (0.0055–0.0129%) and well correlated with the N:P ratio in water. Nitrogen fixation expressed as nitrogenase activity was high in some marshes (17.5 nmol C₂H₄ cm^–2 h^–1) and low (< 5 nmol C₂H₄ cm^–2 h^–1) in others depending mainly on the proportion of heterocyst-forming cyanobacteria (Nostocales, Stigonematales) in the mat. Alkaline phosphatase activity was positively correlated with the N:P ratio of the mat. Experimental addition of phosphorus resulted in significant increase in primary production and nitrogen fixation while it suppressed the APA activity. The presented data clearly showed that oligotrophic marshes of northern Belize are strongly P limited. Increased input of phosphorus would profoundly change their structure and functions.     

Aerobic nitrate and nitrite reduction in continuous cultures of Escherichia coli E4

Nitrate and nitrite was reduced by Escherichia coli E4 in a l-lactate (5 mM) limited culture in a chemostat operated at dissolved oxygen concentrations corresponding to 90–100% air saturation. Nitrate reductase and nitrite reductase activity was regulated by the growth rate, and oxygen and nitrate concentrations. At a low growth rate (0.11 h^–1) nitrate and nitrite reductase activities of 200 nmol · mg^–1 protein · min^–1 and 250 nmol · mg^–1 protein · min^–1 were measured, respectively. At a high growth rate (0.55 h^–1) both enzyme activities were considerably lower (25 and 12 nmol mg^–1 · protein · min^–1). The steady state nitrite concentration in the chemostat was controlled by the combined action of the nitrate and nitrite reductase. Both nitrate and nitrite reductase activity were inversely proportional to the growth rate. The nitrite reductase activity decreased faster with growth rate than the nitrate reductase. The chemostat biomass concentration of E. coli E4, with ammonium either solely or combined with nitrate as a source of nitrogen, remained constant throughout all growth rates and was not affected by nitrite concentrations. Contrary to batch, E. coli E4 was able to grow in continuous cultures on nitrate as the sole source of nitrogen. When cultivated with nitrate as the sole source of nitrogen the chemostat biomass concentration is related to the activity of nitrate and nitrite reductase and hence, inversely proportional to growth rate.     

Relationship of cellular phosphorus in the cyanobacterium Nodularia to phosphorus availability in the Peel-Harvey Estuarine System

A laboratory-determined critical concentration of phosphorus in the cyanobacterium Nodularia spumigena was used to assess the phosphorus sufficiency of field populations in the Peel-Harvey Estuarine System, Western Australia. Nutrient addition bioassays indicated that phosphorus was the limiting nutrient. Since river flow, and thus phosphorus input, to the estuary ceases 1–2 months before the onset of Nodularia blooms, the phosphorus contained in these blooms was derived from sediment release and nutrient recycling. A progressive increase in the mean cellular phosphorus concentration of Nodularia from 1981 to 1983 indicated that sediments were releasing a greater amount of phophorus each year. Although evidence suggests that the biomass of recent Nodularia blooms is now limited by physical factors, the additional phosphorus contained in each bloom will increase the supply of phosphorus to other organisms. Cellular phosphorus concentrations in Nodularia appeared to be correlated with river phosphorus loadings in the Peel Inlet but not in the Harvey Estuary.     

The influence of H2, CO2 and dilution rate on the continuous fermentation of acetone-butanol

Summary The effect of product gases, H₂ and CO₂, on solvent production was studied using a continuous culture of alginate-immobilized Clostridium acetobutylicum. Initially, in order to find the optimum dilution rate for aceton--butanol production in this system, fermentations were carried out at various dilution rates. With 10% H₂ and 10% CO₂ in the sparging gas, a dilution rate of 0.07 h^–1 was found to maximize volumetric productivity (0.58 g·l^–1·h^–1), while the maximum specific productivity of 0.27 g^–·h^–1 occured at 0.12 h^–1. Continuous cultures with vigorous sparging of N₂ produced only acids. It was concluded that in the case of continuous fermentation H₂ is essential for good solvent production, although good solvent production is possible in an H₂-absent environment in the case of batch fermentations. When the fermentation was carried out at atmospheric pressure under H₂-enriched conditions, the presence of CO₂ in the sparging gas did not slow down glucose metabolism; rather it changed the direction of the phosphoroclastic reaction and as a result increased the butanol/acetone ratio.     

The effect of carbon dioxide on the growth kinetics of fructose-limited chemostat cultures of Acetobacterium woodii DSM 1030

The growth of the anaerobic acetogenic bacterium Acetobacterium woodii DSM 1030 was investigated in fructose-limited chemostat cultures. A defined medium was developed which contained fructose, mineral salts, cysteine · HCl and Ca pantothenate (1 mg · 1^–1) supplied in a vitamin supplement. Growth at high dilution rates was dependent on the presence of CO₂ in the gas phase. The ^max was found to be 0.16 h^–1 and the fructose maintenance requirement was 0.1 to 0.13 mmol fructose · (g dry wt)^–1 · h^–1. A growth yield of 61 g dry wt · (mol fructose)^–1, corrected for the cell maintenance requirement and for incorporation of fructose carbon into cell biomass, was determined from the fructose consumption. A corresponding growth yield of 69 g dry wt · (mol fructose)^–1 was calculated from the acetate production assuming that fructose fermentation was homoacetogenic. A Y_ATP of 12.2 to 13.8 g dry wt · (mol ATP)^–1 was calculated from these growth yields using a value of 5 mol ATP · (mol fructose)^–1 as an estimate of the amount of ATP synthesised from fructose fermentation. The addition of yeast extract (0.5 g · 1^–1) to the medium did not influence the ^max or cell yield. After prolonged growth under fructose-limited conditions the requirement of the culture for CO₂ in the gas phase was reduced.Abbreviations YE yeast extract - IC inorganic carbon - D fermenter dilution rate : h^–1 - M_X maintenance requirement for X: mmol X · (g dry wt)^–1 · h^–1 - X may be fructose (Fruct), fructose consumed in energy metabolism (Fruct [E]), acetate (Ac) - ATP CO₂, NH _inf4 ^sup+ or Pi - qX specific rate of utilisation or consumption of X: mmol X · (g dry wt)^–1 · h^–1 - V fermenter volume: litre - rC · Cell, fermenter cell carbon production: mmol C · h^–1 - Y_X yield of cells on X: g dry wt · (mol X)^–1 - Y _infx ^supmax the yield corrected for cell maintenance: g dry wt · (mol X)^–1 - S_ATP stoichiometry of ATP synthesis from fructose: mol ATP · (mol frucose)^–1 - x cell concentration: g dry wt · 1^–1 - specific growth rate : h^–1 - ^max maximum specific growth rate: h^–1     

Nitrogenase activity in Rhodopseudomonas sulfidophila

The marine purple nonsulfur bacterium, Rhodopseudomonas sulfidophila, strain W4, was capable of photosynthetic growth on dinitrogen and malate. Higher growth rates were observed when either glutamate or ammonia replaced dinitrogen as nitrogen source and when bicarbonate was omitted from the culture medium. Although ammonia was released from cells growing on malate and N₂, no nitrogenase activity could be detected unless -ketoglutarate was added to the culture medium. No nitrogenase activity was found in cultures grown in the presence of NH ₄ ⁺ . In cultures grown on glutamate as nitrogen source, nitrogenase and hydrogenase activities were found to be 5.4 nmol C₂H₂ reduced · min^-1 · mg^-1 dry weight and 50 nmol methylene blue reduced · min^-1 · mg^-1 dry weight respectively. Such activities are significantly lower than those observed for other members of the Rhodospirillaceae e.g. Rhodopseudomonas capsulata. However, the hydrogenase activity would be sufficient to recycle all H₂ produced by nitrogenase. It was indeed observed that growing cells did not evolve molecular hydrogen during photoheterotrophic growth and that H₂ stimulated nitrogenase activity in resting cells of R. sulfidophila. The nitrogenase from this bacterium proved to be extremely sensitive to low concentrations of oxygen, half-inhibition occurring at between 1–1.5% O₂ in the gas phase, depending on the bacterial concentration. Light was essential for nitrogenase activity. No activity was found during growth in the dark under extremely low oxygen concentrations (1–2% O₂), which are still sufficient to support good growth. Resting cell suspensions prepared from such cultures were unable to reduce acetylene upon illumination. Optimum nitrogenase activities were broadly defined over the temperature range, 30–38°C, and between pH 6.9 and 8.0. The results are discussed in comparison with the non-marine purple nonsulfur bacterium, R. capsulata, which somewhat resembles R. sulfidophila.     

Benthic community metabolism of three Austrian pre-alpine lakes of different trophic conditions and its oxygen dependency

Benthic community respiration rates of profundal sediments of Fuschlsee (37.6 mg · O₂ · m^–2 · h^–1 — eutrophic), Mondsee (40.19 mg · O₂ · m^–2 · h^–1 — eutrophic) and Attersee (11.5 mg · O₂ · m^–2 · h^–1 — oligo-mesotrophic) were measuredin situ, and in cores. By exposing the sediments to different oxygen levels in the laboratory it was found that benthic community metabolism reduced with decreasing oxygen concentrations. The slope of the regression lines, relating oxygen uptake rates to oxygen concentrations, differed significantly for the different sites investigated. These results were closely related to the trophic conditions of the lakes.     

Morphology,physiology and infectivity of twoFrankia isolates An 1 and An 2 from root nodules ofAlnus nitida

Summary Two different strains, An 1 and An 2, were obtained from root nodules ofAlnus nitida Endl., collected from one locality in the area of its natural habitat near Bahrin, District Swat, Pakistan. The light and electron microscopy of the isolates revealed the occurrence of septate and branched hyphae bearing sporangia and vesicles. The strains differed in their growth requirements, nitrogen-fixing ability and production of extracellular pigments, thus indicating the existence of more than oneFrankia strain in the same locality. In the absence of combined nitrogen in the medium strain An 1 formed vesicles and fixed N₂ (up to 200 nmol C₂H₄. mg protein^–1.h^–1), while strain An 2 under the experimental conditions formed only few vesicles and fixed N₂ at a very low rate (ca 10 nmol C₂H₄. mg protein^–1 .h^–1). The nitrogenase activity of strain An 1 was strongly affected by the O₂ concentration.Frankia An 1 and An 2 were infective and effective onA. nitida andA. glutinosa but not onDatisca cannabina andElaeagnus umbellata. Both An 1 and An 2 strains were more infective and effective onA. glutinosa thanFrankia strains AvcIl and CpI1.     

A comparative study of nitrogen fixation by the Anabaena-Azolla symbiosis and free-living populations of Anabaena spp. in Lake Ngahawa,New Zealand

Summary The symbiotic fern Azolla filiculoides var. rubra, which contains a blue-green nitrogen fixing alga Anabaena azollae, fixed 164 Kg N·ha^-1·ann^-1 in the littoral zone of a small eutrophic lake. Associated planktonic Anabaena spp. blooms, dominated by Anabaena spiroides, fixed 29.5Kg N·ha^-1·ann^-1. Nitrogen fixation in both organisms was not obviously related to ambient dissolved inorganic nitrogen levels. By comparing ¹⁵N–N₂ and acetylene reduction techniques, we determined a ratio of 3 moles C₂H₂ reduced to 1 mole of N₂ fixed. Combining this with results from one diurnal investigation, it was estimated that 24% of the total daily fixation by Azolla occurred at night. Highest nitrogen fixation rates in Azolla occurred when plant density was lowest. Nitrogen fixation by planktonic Anabaena spp. generally paralleled changes in biomass. Frond breakage due to wind caused a decrease in Azolla nitrogen fixation and growth which was followed by a bloom of planktonic Anabaena spp. A second Anabaena spp. bloom was instrumental in the summer decline of Azolla. Maximum growth and nitrogen fixation of both organisms did not occur simultaneously. If physical disruption to the Azolla mat does not occur, it is likely that growth of the population would continue throughout the year.This work was completed at the Department of Scientific and Industrial Research, Freshwater Section, PO Box 415, Taupo, New Zealand, with partial assistance of N.S.F. Grant BMS-74-20745 to C.R. Goldman     

Nitrogen Fixation Associated with the New Zealand Mangrove (Avicennia marina (Forsk.) Vierh. var. resinifera (Forst. f.) Bakh.)

Nitrogenase activity in mangrove forests at two locations in the North Island, New Zealand, was measured by acetylene reduction and ¹⁵N₂ uptake. Nitrogenase activity (C₂H₂ reduction) in surface sediments 0 to 10 mm deep was highly correlated (r = 0.91, n = 17) with the dry weight of decomposing particulate organic matter in the sediment and was independent of light. The activity was not correlated with the dry weight of roots in the top 10 mm of sediment (r = −0.01, n = 13). Seasonal and sample variation in acetylene reduction rates ranged from 0.4 to 50.0 μmol of C₂H₄ m⁻² h⁻¹ under air, and acetylene reduction was depressed in anaerobic atmospheres. Nitrogen fixation rates of decomposing leaves from the surface measured by ¹⁵N₂ uptake ranged from 5.1 to 7.8 nmol of N₂ g (dry weight)⁻¹ h⁻¹, and the mean molar ratio of acetylene reduced to nitrogen fixed was 4.5:1. Anaerobic conditions depressed the nitrogenase activity in decomposing leaves, which was independent of light. Nitrogenase activity was also found to be associated with pneumatophores. This activity was light dependent and was probably attributable to one or more species of Calothrix present as an epiphyte. Rates of activity were generally between 100 and 500 nmol of C₂H₄ pneumatophore⁻¹ h⁻¹ in summer, but values up to 1,500 nmol of C₂H₄ pneumatophore⁻¹ h⁻¹ were obtained.     

Periphyton chemistry and nitrogenase activity in a northern Everglades ecosystem

Cyanobacterial peri­phyton communities are a dominant feature of oligotrophic Everglades marshes, however, little is known regarding the biogeochemical aspects of this ecosystem component. This study was undertaken to investigate the potential for N₂ fixation in the peri­phyton communities of a hydrologically-controlled portion of the northern Everglades marsh (Water Conservation Area 2A, WCA-2A). The objectives of this research were to characterize the temporal patterns of nutrient composition and N₂ fixation of the natural WCA-2A peri­phyton communities and to compare fixation rates of peri­phyton with those of other ecosystem components in both natural and nutrient-impacted WCA-2A areas. In general, N₂ fixation (measured by the acetylene reduction (AR) method) of natural WCA-2A peri­phyton was enhanced under light conditions showing a nitrogenase pattern characteristic of autotrophic cyanobacteria. Winter (November–March) rates of AR expressed per gram organic carbon (gOC) ranged from 147–240 nmol C₂H₂ g OC^–1 h^–1, while summer rates were elevated with an observed peak of 1148 nmol C₂H₂ g OC^–1 h^–1 in July 1998. This translates into an estimated yearly contribution of approximately 10 g N m^–2 to an unimpacted WCA-2A slough ecosystem. Nitrogenase activity did not correlate seasonally with nutrients (Ca, Mg, Fe, N, P, Mn), but closely followed measured N stable isotopic ratios (¹⁵N) in floating peri­phyton. In oligotrophic marsh areas, AR (on a weight basis) decreased in the order floating peri­phyton > benthic peri­phyton floc > soil > water > detrital plant biomass, while highest AR rates were observed for detrital biomass in areas impacted by agricultural discharges.     

H2 oxidation,O2 uptake and CO2 fixation in hydrogen treated soils

In many legume nodules, the H₂ produced as a byproduct of N₂ fixation diffuses out of the nodule and is consumed by the soil. To study the fate of this H₂ in soil, a H₂ treatment system was developed that provided a 300 cm³ sample of a soil:silica sand (2:1) mixture with a H₂ exposure rate (147 nmol H₂ cm^–3hr^–1) similar to that calculated exist in soils located within 1–4 cm of nodules (30–254 nmol H₂ cm^–3hr^–1). After 3 weeks of H₂ pretreatment, the treated soils had a K_m and V_max for H₂ uptake (1028 ppm and 836 nmol cm^–3 hr^–1, respectively) much greater than that of control, air-treated soil (40.2 ppm and 4.35 nmol cm^–3 hr^–1, respectively). In the H₂ treated soils, O₂, CO₂ and H₂ exchange rates were measured simultaneously in the presence of various pH₂. With increasing pH₂, a 5-fold increase was observed in O₂ uptake, and CO₂ evolution declined such that net CO₂ fixation was observed in treatments of 680 ppm H₂ or more. At the H₂ exposure rate used to pretreat the soil, 60% of the electrons from H₂ were passed to O₂, and 40% were used to support CO₂ fixation. The effect of H₂ on the energy and C metabolism of soil may account for the well-known effect of legumes in promoting soil C deposition.     

n-3 PUFA productivity in chemostat cultures of microalgae

Eicosapentaenoic (EPA) and docosahexaenoic (DHA) acid productivities from chemostat cultures of an isolate of Isochrysis galbana have been studied. The productivities reached in the interval of dilution rates between 0.0295 h^–1 and 0.0355 h^–1 were 1.5mg·1^–1·h^–1 for lipids, 300 g·1^–1·h^–1 for EPA and 130g1·1^–1·h^–1 for DHA. Furthermore, light attenuation by mutual shading, and agitation speed influences on growth and fatty acid composition were analysed. A model relating steady-state dilution rates to internal average light intensity has been proposed, the parameter values of which obtained by non-linear regression were: maximum specific growth rate (_max)=0.0426 h^–1; the affinity of cells to light (I_k) = 10.92 W·m^–2; the exponent (n) = 5.13; regression coefficient (r ²)=0.9999. Correspondence to: E. Molina Grima