Acetylene reduction by bryophyte-cyanobacteria associations on a Subantarctic island

Summary Nineteen bryophyte species containing epiphytic cyanobacteria on Marion Island (46°54 S, 37°45 E) were investigated for their capacity to reduce acetylene in the field. Fourteen species exhibited reduction, varying from 0.36 to 310.57 nmol C₂H₂ g^-1 dry weight h^-1. These values are within the range of those reported from northern hemisphere subpolar sites. The highest reduction rate occurred at a highly minerotrophic mire receiving nutrient-rich run-off from an adjacent bird-occupied slope. The lowest non-zero values occurred in moss-balls growing on exposed, wind-swept rocky ridges. Multiple regression analysis indicated a strong correlation between acetylene reduction rate and sample water content, with cyanobacteria abundance the next most important variable. Under the incubation conditions employed neither temperature nor radiation were related to reduction rate. In view of the predominance of bryophyte-dominated communities in the island vegetation it is likely that fixation of N by bryophyte-cyanobacteria associations forms an important contribution of reduced N to the ecosystem.     

Limitations on the Analysis of Acetylene Reduction by Soybean at Low Levels of Acetylene

The analysis of acetylene reduction at low concentrations ofacetylene involves a number of assumptions and both technicaland kinetic complexities. The major difficulty in convertingacetylene reduction rates to apparent N₂ reduction rates isdetermining the K_m for acetylene in the presence of N₂. Thissubstrate competition is dominant over diffusion limitationeffects, but both introduce equivalent deviations in the observedK_m. Because N₂ is a non-linear partial competitive inhibitorof acetylene reduction, correction for its presence is difficult.Two further complications are introduced by the non-linear responseof nitrogenase to acetylene concentration even in the absenceof N₂, and changes in the apparent K_m of acetylene and K₁ ofN₂ as a function of other variables in the enzyme assay. Itis proposed that transient analysis may be used for measurementof diffusion coefficients and calculations of possible diffusionlimitations. It is demonstrated that one proposed model forestimating diffusion limitation (Denison et al., 1983, PlantPhysiology 73, 648–51) confounds substrate competitionwith diffusion limitation. Acetylene reduction, nitrogen fixation, diffusion limitation     

Localization of Associative Cyanobacteria on the Roots of Epiphytic Orchids

This work is the first study of the localization of phototrophic microorganisms in the rhizoplane and velamen of epiphytic orchids, namely, on the aerial and substrate roots of Acampe papillosa and Dendrobium moschatum and on the aerial roots of Phalaenopsis amabilis and Dendrobium phalaenopsis. The composition of the bacterial community on the plant roots depended on the conditions of plant growth. Under conditions simulating the climate of moist tropical forests, the aerial roots proved to be populated with phototrophic microorganisms, among which cyanobacteria predominated. Interlaced fungal hyphae and filamentous cyanobacteria formed a sheath on the surface of the aerial roots. The nitrogen-fixing capacity of the sheath of the aerial roots was studied on the example of P. amabilis.     

The diet of the Wandering Albatross Diomedea exulans at Subantarctic Marion Island

Summary The diet of the Wandering Albatross at Subantarctic Marion Island was studied by inducing recently fed chicks to regurgitate and by stomach flushing adults about to feed chicks. Liquid comprised 70.2% of stomach content mass recovered from chicks. Solid material comprised cephalopods (58.6% by mass), fish (36.5%) and crustacean, cetacean and seabird material as minor items. Twenty-three taxa of cephalopods were identified, the onychcteuthid squid Kondakovia longimana being the most important. Estimated average mass of squid was 694 g with a maximum of over 8 kg. Diet of the Wandering Albatross at Marion Island was broadly similar to that at other studied localities. The high proportion of cephalopods known to float after death in the diet, and the deep-water habits of the few fish identified, suggest that scavenging plays an important role in foraging behaviour.     

环境因子对森林净生态系统生产力的影响

净生态系统生产力(net ecosystem production,NEP)是生态系统净的碳积累速率,可以指示生态系统碳汇/碳源的状态,当NEP为正值时指征生态系统为碳汇,反之则为碳源。在全球环境变化背景下,NEP已作为生态系统碳循环的核心概念被深入研究。本文以NEP为出发点,综述了5个主要非生物环境因子(水分、温度、氮沉降、大气CO2浓度增加和时空尺度)对森林生态系统NEP的影响。从文献分析表明NEP受生态系统本身性质和各环境影响因子及其之间相互作用的调控。本文最后指出,合理运用Meta分析、生态学联网研究、设计和开展长期观测、多尺度与多因子的科学试验在未来研究中的重要性和必要性,相关研究的开展将有利于全面理解和正确评估环境因子对净生态系统生产力的影响,对研究和预测全球变化对陆地生态系统碳循环的影响具有重要意义。     

The Effects of Moisture on Acetylene Reduction by Mats of Blue-green Algae in Sub-tropical Grassland

During the dry winter months desiccation is the main factorpreventing nitrogen fixation by mats of blue-green algae inkikuyu lawns. In the wetter summer months a few days of sunshinewithout rain will dry out the algal mats and depress nitrogenfixation unless the algae are protected by swards of grass.Dry mats resume nitrogen fixation within an hour of becomingdamp and attain their normal rate of fixation within 5 h. Maximumfixation rates occur at 100 per cent relative humidity and withsoil moisture contents of 22 to 42 per cent. Dry algal matsare unable to take up sufficient water as vapour (even in saturatingconditions) to permit resumption of acetylene reduction. Algal-dominatedsoils show greater water retention than adjacent bare soils.     

Effects of Salinity on Acetylene Reduction (Nitrogen Fixation) and Respiration in a Marine Azotobacter

An acetylene-reducing (nitrogen-fixing) bacterium, identified as Azotobacter sp., was isolated from a site in the Canary Creek Marsh, Del. Acetylene reduction activity of the isolate was maximal at 15 to 25‰ NaCl, with no activity observed at 0 or 60‰. Respiration studies showed similar results, with maximal activity occurring at a slightly lower salinity (10 to 20‰ NaCl). The salinities over which peak activity occurred fell within the normal range of in situ salinity (20 to 28‰ total salinity).     

Effects of Fluctuations of Abiotic Factors on the Metabolism of Some Hydrobionts

It has been shown that slight sinusoidal fluctuations of temperature, salinity, and oxygen concentration optimize the growth and energy metabolism of the crustaceanLeander modestus, mollusks Pila sp., and hydrophyte Elodea canadensis. Under the effect of temperature fluctuations, the growth of E. canadensis accelerates, and the rate of photosynthesis increases.     

Action of Water in Depressing Acetylene Reduction by Detached Nodules

Adverse effects of water on acetylene reduction by detached soybean root nodules could not be attributed to ethanol formation, but apparently were associated with nodule tissue damage caused by detachment. Intact nodules were not affected by moisture.     

Further Errors in the Acetylene Reduction Assay: Effects of Plant Disturbance

A flow-through gas system was used to study the effects of disturbanceon nitrogenase (acetylene reduction) activity of nodulated rootsystems of soyabean (Glycine max) and white clover (Trifoliumrepens). Detopping plus removal of the rooting medium (by shaking)produced a substantial decrease in maximum nitrogenase activity.This response is due to a reduction in oxygen flux to the bacteroidscaused by an increase in the oxygen diffusion resistance ofthe nodule. The decrease in maximum nitrogenase activity wasmuch smaller for roots subjected to detopping only. Thus, theeffect of root shaking is more important than that of shootremoval. The effect of detopping plus root shaking on nitrogenase activityoccurred whether the plants were equilibrated and assayed at25°C or 15°C. However, the effect of disturbance onthe oxygen diffusion resistance of the nodules, and thus onnitrogenase activity, was greater at the higher temperature.At the lower temperature the oxygen diffusion resistance ofthe nodules had already been increased in response to the reducedrequirement for oxygen. These nodules were less susceptibleto the effects of disturbance. Thus, comparisons of the effectsof equilibration temperature on nitrogenase activity produceddifferent results depending on whether intact or disturbed systemswere used. With intact systems activity was lower at the lowertemperature but with detopped/shaken roots the lowest activityoccurred at the higher temperature. It is concluded that the use of detopped/shaken roots can producesubstantial errors in the acetylene reduction assay, which makesthe assay invalid even when used for comparative purposes. However,comparisons with rates of ¹⁵N₂ fixation and H₂ production showthat accurate measurements of nitrogenase activity can be obtainedfrom maximum rates of acetylene reduction by intact plants ina flow-through gas system. The continued use of assay proceduresin which cumulated ethylene production from disturbed systemsis measured in closed vessels must be questioned. Key words: Nodules, acetylene, nitrogenase activity     

Effects of Soil Abiotic and Biotic Factors on Tree Seedling Regeneration Following a Boreal Forest Wildfire

Ecosystems - Wildfire disturbance is important for tree regeneration in boreal ecosystems. A considerable amount of literature has been published on how wildfires affect boreal forest regeneration....     

Acetylene Reduction by Soil Cores of Maize and Sorghum in Brazil

Nitrogenase activity was measured by the C₂H₂ reduction method in large soil cores (29 cm in diameter by 20 cm in depth) of maize (Zea mays) and sorghum (Sorghum vulgare). The activity was compared to that obtained by a method in which the roots were removed from the soil and assayed for nitrogenase activity after an overnight preincubation in 1% O₂. In a total of six experiments and 28 soil cores, the nitrogenase activity of the cores was an average of 14 times less than the activity of roots removed from the same cores and preincubated. Nitrogenase activity in the cores was very low and extrapolated to an average nitrogen fixation rate of 2.8 g of N/hectare per day. It was shown that inadequate gas exchange was not a reason for the lower activity in the soil cores, and the core method gave satisfactory results for nitrogenase activity of soybeans (Glycine max) and Paspalum notatum.     

Nitrogen Fixation (Acetylene Reduction) by Epiphytes of Freshwater Macrophytes

The involvement of epiphytic microorganisms in nitrogen fixation was investigated in a shallow freshwater pond near Ithaca, N.Y. The acetylene reduction technique was used to follow diel and seasonal cycles of nitrogen fixation by epiphytes of Myriophyllum spicatum. Acetylene-reducing activity was maximal between noon and 6 p.m., but substantial levels of activity relative to daytime rates continued through the night. Experiments with the seasonal course of activity showed a gradual decline during the autumn months and no activity in January or February. Activity commenced in May, with an abrupt increase to levels between 0.45 and 0.95 nmol of ethylene formed per mg (dry weight) of plant per h. Through most of the summer months, mean rates of acetylene reduction remained between 0.15 and 0.60 nmol/mg (dry weight) per h. It was calculated from diel and seasonal cycles that, in the pond areas studied, epiphytes were capable of adding from 7.5 to 12.5 μg of N per mg of plant per year to the pond. This amount is significant relative to the total amount of nitrogen incorporated into the plant. Blue-green algae (cyanobacteria), particularly Gloeotrichia, appeared to bear prime responsibility for nitrogen fixation, but photosynthetic bacteria of the genus Rhodopseudomonas were isolated from M. spicatum and shown to support high rates of acetylene reduction.     

Asymbiotic Acetylene Reduction by a Fast-Growing Cowpea Rhizobium Strain with Nitrogenase Structural Genes Located on a Symbiotic Plasmid

A procedure was designed which enabled the detection of ex planta nitrogenase activity in the fast-growing cowpea Rhizobium strain IHP100. Nitrogenase activity in agar culture under air occurred at a rate similar to that found for Bradyrhizobium strain CB756 but lower than that for Rhizobium strain ORS571. Hybridization studies showed that both nod and nif genes were located on a 410-kilobase Sym plasmid in strain IHP100.     

Effects of Desiccation on Phosphorus and Potassium Acquisition by a Desiccation-tolerant Moss and Lichen

The hypothesis that desiccation-tolerant mosses and lichensmay be more responsive to nutrient inputs accompanying intermittentdesiccation than mesophytic forest species was investigatedemploying species from semi-arid grassland in Hungary. Shootapices of the moss Syntrichia ruralis and marginal lobes ofthe lichen Cladonia convoluta were maintained for 7 weeks undercontrolled conditions. They were cultivated with or withouta weekly application of the major inorganic macronutrients,and either under constant hydration or with one or two 24 hperiods of desiccation each week. Growth of S. ruralis was stimulatedby nutrient additions, but lower weight increments were achievedwith increasing frequency of desiccation. All samples of thelichen showed positive growth, and no significant treatmenteffects were detected. A large net uptake of P occurred in nutrient-treatedmaterial of both species that was unaffected by the impositionof desiccation treatments. A smaller net uptake of K into theintracellular fraction was also observed when nutrients wereapplied, but in the moss this was against a baseline of decreasingK content. In contrast, more of the original K content was retainedin C. convoluta. In neither species was any clear evidence foundfor inhibition of nutrient uptake by the desiccation episodes.It is suggested that the lack of growth response in the lichenarises from an inability to bring together the additional nutrients,presumably mainly absorbed by the mycobiont, with photosynthateproduced by the photobiont. Copyright 2000 Annals of BotanyCompany Cladonia convoluta, Syntrichia ruralis, desiccation tolerance, mineral nutrition, phosphorus, potassium     

Comparing Time Course Profiles of Immediate Acetylene Reduction by Grasses and Legumes

The time course profiles of C₂H₂ reduction by intact Scirpus olneyi (bulrush), Oryza sativa (rice) and Spartina alterniflora (cordgrass) with roots in atmospheres of N₂ and 30-day-old Glycine max (soybean) in air were all immediately linear. This is the first report of immediately linear rates of C₂H₂ reduction by grass roots removed from soil. The immediately linear profile of C₂H₂ reduction by soil-free grass roots was achieved by preventing contact between the roots and air. Roots of soybeans and S. olneyi receiving pretreatments of O₂ above normal environmental levels for 15 min before assay exhibited a short delay in C₂H₂ reduction. These initially nonlinear rates of C₂H₂ reduction are attributable to transient O₂ inhibition of nitrogenase. Initial nonlinear rates of C₂H₂ reduction were also observed with immature soybean plants and with intact plant assays of O. sativa and S. olneyi in which C₂H₂ was injected into cylinders surrounding the plant tops. These results indicate that, apart from O₂ inhibition of nitrogenase, the diffusion of C₂H₂ and C₂H₄ between the nitrogen-fixing sites and the sampling ports may cause initial nonlinear rates of C₂H₂ reduction. We conclude that in situ plant-associated nitrogenase activity should result in immediate reduction of C₂H₂ and that linear rates are observed when the proper assay conditions are used. Our data suggest that nitrogen fixation is closely associated with the roots of S. olneyi, O. sativa, and S. alterniflora growing in salt marsh sediment.     

Inhibition of Methanogenesis in Marine Sediments by Acetylene and Ethylene: Validity of the Acetylene Reduction Assay for Anaerobic Microcosms

Methanogenesis was irreversibly inhibited in sediments by concentrations of acetylene employed in nitrogen fixation assays (1 to 20%, vol/vol). Ethylene, but not ethane, also stopped methane production, and the inhibition was reversed by gassing with hydrogen.     

The influence of seabird manuring on the phosphorus status of Marion Island (Subantarctic) soils

Summary Manuring by penguins, wandering albatrosses and burrowing petrel and prion species substantially enhances P concentrations in Marion Island soils and plants. Non-manured soils contain low levels of plant-available P on a soil volume basis but the evidence suggests that the island soils are primarily deficient in N rather than P.     

Acetylene Reduction Assays for Nitrogen Fixation Freshwaters: a Note of Caution

Lake water samples were observed to transform [14-C]ethylene into water-soluble compounds that were undetectable by conventional acetylene reduction assay procedures. Methane oxidizing bacteria, which are known to be common in freshwaters, appeared to be responsible for this activity. As much as 28 percent of added ethylene has been observed to be transformed and this figure is probably an underestimate. It is suggested that acetylene reduction assays may not be accurately applied to samples containing methane oxidizing bacteria.     

Effects of Abiotic Factors on the Phylogenetic Diversity of Bacterial Communities in Acidic Thermal Springs

Acidic thermal springs offer ideal environments for studying processes underlying extremophile microbial diversity. We used a carefully designed comparative analysis of acidic thermal springs in Yellowstone National Park to determine how abiotic factors (chemistry and temperature) shape acidophile microbial communities. Small-subunit rRNA gene sequences were PCR amplified, cloned, and sequenced, by using evolutionarily conserved bacterium-specific primers, directly from environmental DNA extracted from Amphitheater Springs and Roaring Mountain sediment samples. Energy-dispersive X-ray spectroscopy, X-ray diffraction, and colorimetric assays were used to analyze sediment chemistry, while an optical emission spectrometer was used to evaluate water chemistry and electronic probes were used to measure the pH, temperature, and E_h of the spring waters. Phylogenetic-statistical analyses found exceptionally strong correlations between bacterial community composition and sediment mineral chemistry, followed by weaker but significant correlations with temperature gradients. For example, sulfur-rich sediment samples contained a high diversity of uncultured organisms related to Hydrogenobaculum spp., while iron-rich sediments were dominated by uncultured organisms related to a diverse array of gram-positive iron oxidizers. A detailed analysis of redox chemistry indicated that the available energy sources and electron acceptors were sufficient to support the metabolic potential of Hydrogenobaculum spp. and iron oxidizers, respectively. Principal-component analysis found that two factors explained 95% of the genetic diversity, with most of the variance attributable to mineral chemistry and a smaller fraction attributable to temperature.