Factors Limiting Microbial Growth and Activity at a Proposed High-Level Nuclear Repository, Yucca Mountain, Nevada

As part of the characterization of Yucca Mountain, Nev., as a potential repository for high-level nuclear waste, volcanic tuff was analyzed for microbial abundance and activity. Tuff was collected aseptically from nine sites along a tunnel in Yucca Mountain. Microbial abundance was generally low: direct microscopic cell counts were near detection limits at all sites (3.2 x 10(sup4) to 2.0 x 10(sup5) cells g(sup-1) [dry weight]); plate counts of aerobic heterotrophs ranged from 1.0 x 10(sup1) to 3.2 x 10(sup3) CFU g(sup-1) (dry weight). Phospholipid fatty acid concentrations (0.1 to 3.7 pmol g(sup-1)) also indicated low microbial biomasses; diglyceride fatty acid concentrations, indicative of dead cells, were in a similar range (0.2 to 2.3 pmol g(sup-1)). Potential microbial activity was quantified as (sup14)CO(inf2) production in microcosms containing radiolabeled substrates (glucose, acetate, and glutamic acid); amendments with water and nutrient solutions (N and P) were used to test factors potentially limiting this activity. Similarly, the potential for microbial growth and the factors limiting growth were determined by performing plate counts before and after incubating volcanic tuff samples for 24 h under various conditions: ambient moisture, water-amended, and amended with various nutrient solutions (N, P, and organic C). A high potential for microbial activity was demonstrated by high rates of substrate mineralization (as much as 70% of added organic C in 3 weeks). Water was the major limiting factor to growth and microbial activity, while amendments with N and P resulted in little further stimulation. Organic C amendments stimulated growth more than water alone.     

Geochemically Generated,Energy-Rich Substrates and Indigenous Microorganisms in Deep,Ancient Groundwater

Recent studies have shown that the biosphere extends to depths that exceed 3 km, raising questions regarding the age of the microbes in these deep ecosystems and their sources of energy for metabolism. Abiogenic energy sources that are derived from in situ, purely geochemical sources and thus independent from photosynthesis have been suggested. We sampled saline fracture water emanating from a 3.1-km deep borehole in a Au mine in the Witwatersrand Basin of South Africa and characterized the chemical constituents (including stable isotopes), groundwater age, and indigenous microorganisms. Salinity data and ratios of dissolved noble gases indicate that extremely ancient (2.0 Ga) saline fracture water has mixed with meteoric water to yield an average subsurface residence time of 20–160 Ma, the oldest age of any waters collected to date in the Witwatersrand Basin. H₂ isotope data suggest the water originated from a depth of 4 to 5 km. Sulfur isotope fractionation indicates biological sulfate reduction. Calculations of free energies and steady state energy fluxes based on water chemistry data also support sulfate reduction as the dominant terminal electron accepting process. Lipid and flow cytometry data indicate a sparse microbial community (10³ cells ml^?1), despite the presence of relatively high concentrations of energy-rich compounds (H₂, CH₄, CO, ethane, propane, butane, and acetate). The H₂ can be explained by radiolysis of water. Stable isotopic signatures of the CH₄ and short chain hydrocarbons indicate abiogenic synthesis. The persistence of energy-rich compounds suggests that other factors are limiting to microbial metabolism and growth, e.g., availability of an inorganic nutrient, such as Fe or phosphate.     

Effect of gamma radiation on native endolithic microorganisms from a radioactive waste deposit site.

A time-course experiment was conducted to evaluate the effects of gamma radiation on the indigenous microbiota present in rock obtained from Yucca Mountain, Nevada Test Site. Microcosms were constructed by placing pulverized Yucca Mountain rock in polystyrene cylinders. Continuous exposure (96 h) at a dose rate of 1.63 Gy/min was used to mimic the near-field environment surrounding waste canisters. The expected maximum surface dose rate from one unbreached canister designed to contain spent nuclear fuels is 0.06 Gy/min. Considering the current repository packing design, multiple canisters within one vault, the cumulative dose rate may well approach that used in this experiment. The microbial communities were characterized after receiving cumulative doses of 0, 0.098, 0. 58, 2.33, 4.67, 7.01 and 9.34 kGy. Radiation-resistant microorganisms in the pulverized rock became viable but nonculturable (VBNC) after a cumulative dose of 2.33 kGy. VBNC microorganisms lose the ability to grow on media on which they have routinely been cultured in response to the environmental stress imposed (i.e. radiation) but can be detected throughout the time course using direct fluorescence microscopy techniques. Two representative exopolysaccharide-producing isolates from Yucca Mountain were exposed to the same radiation regimen in sand microcosms. One isolate was much more radiation-resistant than the other, but both had greater resistance than the general microbial community based on culturable counts. However, when respiring cell counts (VBNC) were compared after irradiation, the results would indicate much more radiation resistance of the individual isolates and the microbial community in general. These results have significant implications for underground storage of nuclear waste as they indicate that indigenous microorganisms are capable of surviving gamma irradiation in a VBNC state.     

The coupling of biological iron cycling and mineral weathering during saprolite formation, Luquillo Mountains, Puerto Rico

Corestones of quartz diorite bedrock in the Rio Icacos watershed in Puerto Rico weather spheroidally to form concentric sets of partially weathered rock layers (referred to here as rindlets) that slowly transform to saprolite. The rindlet zone (0.2–2 m thick) is overlain by saprolite (2–8 m) topped by soil (0.5–1 m). With the objective of understanding interactions between weathering, substrate availability, and resident micro‐organisms, we made geochemical and microbiological measurements as a function of depth in 5 m of regolith (soil + saprolite). We employed direct microscopic counting of total cell densities; enumeration of culturable aerobic heterotrophs; extraction of microbial DNA for yield calculations; and biochemical tests for iron‐oxidizing bacteria. Total cell densities, which ranged from 2.5 × 10⁶ to 1.6 × 10¹⁰ g^?1 regolith, were higher than 10⁸ g^?1 at three depths: in the upper 1 m, at 2.1 m, and between 3.7 and 4.9 m, just above the rindlet zone. High proportions of inactive or unculturable cells were indicated throughout the profile by very low percentages of culturable heterotrophs (0.0004% to 0.02% of total cell densities). The observed increases in total and culturable cells and DNA yields at lower depths were not correlated with organic carbon or total iron but were correlated with moisture and HCl‐extractable iron. Biochemical tests for aerobic iron‐oxidizers were also positive at 0.15–0.6 m, at 2.1–2.4 m, and at 4.9 m depths. To interpret microbial populations within the context of weathering reactions, we developed a model for estimating growth rates of lithoautotrophs and heterotrophs based on measured substrate fluxes. The calculations and observations are consistent with a model wherein electron donor flux driving bacterial growth at the saprolite–bedrock interface is dominated by Fe(II) and where autotrophic iron‐oxidizing bacteria support the heterotrophic population and contribute to bedrock disaggregation and saprolite formation.     

Microbial community structure and biogeochemistry of Miocene subsurface sediments: implications for long-term microbial survival

Thirty closely spaced cores were obtained from Miocene-aged fluvial, lacustrine and palaeosol subsurface sediments ranging in depth from 173 to 197 m at a site in south-central Washington to investigate the size and composition of the microbial community in relation to sediment geochemical and geophysical properties. Total phospholipid fatty acid (PLFA) analysis indicated that the greatest concentrations of microbial bio-mass were in low-permeability lacustrine sediments that also contained high concentrations of organic carbon. Community structure, based on lipid analyses and on in situ hybridization of bacterial cells with 16S RNA-directed DNA probes, also revealed the presence of metabolically active bacteria that respire sulphate and/or Fe(III) in the lacustrine sediments. Concentrations of pore water sulphate were low (4–8 mg/L) and HCI-extractable Fe was predominantly Fe(II) in the same samples where total biomass and organic carbon were highest. The low hydraulic conductivity (10^-6 to < 10^-9 cm/s) of these sediments has likely contributed to the long term maintenance of both bacteria and organic carbon by limiting the supply of soluble electron acceptors for microbial respiration. These results suggest that the current subsurface microbial population was derived from organisms that were present during lake sedimentation = 6–8 million years ago.     

Correlation between viral production and carbon mineralization under nitrate-reducing conditions in aquifer sediment

A variety of microbially mediated metabolic pathways impact biogeochemical cycling in terrestrial subsurface environments. However, the role that viruses have in influencing microbial mortality and microbial community structure is poorly understood. Here we investigated the production of viruses and change in microbial community structure within shallow alluvial aquifer sediment slurries amended with ¹³C-labeled acetate and nitrate. Biostimulation resulted in production of viruses concurrent with acetate oxidation, ¹³CO₂ production and nitrate reduction. Interestingly, change in viral abundance was positively correlated to acetate consumption (r²=0.6252, P<0.05) and ¹³CO₂ production (r²=0.6572, P<0.05); whereas change in cell abundance was not correlated to acetate consumption or ¹³CO₂ production. Viral-mediated cell lysis has implications for microbial community structure. Betaproteobacteria predominated microbial community composition (62% of paired-end reads) upon inoculation but decreased in relative abundance and was negatively correlated to changes in viral abundance (r²=0.5036, P<0.05). As members of the Betaproteobacteria decreased, Gammaproteobacteria, specifically Pseudomonas spp., increased in relative abundance (82% of paired-end reads) and was positively correlated with the change in viral abundance (r²=0.5368, P<0.05). A nitrate-reducing bacterium, Pseudomonas sp. strain Alda10, was isolated from these sediments and produced viral-like particles with a filamentous morphology that did not result in cell lysis. Together, these results indicate that viruses are linked to carbon biogeochemistry and community structure in terrestrial subsurface sediments. The subsequent cell lysis has the potential to alter available carbon pools in subsurface environments, additionally controlling microbial community structure from the bottom-up.     

Analysis of copper corrosion in compacted bentonite clay as a function of clay density and growth conditions for sulfate‐reducing bacteria

Aims: To investigate the relationships between sulfate‐reducing bacteria (SRB), growth conditions, bentonite densities and copper sulfide generation under circumstances relevant to underground, high‐level radioactive waste repositories. Methods and Results: Experiments took place 450 m underground, connected under in situ pressure to groundwater containing SRB. The microbial reduction of sulfate to sulfide and subsequent corrosion of copper test plates buried in compacted bentonite were analysed using radioactive sulfur (³⁵SO₄^2?) as tracer. Mass distribution of copper sulfide on the plates indicated a diffusive process. The relationship between average diffusion coefficients (D_s) and tested density (ρ) was linear. D_s (m² s^?1) = ?0·004 × ρ (kg m^?3) + 8·2, decreasing by 0·2 D_s units per 50 kg m^?3 increase in density, from 1·2 × 10^?11 m² s^?1 at 1750 kg m^?3 to 0·2 × 10^?11 m² s^?1 at 2000 kg m^?3. Conclusions: It is possible that sulfide corrosion of waste canisters in future radioactive waste repositories depends mainly on sulfide concentration at the boundary between groundwater and the buffer, which in turn depends on SRB growth conditions (e.g., sulfate accessibility, carbon availability and electron donors) and geochemical parameters (e.g., presence of ferrous iron, which immobilizes sulfide). Maintaining high bentonite density is also important in mitigating canister corrosion. Significance and Impact of the Study: The sulfide diffusion coefficients can be used in safety calculations regarding waste canister corrosion. The work supports findings that microbial activity in compacted bentonite will be restricted. The study emphasizes the importance of growth conditions for sulfate reduction at the groundwater boundary of the bentonite buffer and linked sulfide production.     

Influence of temperature and process technology on the occurrence of Aeromonas species and hygienic indicator organisms in drinking water production plants

The occurrence of Aeromonas spp. and hygienic indicator organisms in raw and treated waters of five drinking water production plants in Flanders (Belgium) was surveyed over a period of 17 months. Aeromonads were isolated on ampicillin-dextrin agar (ADA) and further identified by gas-liquid chromatographic analysis of their cellular fatty acid methyl ester (FAME) content. ADA medium was found to be highly specific for the enumeration of Aeromonas spp. In general, Aeromonas counts were very low in untreated groundwater but numbered 10⁴–10⁶ colony-forming units per liter in open storage reservoirs for surface water. Aeromonas spp. were seasonally distributed with maximal densities occurring during the summer. The ecology of Aeromonas in the different waters was studied in relation to the physical, chemical, and microbiological water characteristics. Strongly positive correlations were observed between Aeromonas densities and heterotrophic plate counts, whereas a clearly negative relationship was found with dissolved oxygen. On average, 99.7% of the aeromonads were removed by flocculation-decantation followed by breakpoint chlorination, whereas 98.9% were removed by slow sand filtration. Flocculation-decantation without breakpoint chlorination did not reduce the microbial numbers. At three of four drinking water production plants tested, rapid sand filtration decreased the number of aeromonads and hygienic indicator organisms. At one plant, however, the numbers of Aeromonas and hygienic indicator organisms were high in the sand filter effluents. Increased numbers of aeromonads were also counted in the effluent of the activated carbon filters. Hence, inactivation of Aeromonas spp. by the current process technology appears not sufficient to exclude postchlorination. The survival of aeromonads in certain filter systems may be due to the growth of these bacteria on biodegradable organic material, provided by the decomposition from bacteria, algae, or other sources. Correspondence to: W. Verstraete     

Observations pertaining to the origin and ecology of microorganisms recovered from the deep subsurface of Taylorsville Basin,Virginia

To understand the conditions under which microorganisms exist in deep hydrocarbon reservoirs, sidewall cores were collected from a natural gas‐bearing formation, 2800 m below the surface in Taylorsville Basin, Virginia. Data from chemical and microbial tracers and controls indicate that the interiors of some sidewall cores contained microorganisms indigenous to the rock formation. The cultured microorganisms were composed primarily of saline‐tolerant, thermophilic fermenting, Fe(III)‐reducing, and sulfate‐reducing bacteria (1 to 10⁴ cells/g). The physiological capabilities of the cultured microorganisms are compatible with the temperature (76°C), pressure (32 MPa), and salinity (≈0.8 wt.% NaCl equivalent) in the sampled interval. The petrological data indicated that the strata contain intercrystalline pores of micrometer size, that occur between late diagenetic cement in siltstone and within cross‐cutting, mineralized fractures in shale. These pores made up only 0.04% of the rock volume, were mostly gas‐filled, and were interconnected by pore throats with diameters <0.04 μm. Because the pore throats are smaller than known bacteria, the cultured microorganisms were probably trapped within the larger pores containing alkaline, brackish, formation water. The total phospholipid fatty acid concentration of the rock samples yielded a cellular concentration equivalent to 4 x 10⁵ cells/g, much greater than had been determined by enumeration of the cultured bacteria. This may have resulted from either inhibition of dephosphorolation reactions within pores filled with reduced gases, such as methane, or the inability to culture >0.1% of the viable bacteria. The recovery of living bacteria from such an austere environment represents one of the most remarkable examples of microbial survival yet reported.     

Interactions between Seagrass Complexity,Hydrodynamic Flow and Biomixing Alter Food Availability for Associated Filter-Feeding Organisms

Seagrass shoots interact with hydrodynamic forces and thereby a positively or negatively influence the survival of associated species. The modification of these forces indirectly alters the physical transport and flux of edible particles within seagrass meadows, which will influence the growth and survivorship of associated filter-feeding organisms. The present work contributes to gaining insight into the mechanisms controlling the availability of resources for filter feeders inhabiting seagrass canopies, both from physical (influenced by seagrass density and patchiness) and biological (regulated by filter feeder density) perspectives. A factorial experiment was conducted in a large racetrack flume, which combined changes in hydrodynamic conditions, chlorophyll a concentration in the water and food intake rate (FIR) in a model active filter-feeding organism (the cockle). Results showed that seagrass density and patchiness modified both hydrodynamic forces and availability of resources for filter feeders. Chlorophyll a water content decreased to 50% of the initial value when densities of both seagrass shoots and cockles were high. Also, filter feeder density controlled resource availability within seagrass patches, depending on its spatial position within the racetrack flume. Under high density of filter-feeding organisms, chlorophyll a levels were lower between patches. This suggests that the pumping activity of cockles (i.e. biomixing) is an emergent key factor affecting both resource availability and FIR for filter feeders in dense canopies. Applying our results to natural conditions, we suggest the existence of a direct correlation between habitat complexity (i.e. shoot density and degree of patchiness) and filter feeders density. Fragmented and low-density patches seem to offer both greater protection from hydrodynamic forces and higher resource availability. In denser patches, however, resources are allocated mostly within the canopy, which would benefit filter feeders if they occurred at low densities, but would be limiting when filter feeder were at high densities.     

Biomass and organic acids in sandstone of a weathering building: Production by bacterial and fungal isolates

Ten fungal and nine bacterial strains were isolated from a weathering sandstone building. Their growth, organic acid production, and acidification capacity were assessed in culture under nutritional conditions similar to those in situ. Biomass (10–50 nmol phospholipid-PO₄g⁻¹) within the rock was small compared to soils. The isolated organisms were able to produce high amounts of those acids found in the sandstone, but acid production did not cause a drastic reduction in culture pH. It is suggested that the importance of acidification in microbial degradation of sandstone has been overestimated and that, under in situ pH and nutritional conditions, cation chelation by microbially produced organic acid anions may be more relevant to the weathering process.     

Hydrogeologic parameters affecting vadose‐zone microbial distributions

Abstract

The vadose zone and its contaminant‐attenuating processes are physically interposed between surface contamination and groundwater supplies. Given the potential role of microorganisms in mediating vadose‐zone chemical processes, it is vital to understand vadose microbial distributions and factors controlling those distributions. Vadose and shallow saturated zone sediments obtained from cores drilled to approximately 8 m below the surface at two hydrogeologically contrasting sites, named Dalmeny and Washington State University (WSU), were examined for culturable heterotrophic bacteria, total organic carbon (TOC), and sediment texture. Pore‐water elutions were analyzed for dissolved organic carbon, sulfate, and inorganic nitrogen species. Numbers of cultured bacteria (10³‐10⁷ g^?1) generally decreased with depth at both sites. The TOC decreased uniformly with depth at WSU where soil processes are the sole carbon source; at Dalmeny, where both soil and kerogen carbon are present, TOC was higher and relatively constant with depth. Numbers of distinct colony types at Dalmeny did not decline below the solum. Bacteria at Dalmeny were more numerous, exhibited greater numbers of colony types, and were metabolically more flexible than those at WSU. The smooth decline of numbers with depth at WSU paralleled and may be caused by the TOC decline with distance from a solum source. Sediment permeability and pore‐water flux did not control bulk populations as suggested in previous studies; this may be explained by bacterial residence on fracture surfaces in low‐permeability materials. Psychrotolerant organisms did not appear to be as abundant as mean ambient temperatures might suggest.     

Landscape Distribution of Microbial Activity in the McMurdo Dry Valleys: Linked Biotic Processes, Hydrology, and Geochemistry in a Cold Desert Ecosystem

In desert ecosystems, microbial activity and associated nutrient cycles are driven primarily by water availability and secondarily by nutrient availability. This is especially apparent in the extremely low productivity cold deserts of the McMurdo Dry Valleys, Antarctica. In this region, sediments near streams and lakes provide the seasonally wet conditions necessary for microbial activity and nutrient cycling and thus transfer energy to higher organisms. However, aside from a few studies of soil respiration, rates of microbial activity throughout the region remain unexplored. We measured extracellular enzyme activity potentials (alkaline phosphatase, leucine-aminopeptidase, beta-glucosidase, phenol oxidase, and peroxidase) in soils adjacent to lakes and streams, expecting activity to be primarily related to soil water content, as well as time of season and organic matter supply. Phosphatase and beta-glucosidase activities were higher in shoreline than upland soils; however, potential rates were not correlated with soil water content. Instead, soil organic matter, salinity, and pH were the best predictors of microbial activity. Microbial nutrient limitation metrics estimated from extracellular enzyme activity were correlated with pH and salinity and exhibited similar patterns to previously published trends in soil P and N content. Compared to other terrestrial ecosystems, organic matter specific rates for leucine-aminopeptidase and oxidative enzyme activities were high, typical of alkaline desert soils. Phosphatase activity was close to the global mean whereas beta-glucosidase activity was extremely low, which may reflect the lack of vascular plant derived organic matter in the Dry Valleys. In this cold desert ecosystem, water availability promotes microbial activity, and microbial nutrient cycling potentials are related to soil geochemistry. Author contributions:   LHZ performed research, analyzed data, and wrote the paper; RLS contributed new methods and wrote the paper; JEB conceived/designed study, performed research and analyzed data; MNG conceived/designed study and performed research; CTV conceived/designed study and performed research.     

Insect outbreaks increase populations and facilitate reproduction in a cavity‐dependent songbird,the Mountain Chickadee Poecile gambeli

Large‐scale insect outbreaks, typical of temperate ecosystems, may increase food availability and influence species interactions in insectivorous bird communities. We assessed how population densities of a secondary cavity nester, the Mountain Chickadee Poecile gambeli, varied with densities of avian cavity excavators, potential competitors and nest predators, during two large‐scale insect outbreaks of Western Spruce Budworm Choristoneura occidentalis and Mountain Pine Beetle Dendroctonus ponderosae. At the regional level, Mountain Chickadee densities doubled with increases in budworm and beetle availability, then declined with decreases in beetle availability, despite high budworm supply. At the site level, densities of excavators were the best predictor of site‐level variation in Chickadee densities in the following year. During and after the beetle outbreak, Mountain Chickadees used more cavities excavated by Red‐breasted Nuthatches Sitta canadensis and Downy Woodpeckers Picoides pubescens, both of which are bark insectivores that increased in densities concomitantly with the beetle outbreak and whose foraging activities may have facilitated the Mountain Chickadees' access to bark beetles. Thus, Mountain Chickadees showed a numerical response to the food pulse at the regional level, but a functional response to the pulse of nesting cavities at the site level. Plasticity in resource selection and heterospecific attraction may allow Mountain Chickadees to respond to resource pulses in highly variable environments.     

The Availability of Condensed Phosphates for the Algal Strain Scenedesmus obtusiusculus Chodat,with Special Regard to Conditions Characteristic in Lake Balaton

The limiting factor in the water of Lake Balaton was calculated by means of Verduin's equation. If only the orthophosphate-phosphorus concentration is inserted into the equation, phosphorus is the primary limiting factor. If, however, total phosphorus is considered, the factor intensity of nitrogen will be the least, i. e. nitrogen will be the primary limiting-factor of plant growth. According to measurements during 1976, average values for the total dissolved phosphorus and orthophosphate-phosphorus content of Balaton Lake water were 15,66 mg/m³ and 7.66 mg/m³, respectively. Experiments with the algal strain Scenedesmus obtusiusculus Chod. (Chlorophyceae, Chlorococcales). were designed to test the availability of the condensed phosphorus form for the algae. The experiments were performed partly with synthetic polyphosphates, partly, with polyphosphates isolated from Balaton Lake water. The results showed, contrary to our expectations, that phosphates present in the condensed form (irrespective of their structure and degree of condensation). were not utilized by the algae under sterile conditions, i. e. in the absence of bacterial activity. In the light of the above it is recommended to consider only orthophosphate-phosphorus when calculating the limiting factor.     

Bacteria associated with deep,alkaline, anaerobic groundwaters in Southeast Washington

The microbial diversity in two deep, confined aquifers, the Grande Ronde (1270 m) and the Priest Rapids (316 m), Hanford Reservation, Washington, USA, was investigated by sampling from artesian wells. These basaltic aquifers were alkaline (pH 8.5 to 10.5) and anaerobic (Eh –200 to –450 mV). The wells were allowed to free-flow until pH and Eh stabilized, then the microflora was sampled with water filtration and flow-through sandtrap methods. Direct microscopic counts showed 7.6 × 10⁵ and 3.6 × 10³ bacteria ml^–1 in water from the Grande Ronde and Priest Rapids aquifers, respectively. The sand filter method yielded 5.7 × 10⁸ and 1.1 × 10⁵ cells g^–1 wet weight of sand. The numbers of bacteria did not decrease as increasing volumes of water were flushed out. The heterotrophic diversity of these bacterial populations was assessed using enrichments for 20 functional groups. These groups were defined by their ability to grow in a matrix of five different electron acceptors (O₂, Fe(III), NO₃ ^–, SO₄ ^2–, HCO₃ ^–) and four groups of electron donors (fermentation products, monomers, polymers, aromatics) in a mineral salts medium at pH 9.5. Growth was assessed by protein production. Culture media were subsequently analyzed to determine substrate utilization patterns. Substrate utilization patterns proved to be more reliable indicators of the presence of a particular physiological group than was protein production. The sand-trap method obtained a greater diversity of bacteria than did water filtration, presumably by enriching the proportion of normally sessile bacteria relative to planktonic bacteria. Substrate utilization patterns were different for microflora from the two aquifers and corresponded to their different geochemistries. Activities in the filtered water enrichments more closely matched those predicted by aquifer geochemistry than did the sand-trap enrichments. The greatest activities were found in Fe(III)-reducing enrichments from both wells, SO₄-reducing enrichments from the Grande Ronde aquifer, and methanogenic enrichments from the Priest Rapids aquifer. Organisms from these aquifers may be useful for high-pH bioremediation applications as well as production of biotechnological products. These organisms may also be useful for modeling potential reactions near buried concrete, as might be found in subsurface waste depositories. Offprint requests to: T. O. Stevens.     

叶际微生物研究进展

植物的叶际是一个复杂的生态系统,微生物的生存环境条件严苛。其可被利用的营养成分较少,温湿度波动大。此外,较强的紫外线辐射对于叶际微生物的生存也有很大影响。但是植物叶际却有着丰富的微生物多样性,其中还有许多有益细菌和真菌。它们通过和植物寄主的互作,改善着叶际微生物的栖居环境;其对植物病原体的拮抗亦可提高植物的抗病性。植物叶际的微生物还可以产生激素以促进植物生长,还有一些微生物可以利用农药等污染有机物作为营养物质,在污染物的环境生物修复方面显示巨大的潜力。此外,叶际微生物作为一种生态学指标在生态稳定与环境安全评价中开始发挥显著的作用。     

Establishment of Microbial Eukaryotic Enrichment Cultures from a Chemically Stratified Antarctic Lake and Assessment of Carbon Fixation Potential

Lake Bonney is one of numerous permanently ice-covered lakes located in the McMurdo Dry Valleys, Antarctica. The perennial ice cover maintains a chemically stratified water column and unlike other inland bodies of water, largely prevents external input of carbon and nutrients from streams. Biota are exposed to numerous environmental stresses, including year-round severe nutrient deficiency, low temperatures, extreme shade, hypersalinity, and 24-hour darkness during the winter ¹. These extreme environmental conditions limit the biota in Lake Bonney almost exclusively to microorganisms ².Single-celled microbial eukaryotes (called "protists") are important players in global biogeochemical cycling ³ and play important ecological roles in the cycling of carbon in the dry valley lakes, occupying both primary and tertiary roles in the aquatic food web. In the dry valley aquatic food web, protists that fix inorganic carbon (autotrophy) are the major producers of organic carbon for organotrophic organisms ^{4, 2}. Phagotrophic or heterotrophic protists capable of ingesting bacteria and smaller protists act as the top predators in the food web ⁵. Last, an unknown proportion of the protist population is capable of combined mixotrophic metabolism ^{6, 7}. Mixotrophy in protists involves the ability to combine photosynthetic capability with phagotrophic ingestion of prey microorganisms. This form of mixotrophy differs from mixotrophic metabolism in bacterial species, which generally involves uptake dissolved carbon molecules. There are currently very few protist isolates from permanently ice-capped polar lakes, and studies of protist diversity and ecology in this extreme environment have been limited ^{8, 4, 9, 10, 5}. A better understanding of protist metabolic versatility in the simple dry valley lake food web will aid in the development of models for the role of protists in the global carbon cycle.We employed an enrichment culture approach to isolate potentially phototrophic and mixotrophic protists from Lake Bonney. Sampling depths in the water column were chosen based on the location of primary production maxima and protist phylogenetic diversity ^{4, 11}, as well as variability in major abiotic factors affecting protist trophic modes: shallow sampling depths are limited for major nutrients, while deeper sampling depths are limited by light availability. In addition, lake water samples were supplemented with multiple types of growth media to promote the growth of a variety of phototrophic organisms.RubisCO catalyzes the rate limiting step in the Calvin Benson Bassham (CBB) cycle, the major pathway by which autotrophic organisms fix inorganic carbon and provide organic carbon for higher trophic levels in aquatic and terrestrial food webs ¹². In this study, we applied a radioisotope assay modified for filtered samples ¹³ to monitor maximum carboxylase activity as a proxy for carbon fixation potential and metabolic versatility in the Lake Bonney enrichment cultures.     

Bacterial Diversity in an Alpine Debris-Free and Debris-Cover Accumulation Zone Glacier Ice,North Sikkim,India

The Himalayas are water tower for billions of people; however in recent years due to climate change several glaciers of Himalaya are receding or getting extinct which can lead to water scarcity and political tensions. Thus, it requires immediate attention and necessary evaluation of all the environmental parameters which can lead to conservation of Himalayan glaciers. This study is the first attempt to investigate the bacterial diversity from debris-free Changme Khang (CKG) and debris-cover Changme Khangpu (CK) glacier, North Sikkim, India. The abundance of culturable bacteria in CKG glaciers was 1.5?×?10⁴ cells/mL and CK glacier 1.5?×?10⁵ cells/mL. A total of 50 isolates were isolated from both the glacier under aerobic growth condition. The majority of the isolates from both the glaciers were psychrotolerant according to their growth temperature. Optimum growth temperatures of the isolates were between 15 and 20 °C, pH 6–8 and NaCl 0–2%. The phylogenetic studies of 16S RNA gene sequence suggest that, these 21 isolates can be assigned within four phyla/class, i.e., Firmicutes, Beta-proteobacteria, Gamma-proteobacteria and Actinobacteria. The dominant phyla were Firmicutes 71.42% followed by Actinobacteria 14.28%, Alpha-proteobacteria 9.52% and Beta-proteobacteria 4.76%. The isolate Bacillus thuringiensis strain CKG2 showed the highest protease activity (2.24 unit/mL/min). Considering the fast rate at which Himalayan glaciers are melting and availability of limited number of research, there is urgent need to study the microbial communities confined in such environments.     

Carbon addition interacts with water availability to reduce invasive forb establishment in a semi-arid grassland

Increases in nitrogen (N) availability can favor fast-growing invasive species over slow-growing native species. One way to reduce N availability is to add labile carbon (C) to the soil, which can lead to microbial immobilization of plant available N. This method has been used, with widely varying degrees of success, to both study and control plant invasions. One reason that C addition might not work as expected is that N is not always the limiting resource for plant growth. For example, if plant growth is limited by water, changes in N availability might have little effect on invasion. Here I ask whether effects of C addition on N availability, resident plant biomass, and invasion depend on water availability in semi-arid mixedgrass prairie. Six invasive species were seeded into plots treated with a factorial combination of water (ambient or added) and N (+C, control or +N). Carbon addition reduced capture of mineral N by resin probes (by an average of 73%), and reduced biomass of resident species (from 336 g m⁻² to 203 g m⁻²), both with and without added water. In contrast, because there was little invasion in ambient-water plots, C addition reduced invasion only in added-water plots. Given added water, C addition reduced biomass of Centaurea diffusa by 95%, and prevented invasion by Gypsophila paniculata and Linaria dalmatica. Mechanisms by which C addition reduced invasion varied by species, with added C reducing the growth of individual C. diffusa plants, but reducing numbers of G. paniculata and L. dalmatica individuals.