Antibiotic-resistantArthrobacter sp. andPseudomonas sp. responses in the rhizosphere of blue grama after herbage removal

Summary Streptomycin-resistant Pseudomonas and Arthrobacter were isolated from semi-arid grassland soil and their relative responses in the rhizosphere of blue grama (Bouteloua gracilis) subjected to herbage removal were evaluated. Using plants grown in normal soil, the two bacteria showed differential responses to herbage removal, which were most marked in the rhizoplane, where the Pseudomonas showed a two-log unit increase over a 60 hour period, while Arthrobacter, in contrast, exhibited a one-log unit decrease in viable counts for at least 48 hours after defoliation, responses which are similar to those observed in root exudate medium experiments by earlier workers. These results suggest that the rhizoplane may be a critical environment for interaction of these two types of microorganisms, and that sequential responses of the root-associated soil microorganisms may occur after herbage removal from this important rangeland plant. These responses are most likely associated with increased exudate release following herbage removal, which has been best documented using blue grama grown under sterile conditions.     

Abbau von 14C-, 3H- und 36Cl-markiertem γ-Hexachlorcyclohexan durch anaerobe Bodenmikroorganismen

Zusammenfassung Durch eine anaerobe Mischflora aus Ackerboden wurde -Hexachlorcyclohexan (-HCH) in 4–5 Tagen zu 90% abgebaut. Dabei erfolgte eine schnelle Abspaltung des Chlors in Form von Chloridionen und danach eine Freisetzung des C- und H-Anteiles in Form flüchtiger Verbindungen, in denen kein Chlor und auch kein CO₂ nachzuweisen war.Die Verwendung von ¹⁴C/³H- und ³⁶Cl/³H-doppelmarkiertem -HCH zeigte, daß die Cl- und H-Abspaltung nicht im Verhältnis von 1:1 erfolgte, sondern mehr Cl als H abgespalten wurde. Die flüchtigen Verbindungen enthielten andererseits höhere ¹⁴C- als ³H-Anteile. Gaschromatographische Untersuchungen zeigten ebenfalls eine rasche Verminderung des -HCH und die Bildung verschiedener Metabolite. Es wurde jedoch kein -Pentachlorcyclohexen nachgewiesen. Bei steigenden O₂-Gehalten in der Gasphase verminderte sich der -HCH-Abbau. Jedoch fanden auch noch bei 5% O₂ Chlorabspaltung und die Freisetzung flüchtiger Metabolite statt.-HCH wurde ebenfalls, jedoch langsamer, durch die anaerobe Mischflora abgebaut. Auch hier wurde Chlorid abgespalten, und es traten ebenfalls flüchtige Verbindungen auf, die kein Chlor enthielten.

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Degradation of ¹⁴C-, ³H- and ³⁶Cl-labelled -hexachlorocyclohexane by anaerobic soil microorganisms

Up to 90% of the -Hexachlorocyclohexane (-HCH) applied to an anaerobic mixed bacterial flora enriched from an arable soil were degraded within 4–5 days. Degradation resulted in a rapid release of chloride and in formation of chlorine-free volatile metabolites. CO₂ formation from the molecule was not detected.Investigations with ¹⁴C/³H- and ³⁶Cl/³H double-labelled -HCH indicated that the release of Cl and H did not occur in the ratio of 1:1. More Cl than H was split off. The volatile compounds contained more ¹⁴C than ³H. Gas chromatographic studies also showed the rapid decrease of -HCH and the formation of several metabolites. -Pentachlorocyclohexene was not detected. Increasing O₂-contents in the gas phase of cultures resulted in decreases of the compound's degradation. Release of chloride and of volatile metabolites were observed with O₂ contents in the gas phase up to 5%.-HCH was also, but more slowly as with -HCH, degraded by the anaerobic mixed flora. Chloride was released and volatile, chlorine-free metabolites were found.

     

A Simple Novel Agar Diffusion Method for Isolation of Indigenous Microalgae Chlamydomonas sp. CRP7 and Chlorella sp. CB4 from Operational Swampy Top Soil

A simple agar diffusion method is developed where pure colony of Chlamydomonas sp. CRP7 was isolated from Chlorella sp. CB4 mixtures by passing through agar migration with a light exposure of 6,000 lux for 7 h. The main concept behind it is that Chlamydomonas has flagella and the rhodopsin pigment is attracted towards light. Thus the above two microalgae species can be separated from the mixtures as eye spot serves as a navigator and flagella serves as a propeller for Chlamydomonas spp. Further the genomic DNA was isolated and purified from the above mentioned two species after the separation from the mixtures. PCR amplification was carried out for ITS1, 5.8S and ITS2 regions. The amplified products were sequenced and the sequence analysis confirmed that they belong to Chlamydomonas sp. and Chlorella sp. This is an important augmentation for isolation and separation of microalgae.     

What is the role of arbuscular mycorrhizal fungi in plant-to-ecosystem responses to Elevated atmospheric CO2?

 We advocate the concept of an arbuscular mycorrhiza (AM) as a temporally and spatially complex symbiosis representing a suite of hosts and fungi, as against the more traditional "dual organism" view. We use the hierarchical framework presented in Fig. 1 as a basis for organizing many unanswered questions, and several questions that have not been asked, concerning the role of AM in responses to elevated atmospheric CO₂. We include the following levels: plant host, plant population, plant community, functional group and ecosystem. Measurements of the contributions of AM fungi at the various levels require the use of different response variables. For example, hyphal nutrient translocation rates or percent AM root infection may be important measures at the individual plant level, but hyphal biomass or glomalin production and turnover are more relevant at the ecosystem level. There is a discrepancy between our knowledge of the multifaceted role of AM fungi in plant and ecosystem ecology and most of the current research aimed at elucidating the importance of this symbiosis in global-change scenarios. Our framework for more integrated and multifactorial research on mycorrhizal involvement in regulating CO₂ responses may also serve to enhance communication between researchers working at different scales on large global-change ecosystem projects. Accepted: 12 February 1999     

Development of a 16S rRNA gene-based prototype microarray for the detection of selected actinomycetes genera

Actinomycetes are known for their secondary metabolites, which have been successfully used as drugs in human and veterinary medicines. However, information on the distribution of this group of Gram-positive bacteria in diverse ecosystems and a comprehension of their activities in ecosystem processes are still scarce. We have developed a 16S rRNA-based taxonomic microarray that targets key actinomycetes at the genus level. In total, 113 actinomycete 16S rRNA probes, corresponding to 55 of the 202 described genera, were designed. The microarray accuracy was evaluated by comparing signal intensities with probe/target-weighted mismatch values and the Gibbs energy of the probe/target duplex formation by hybridizing 17 non-actinomycete and 29 actinomycete strains/clones with the probe set. The validation proved that the probe set was specific, with only 1.3% of false results. The incomplete coverage of actinomycetes by a genus-specific probe was caused by the limited number of 16S rRNA gene sequences in databases or insufficient 16S rRNA gene polymorphism. The microarray enabled discrimination between actinomycete communities from three forest soil samples collected at one site. Cloning and sequencing of 16S rRNA genes from one of the soil samples confirmed the microarray results. We propose that this newly constructed microarray will be a valuable tool for genus-level comparisons of actinomycete communities in various ecological conditions.     

A conceptual model of organo-mineral interactions in soils: self-assembly of organic molecular fragments into zonal structures on mineral surfaces

In this paper, we propose a structure for organo-mineral associations in soils based on recent insights concerning the molecular structure of soil organic matter (SOM), and on extensive published evidence from empirical studies of organo-mineral interfaces. Our conceptual model assumes that SOM consists of a heterogeneous mixture of compounds that display a range of amphiphilic or surfactant-like properties, and are capable of self-organization in aqueous solution. An extension of this self-organizational behavior in solution, we suggest that SOM sorbs to mineral surfaces in a discrete zonal sequence. In the contact zone, the formation of particularly strong organo-mineral associations appears to be favored by situations where either (i) polar organic functional groups of amphiphiles interact via ligand exchange with singly coordinated mineral hydroxyls, forming stable inner-sphere complexes, or (ii) proteinaceous materials unfold upon adsorption, thus increasing adhesive strength by adding hydrophobic interactions to electrostatic binding. Entropic considerations dictate that exposed hydrophobic portions of amphiphilic molecules adsorbed directly to mineral surfaces be shielded from the polar aqueous phase through association with hydrophobic moieties of other amphiphilic molecules. This process can create a membrane-like bilayer containing a hydrophobic zone, whose components may exchange more easily with the surrounding soil solution than those in the contact zone, but which are still retained with considerable force. Sorbed to the hydrophilic exterior of hemimicellar coatings, or to adsorbed proteins, are organic molecules forming an outer region, or kinetic zone, that is loosely retained by cation bridging, hydrogen bonding, and other interactions. Organic material in the kinetic zone may experience high exchange rates with the surrounding soil solution, leading to short residence times for individual molecular fragments. The thickness of this outer region would depend more on input than on the availability of binding sites, and would largely be controlled by exchange kinetics. Movement of organics into and out of this outer region can thus be viewed as similar to a phase-partitioning process. The zonal concept of organo-mineral interactions presented here offers a new basis for understanding and predicting the retention of organic compounds, including contaminants, in soils and sediments.     

广西木论喀斯特森林土壤养分水平与植被及地形的关系

研究了广西木论喀斯特森林原生植被和次生植被的土壤养分特征,测定了陡坡、缓坡、洼地3种主要地形内林地0～5 cm表层土壤中的有机质(SOM)、氮、磷、钾含量,以及它们在石土面、土面两类主要小生境间的差异.结果表明:林分成熟度高、优势种不明显的原生林积累了高水平的SOM,缓坡石土面最高,达345 g kg-1,洼地的含量较低,但仍在80 g kg-1以上;土壤全氮(TN)、碱解氮(AN)含量也较高.以红背山麻杆(Alchornea trewioides)、广西野桐(Mallotus conspuyrcatus)、伞花木(Eurycorymbus cavderiei)等为优势种的次生林,物种多样性较原生林低,SOM、TN、AN含量也低,缓坡石土面和土面小生境的SOM分别为110 g kg-1和77 g kg-1,是原生林的32%和35%,下降幅度最大,其次为陡坡,而洼地的差异不明显.全磷(TP)、有效磷(AP)、全钾(TK)、有效钾(AK)含量在两林型间差异无明显规律.原生林和次生林土壤养分含量(除TK外)以缓坡>陡坡>洼地;但次生林中三者间无显著差异.同类型样地内小生境土壤养分含量(除TK外)均为石土面含量高于土面.     

连作对三七种子萌发及种苗生长的影响

为进一步明确三七的连作效应,以不同种植年限及不同空间分布的土壤为栽培基质,研究了连作对三七种子萌发及种苗生长的影响。结果表明:连作使三七种子萌发的最大速度增加,种子的发芽势变化不大,而发芽率、发芽指数和快速发芽期则呈明显降低或变短的趋势;与根区外土相比,根区土和根区下土使三七种子的发芽势、发芽率和发芽指数显著降低,对种子萌发的最大速度也有降低的作用;种植1年三七的土壤(1年土)对后茬三七种苗的生长并无明显障碍效应,2年土比3年土具有更强的抑制三七种苗生长的作用。连作对三七种子萌发及种苗生长均会产生明显的障碍效应,自毒作用可能只是造成三七连作障碍的原因之一。     

甲氰菊酯和辛硫磷及其混剂的土壤微生物降解

The degradation rates of fenpropathrin, phoxim and their mixture in un-sterilized soil were much quicker-than those in sterilized soil, which indicated that soil microorganisms played a significant role in the degradation pro-cessin soil. The half-life (T0.5) in un-sterilized soil was 56.2 d for fenpropathrin, 57.8 d for mixed fen-propathrin, 48.2 d for phoxim, and 41.7 d for mixed phoxim. The corresponding half-life (T0.5) in sterilizedsoil was 135.1 d, 147.3 d, 123.6 d, and 126.2 d, respectively. There were no significant differences for degradation rates between single use and mixed use of fenpropathrin and nhoxirn.