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The potential of soil microorganisms to enhance the retention of (137)Cs and (85)Sr in organic systems was assessed in a series of experiments. A biologically active, 'mineral-free', organic material, produced under laboratory conditions from leaves, was used as the uptake matrix in all experiments to minimise potential interference from competing clay minerals. Biological uptake and release were differentiated from abiotic processes by comparing the sorption of radionuclides in sterilised organic material with sterile material inoculated with soil extracts or single fungal strains. Our results show conclusively that living components of soil systems are of primary importance in the uptake of radionuclides in organic material. The presence of soil microorganisms significantly enhanced the retention of Cs in organic systems and approximately 70% of the Cs spike was strongly (irreversibly) bound (remained non-extractable) in the presence of microorganisms compared to only approximately 10% in abiotic systems. Sorption of (85)Sr was not significantly influenced by the presence of soil microorganisms. A non-linear temperature response was observed for the retention in biotic systems with increased uptake at between 10 and 30 degrees C and lower retention at temperatures above or below the optimum range. The optimum temperatures for biological uptake were between 15 and 20 degrees C for Cs, and 25 and 30 degrees C for Sr. Our results indicate that single strains of soil and saprotrophic fungi make an important contribution to the sorption of Cs and Sr in organic systems, but can only account for part of the strong, irreversible binding observed in biotic systems. Single strains of soil fungi increased the amount of non-extractable (137)Cs (by approximately 30%) and (85)Sr (by approximately 20%) in the organic systems as compared to abiotic systems, but the major fraction of (137)Cs and (85)Sr sorbed in systems inoculated with saprotrophic fungi remained extractable.

An investigation was conducted to determine the impact of acute doses of gamma radiation on the microbial community structure of a Holcus lanatus dominated grassland soil. Mesocosms containing soil and established grass were irradiated using a sealed (137)Cs source (7.0 Gy min(-1)). Doses ranged from 5 to 160 Gy, analyses were conducted on the day of irradiation, then 7 and 30 days later. Plant growth and arbuscular mycorrhizal fungal colonisation of roots were reduced by irradiation. Gram-negative bacteria, and microbial metabolic capacity were also negatively affected by treatment. Microbial biomass measured by phospholipid fatty acid (PLFA) analysis, showed an increase at doses above 20 Gy, 7 and 30 days after treatment. Proportions of Gram-positive bacterial and fungal PLFAs fluctuated inversely to each other, in response to both sampling time and radiation dose. We hypothesise that many of the observed soil microbial responses are indirect effects mediated by the influence of ionising radiation on the plants in this system.

An assessment of the biodiversity of soils was a component of the Countryside Survey 2000 (CS2000). This was the first integrated survey of soil biota and chemical properties at a national scale. A total of 1052 soil samples were collected across Great Britain during CS2000 and analysed for a range of soil microbial and invertebrate characteristics resulting in the production of a series of robust datasets. A principal objective was to use these datasets to investigate relationships between soil biota and environmental factors such as geographical location, vegetation, land use, land cover, soil type and pollutant levels as first stages in characterising the inherent biodiversity of British soils and investigating the potential of soil biodiversity as indicators of soil health at a regional or national scale. Preliminary results for culturable heterotrophic, invertebrate taxa, Acari, Collembola and Oribatid mites are presented here to illustrate the nature of the data collected and the patterns of soil biodiversity in relation to large-scale regional, vegetation and soil characteristics across the British countryside.

The genetic diversity of 128 carbofuran-degrading bacteria was determined by ARDRA (amplified ribosomal DNA restriction analysis) of 16S rDNA and restriction fragment length polymorphism analysis of the 16S-23S rDNA spacer region (IGS) using five endonucleases. The isolates were distributed in 26 distinct ARDRA groups and 45 IGS types revealing a high level of microbial diversity confirmed by ARDRA clustering and sequencing of 16S rDNA. The occurrence of a methylcarbamate-degrading gene (mcd) was monitored by polymerase chain reaction amplification using specific primers. The mcd gene was detected only in 58 bacteria and there was no clear relationship between the presence of this gene and the phylogenetic position of the strain.

Well-drained non-agricultural soils mediate the oxidation of methane directly from the atmosphere, contributing 5 to 10% towards the global methane sink. Studies of methane oxidation kinetics in soil infer the activity of two methanotrophic populations: one that is only active at high methane concentrations (low affinity) and another that tolerates atmospheric levels of methane (high affinity). The activity of the latter has not been demonstrated by cultured laboratory strains of methanotrophs, leaving the microbiology of methane oxidation at atmospheric concentrations unclear. Here we describe a new pulse-chase experiment using long-term enrichment with 12CH4 followed by short-term exposure to 13CH4 to isotopically label methanotrophs in a soil from a temperate forest. Analysis of labelled phospholipid fatty acids (PLFAs) provided unambiguous evidence of methane assimilation at true atmospheric concentrations (1.8-3.6 p.p.m.v.). High proportions of 13C-labelled C18 fatty acids and the co-occurrence of a labelled, branched C17 fatty acid indicated that a new methanotroph, similar at the PLFA level to known type II methanotrophs, was the predominant soil micro-organism responsible for atmospheric methane oxidation.

Microorganisms are responsible for driving the biogeochemical cycling of elements on Earth. Despite their importance and vast diversity, the taxonomic identity of the microorganisms involved in any specific process has usually been confined to that small fraction of the microbiota that has been isolated and cultivated. The recent coupling of molecular biological methods with stable-isotope abundance in biomarkers has provided a cultivation-independent means of linking the identity of bacteria with their function in the environment. Here we show that 13C-DNA, produced during the growth of metabolically distinct microbial groups on a 13C-enriched carbon source, can be resolved from 12C-DNA by density-gradient centrifugation. DNA isolated from the target group of microorganisms can be characterized taxonomically and functionally by gene probing and sequence analysis. Application of this technique to investigate methanol-utilizing microorganisms in soil demonstrated the involvement of members of two phylogenetically distinct groups of eubacteria; the alpha-proteobacterial and Acidobacterium lineages. Stable-isotope probing thus offers a powerful new technique for identifying microorganisms that are actively involved in specific metabolic processes under conditions which approach those occurring in situ.