Biodegradability of natural dissolved organic matter collected from a UK moorland stream.

The fate of dissolved organic matter (DOM) exported from headwaters is a large uncertainty in global carbon models and catchment biogeochemical process understanding. We examined the biodegradability of stream DOM collected during different flow conditions (n = 12) from a heather-dominated moorland headwater in NE Scotland. Freeze-dried DOM isolates were characterised, re-dissolved to 10 mg C L(-1), inoculated with indigenous stream sediment microbes and incubated, with and without added nutrients, to assess decomposition rates at different times up to 41 days. Biodegradable DOM ranged from 5.0 to 19% of the total transported DOM, representing 8.54 kg C ha(-1) yr(-1) (11.1% of the total DOC flux, calculated as 77.2 ± 39.0 kg C ha(-1) yr(-1)). No simple patterns with flow rate were apparent but accumulated antecedent rainfall, specific UV absorbance of DOM and (15)N content were significant predictors of the proportion of organic matter decomposed. In headwater streams draining organic-rich catchments, in-stream DOM decomposition processes act as a secondary control on the spatial variability of carbon species, and are important for establishing accuracy of aquatic carbon fluxes and cycling budgets. Moreover, biologically-mediated DOM decomposition represents a net 'climate forcing effect' via the soil-stream-atmosphere pathway, drives downstream ecosystem metabolism and should be incorporated in carbon predictive modelling and ecosystem process studies.

The biogeochemical reactivity of suspended particulate matter at nested sites in the Dee basin, NE Scotland.

Variation in the organic matter content associated with suspended particulate matter (SPM) is an often overlooked component of carbon cycling within freshwater riverine systems. The potential biogeochemical reactivity of particulate organic carbon (POC) that affect its interactions and fate, i.e. respired and lost to the atmosphere along river continua or ultimately exported to estuarine and oceanic pools was assessed. Eleven contrasting sites draining nested catchments (5-1837 km(2)) in the River Dee basin, NE Scotland were sampled during summer 2008 to evaluate spatio-temporal variations in quantity and quality (biogeochemical reactivity) of SPM during relatively low flow conditions. Mean SPM concentrations increased from 0.21 to 1.22 mg L(-1) between the uppermost and lowest mainstem sites. Individually, POC concentrations ranged from 0.08 to 0.55 mg L(-1) and accounted for ca. 3-15% of total aqueous organic carbon transported. The POC content was partitioned into autotrophic (2.78-73.0 mg C g(-1) SPM) and detrital (119-388 mg C g(-1) SPM) biomass carbon content. The particulate respired CO(2)-C as a % of the total carbon associated with SPM, measured by MicroResp™ over 18 h, varied in recalcitrance from 0.49% at peat-dominated sites to 3.20% at the lowermost mainstem site. Significant (p<0.05) relationships were observed between SPM biogeochemical reactivity measures (% respired CO(2)-C; chlorophyll α; bioavailable-phosphorus) and arable and improved grassland area, associated with increasing biological productivity downstream. Compositional characteristics and in-stream processing of SPM appear to be related to contributory land use pressures, that influence SPM characteristics and biogeochemistry (C:N:P stoichiometry) of its surrounding aqueous environment. As moorland influences declined, nutrient inputs from arable and improved grasslands increasingly affected the biogeochemical content and reactivity of both dissolved and particulate matter. This increases the potential for recycling of the organic matter that is either transported from upstream or entering further along the riverine continuum.

Application of luminescent biosensors for monitoring the degradation and toxicity of BTEX compounds in soils.

AIMS: To assess the changes in acute toxicity and biodegradation of benzene, toluene, ethylbenzene and xylene (collectively referred to as BTEX) compounds in soil over time and compare the performances of biological and chemical techniques. METHODS AND RESULTS: Biological methods (lux-based bacterial biosensors, basal respiration and dehydrogenase activity) were related to changes in the concentration of the target compounds. There was an initial increase in toxicity determined by the constitutively expressed biosensor, followed by a continual reduction as degradation proceeded. The biosensor with the BTEX-specific promoter was most induced when BTEX concentrations were highest. The treatment with nutrient amendment had a significant increase in microbial activity, while the sterile control produced the lowest level of degradation. SIGNIFICANCE AND IMPACT OF THE STUDY: Luminescent biosensors were able to monitor changes in contaminant toxicity and bioavailability in aqueous extracts from BTEX-impacted soils as degradation proceeded. The integration of biological tests with chemical analysis enables a fuller understanding of the biodegradation processes occurring at their relative rates. CONCLUSIONS: The biological methods were successfully used in assessing the performance of different treatments for enhancing natural attenuation of BTEX from contaminated soils. While, chemical analysis showed biodegradation of parent BTEX compounds in biologically active soils, the biosensor assays reported on changes in bioavailability and potentially toxic intermediate fractions as they estimated the integrative effect of contaminants.

Comparative evaluation of a bioluminescent bacterial assay in terrestrial ecotoxicity testing.

Despite the widespread and successful use of luminescence-based bioassays in water testing, their applications to soils and sediments is less proven. In part this is because such bioassays have mainly been carried out in an aqueous-based medium and, as such, favour contaminants that are readily water-soluble. In this study, aqueous solutions and soils contaminated with heavy metals (HM), polar organic contaminants and hydrophobic organic contaminants (HOCs) were tested using a range of luminescence-based bioassays (Vibrio fischeri, Escherichia coli HB101 pUCD607 and Pseudomonas fluorescens 10586r pUCD607). For the first two chemical groups, the assays were highly reproducible when optimised extraction procedures were employed but for HOCs the bioassay response was poor. Quantitative structure-activity relationships (QSARs) obtained from aqueous solutions had a linear response although correlation for the chemicals tested using bacterial bioassays was significantly less sensitive than that of sublethal tests for Tetrahymena pyriformis. Bacterial and Dendrobaena veneta bioassay responses to extracts from HM amended soils showed that a clear relationship between trophic levels could be obtained. There is no doubt that the wide range of bioluminescent-based bioassays offers complementary applications to traditional testing techniques but there is a significant need to justify and optimise the extraction protocol prior to application.

Degradation and toxicity of phenyltin compounds in soil.

Although the fate of organotins has been widely studied in the marine environment, fewer studies have considered their impact in terrestrial systems. The degradation and toxicity of triphenyltin in autoclaved, autoclaved-reinoculated and non-sterilised soil was studied in a 231 day incubation experiment following a single application. Degradation and toxicity of phenyltin compounds in soil was monitored using both chemical and microbial (lux-based bacterial biosensors) methods. Degradation was significantly slower in the sterile soil when compared to non-sterilised soils. In the non-sterilised treatment, the half-life of triphenyltin was 27 and 33 days at amendments of 10 and 20 mg Sn kg(-1), respectively. As initial triphenyltin degradation occurred, there was a commensurate increase in toxicity, reflecting the fact that metabolites produced may be both more bioavailable and toxic to the target receptor. Over time, the toxicity reduced as degradation proceeded. The toxicity impact on non-target receptors for these compounds may be significant.

Linking biosensor responses to Cd, Cu and Zn partitioning in soils.

Soils bind heavy metals according to fundamental physico-chemical parameters. Bioassays, using bacterial biosensors, were performed in pore waters extracted from 19 contrasting soils individually amended with Cd, Cu and Zn concentrations related to the EU Sewage Sludge Directive. The biosensors were responsive to pore waters extracted from Zn amended soils but less so to those of Cu and showed no toxicity to pore water Cd at these environmentally relevant amended concentrations. Across the range of soils, the solid-solution heavy metal partitioning coefficient (K(d)) decreased (p<0.01) with increasing amendments of Cu and Zn; Cu exhibited the highest K(d) values. Gompertz functions of Cu and Zn, K(d) values against luminescence explained the relationship between heavy metals and biosensors. Consequently, biosensors provide a link between biologically defined hazard assessments of metals and standard soil-metal physico-chemical parameters for determining critical metal loadings in soils.