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Calcium alginate tympanic membrane perforation patches offer a significant advantage in the repair of chronic perforations over traditional techniques in the chinchilla perforation model and may offer attractive opportunities in the clinical setting.

Genetic exchange between bacteria in natural habitats is believed to be strongly influenced by availability of growth substrates and growth rate. To test this hypothesis, conjugal transfer of an RP4 derivative between Pseudomonas fluorescens and Serratia sp. was studied in a simple sand/plant microcosm and was related to availability of root exudates and bacterial metabolic activity. The presence of the plant (Echinochlora crusgalli) greatly stimulated transfer. Average transfer ratios (T/ DWR) were 8.9U10 3II in the rhizosphere and 5.5U10 3IS in sand unaffected by the plant root. The bacteria used root exudates as growth substrate and depending on cell density, the metabolic activity ([ Q H]leucine uptake) of the cells was higher in the rhizosphere than in the sand (around 1.0U10 3P and 0.5U10 3P fmol leucine CFU 3I h 3I , respectively). Thus, an apparent correlation between metabolic activity and transfer was observed. Additional experiments, however, suggested that there was no causal relationship between the two. When incubated in a sand microcosm containing root exudates, the metabolic activity of the bacteria increased while conjugal transfer ratios remained constant. Hence, contrary to the accepted view, root exudates and metabolic activity did not appear to be responsible for the stimulation of conjugal transfer in the rhizosphere. z 1998 Federation of European Microbiological Societies. Published by Elsevier Science B.V.

Methods are described for toxicity testing of water and sediment with two varieties of the freshwater marsh plant Echinochloa crusgal/i (Linneaus) Palisot de Beauvois (Poaceae), and complex effluents. Two tests are described: a seed germination and early seedling growth test in water, and a survival and seedling growth test in natural and synthetic sediments. Effects of effluents from a sewage treatment plant, tannery, textile mill, pulp and paper mill, coking plant and sewage treatment plant included inhibition of germination, chlorophyll synthesis and growth. The tests with rooted marsh plants were sensitive to pollutants and detected toxicity of a range of pollutants in water and sediment. Synthetic sediments similar to natural sediments allowed toxicity tests to be done under carefully controlled conditions of particle size distribution, organic content, pH, electrode potential (Eh) and cation exchange capacity (CEC).

Acid precipitation falls on vast areas of forested land, including most of the eastern deciduous forest of the United States. Forest productivity, ground‐water quality, and surface waters might all be affected. To document and quantify ecosystem response to the onset of acid precipitation, simulated sulfuric acid (H2SO4) rain was applied to model forest plots of sugar maple (Acer saccharum Marsh.) and red alder (Alnus rubra Bong). One set of four plots (two alder and two maple) received a control rain consisting of a stock solution equilibrated with atmospheric CO2 to approximately pH 5.7. For three sets of four plots, sufficient H2SO4 was added to the stock solution to lower the pH to 4.0, 3.5, or 3.0. Nozzles were used to apply simulated acid rain at the rate of 2.8 mm/hour (maple) or 3.7 mm/hour (alder) for 3 hours/day, 3 days/week, throughout the year. Water samples were collected above and below the canopy, below the litter layer, and from 20‐cm and 1‐m depths below the surface of the soil. While constituents of the throughfall were similar to those of the simulated rain, the litter leachate (the actual input to the soil) had consistently higher concentrations of SO42−, Ca2+, and Mg2+, and higher pH, than the acid rain. For the first 6 months, SO42− adsorption by the soil prevented any apparent differences in the SO42−, Ca2+, or Mg2+ concentrations or the pH of samples taken from a depth of 20 cm from plots receiving either acid or control treatments. Then, SO42− concentrations at this depth on plots receiving the most acidic rain (pH 3.0) became increasingly higher than on the other plots until, after 3 years, they were approximately equal to SO42− concentrations in the rain. Soil solutions from the pH 3.5 and 4.0 treatments responded similarly starting 10 months and 1 year, respectively, after initiation of exposure to acid rain. Increased Ca2+, Mg2+, and, to a lesser extent, K+ concentrations and lowered pH in solutions from the 20‐cm depths occurred simultaneously with increased SO42− concentrations. At a depth of 1 m, no acid‐rain‐related effects were evident after 2.5 years exposure to pH 3.0 H2SO4 rain. Results showed that a hardwood canopy and litter layer can alter the input of chemicals to the soil from acid rain, that the relative sensitivity of ions to mobilization by acid rain is not the same for all components of an ecosystem, and that acid rain can alter the chemical composition of water within the root zone within a few years, even if the soil is a strong sulfate adsorber.