Leaching of nitrate from soils and sediments can be reduced in anoxic environments due to denitrification to N 2 O/N 2 or reduction of nitrate to ammonium. While microbial dissimilatory reduction of nitrate to ammonia is well known, it is shown here that this conversion can also proceed at appreciable rates in abiotic systems in the presence of green rust compounds [Fe II 4 Fe III 2 (OH) 12 SO 4 ‚yH 2 O]. In the reaction nitrate is stoichiometrically reduced to ammonium, and magnetite (Fe 3 O 4 ) is the sole Fe-containing product. At a constant pH of approximately 8.25 and 25 °C, the rate expression is given as: dwhere k ) 4.93 × 10 -5 ( 0.39 × 10 -5 L mol -1 s -1 . In anoxic soils and sediments, this reaction may also lead to a nitrate to ammonium reduction, at rates of similar magnitude or even higher than microbial reduction rates. Hence green rust should be considered a possible important reductant for nitrate reduction to ammonium in subsoils, sediments, or aquifers where microbially mediated reduction rates are small.

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Seasonal variation of chlorophyll content, photosynthesis, 0, respiration, and denitrification was measured under light and dark conditions in the sediment of a nutrient-rich Danish lowland stream. Exponential growth of benthic microalgae was observed in early spring (April-May) and photosynthetic capacity persisted until fall. The benthic algae were a major C source for heterotrophic activity as indicated by a close correlation between 0, respiration and Chl content in the sediment. Denitrilication activity was related to Chl content, NO,-availability, and 0, conditions. Diffusion from the overlying water was always the major NO,-source for denitrification. Under lighted conditions, photosynthetic 0, production increased the oxic zone and reduced denitrification activity by up to 85% in spring. A simple diffusion-reaction model allowed denitrification rates to be estimated from 0, respiration rates and concentrations of O2 and N03-in the stream water. Throughout the season, estimated denitrification rates correlated well with those actually measured. The model demonstrated that denitrification activity was controlled primarily by the thickness of the oxic surface layer which served as a diffusion barrier for NO,-to the denitrification zone.

.Abstract Membrane-covered platinum electrodes with a tip diameter of 2-8 pm were used for an amperometric assay of dissolved oxygen in marine sediments. The oxygen profile extended to 3-5mm depth in nonilluminated sediment; even at high light intensities and at low temperatures it did not extend below lo-mm depth in a homogeneous sandy sediment. Oxygen profiles recorded during light-dark cycles were used to estimate the rates of oxygen production and consumption and also to calculate the apparent diffusion coefficient for oxygen in the sediment. Apparently macrofaunal activity, rather than molecular diffusion and water turbulence, was important for the occasional transport of oxygen into deeper layers and thus for the provision of oxidized conditions (positive redox potential) down to 5-10 cm below the sediment surface.

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The addition of 20 mM MoO2-(molybdate) to a reduced marine sediment completely inhibited the SOreduction activity by about 50 nmol g-1 h-' (wet sediment). Acetate accumulated at a constant rate of about 25 nmol g-1 h-1 immediately afterMoO4addition and gave a measure of the preceding utilization rate of acetate by the SO4-reducing bacteria. Similarly, propionate and butyrate (including isobutyrate) accumulated at constant rates of 3 to 7 and 2 to 4 nmol g' h-1, respectively. The rate of H2 accumulation was variable, and a range of 0 to 16 nmol g-1 h-' was recorded. An immediate increase of the methanogenic activity by 2 to 3 nmol g-1 h-1 was apparently due to a release of the competition for H2 by the absence of SO4reduction. If propionate and butyrate were completely oxidized by the SO4-reducing bacteria, the stoichiometry of the reactions would indicate that H2, acetate, propionate, and butyrate account for 5 to 10, 40 to 50, 10 to 20, and 10 to 20%, respectively, of the electron donors for the

Pseudomonas fluorescens DR54 showed antagonistic properties against plant pathogenic Pythium ultimum and Rhizoctonia solani both in vitro and in planta. Antifungal activity was extractable from spent growth media, and fractionation by semi‐preparative HPLC resulted in isolation of an active compound, which was identified as a new bacterial cyclic lipodepsipeptide, viscosinamide, using 1D and 2D 1H‐, 13C‐NMR and mass spectrometry. The new antibiotic has biosurfactant properties but differs from the known biosurfactant, viscosin, by containing glutamine rather than glutamate at the amino acid position 2 (AA2). No viscosin production was observed, however, when Ps. fluorescens DR54 was cultured in media enriched with glutamate. In vitro tests showed that purified viscosinamide also reduced fungal growth and aerial mycelium development of both P. ultimum and R. solani. Viscosinamide production by Ps. fluorescens DR54 was tightly coupled to cell proliferation in the batch cultures, as the viscosinamide produced per cell mass unit approached a constant value. In batch cultures with variable initial C, N or P nutrient levels, there were no indications of elevated viscosinamide production during starvation or maintenance of the cultures in stationary phase. Analysis of cellular fractions and spent growth media showed that a major fraction of the viscosinamide produced remained bound to the cell membrane of Ps. fluorescens DR54. The isolation, determination of structure and production characteristics of the new compound with both biosurfactant and antibiotic properties have promising perspectives for the application of Ps. fluorescens DR54 in biological control.

Expression of the functional gene tfdA involved in degradation of phenoxyacetic acids such as 2,4-dichlorophenoxyacetic acid (2,4-D) and 4-chloro-2-methylphenoxyacetic acid (MCPA) was investigated during degradation scenarios in natural unseeded soil samples. The results illustrate how messenger RNA (mRNA)-based analysis is well suited to quantitatively study the activity of specific microbial populations in soil using phenoxyacetic acid biodegradation as a model system. Via quantitative real-time PCR, a clear response to the presence of phenoxy acids was shown during degradation in soil amended with 20 mg 2,4-D or MCPA per kg soil. Further, we found a relatively high degree of correlation between expression of the functional gene and the rates of mineralization. Melting curve analyses of real-time PCR products, supported by tfdA-denaturing gradient gel electrophoresis analysis showed that, although only class I tfdA genes were apparent in the indigenous microbial population, class III tfdA genes became predominant during incubation, and were the only genes expressed during degradation of MCPA in soil. In contrast, both classes were expressed during degradation of the structurally similar compound 2,4-D. The ability to quantify microbial transcripts directly in environmental samples will have a profound impact on our understanding of microbial processes in the environment in future studies.