Previous studies have documented dissimilatory growth of bacteria on solid Mn 4ϩ oxide, but Mn 3ϩ oxides have not been previously studied; here we have demonstrated for the first time the bacterial reduction of manganite. Strain MR-4 of Shewanella putrefaciens was able to grow on and rapidly reduce insoluble needle-shaped crystals of synthetic manganite (MnOOH), converting them to soluble Mn 2ϩ in the process. The rate of Mn 3ϩ reduction was optimal at pH of 7.0 and 26 ЊC consistent with an enzymatic reaction. In addition the rates of reduction were in proportion to the amount of manganite added, but nearly independent of the cell concentration present (e.g., cell number had only a small effect on the rate of Mn 3ϩ reduction at early stages of growth) suggesting that surface properties were dictating the rates of metal reduction. This thesis was supported by major differences in reduction rates when Mn oxides of different surface areas were studied. Removal of the carbon source (formate or lactate) or addition of metabolic inhibitors reduced the rate of metal reduction. No Mn 3ϩ reduction was observed when the samples were oxygenated, nor when the cells were separated from the Mn 3ϩ oxide by a dialysis membrane. Environmental scanning electron microscopy (ESEM) images showed close contact of the cells with the needle-shaped crystals during early stages of reduction. In later stages, the closely associated cells were coated with a layer of extracellular polymeric material that obscured the cells when viewed by ESEM. When manganite crystals were dried and gold coated, and viewed by standard scanning electron microscopy (SEM), abundant bacteria could be seen on the surface of the metal oxide in a thin biofilm-like layer. The layer of extracellular polymer is a new finding, and neither the composition, function, nor importance in the manganese reduction process have been elucidated.
SUMMARYIn pot experiments with oats, the supply of nitrogen and simazine was varied in such a way that t0 quantities of nitrogen and 6 quantities of simazine were combined factorially. Green oat plants, oat grain, and oat straw were examined for content of dry matter, total nitrogen as weil as for phosphorus, potassium, sodium, calcium, and magnesium. Furthermore the grain was analyzed for amino acids.The investigations have shown that the course of the yield curve for nitrogen can be varied by application of simazine in such a way that it becomes sigmoid. The courses of the respective yield and concentration curves show that an unfavourable ratio of the plant nutrients applied, for example with the very lowest or the very highest nitrogen supplies, causes a high concentration of nitrogen, i.e. a low utilization of the absorbed nitrogen. This low utilization is strengthened by the application of simazine. Reduced yield of dry matter, either due to unfavourable ratios of the plant nutrients applied or to the application of a metabolic inhibitor -in this case simazine -causes in both cases an increase in the nitrogen concentration, i.e. a falling utilization of the absorbed nitrogen. The curves were, broadly speaking, affected in the same way, whether due to varied ratios of the plant nutrients applied, or to varied applications of simazine.With nitrogen supplies producing maximum yield at maturity, the amount of nitrogen absorbed is the same, independent of the application of simazine, but the utilization of the quantity of nitrogen absorbed in the production of dry marter falls when simazine is applied. The fall in the utilization is less pronounced at a nitrogen supply of 3 g per pot and above.Increasing nitrogen supplies cause an increase in the glutamic acid, histidine and arginine content of the protein of the oat grain, while the content of the other amino acids is comparatively unaffected. In the treatment without nitrogen, simazine had the same effect on the amino acid composition of the protein as the application oi nitrogen, while in the other treatments where 476
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