High-resolution NO3
− profiles in freshwater sediment covered with benthic diatoms were obtained with a new microscale NO3
− biosensor characterized by absence of interference from chemical species other than NO2
− and N2O. Analysis of the microprofiles obtained indicated no nitrification during darkness, high rates of nitrification and a tight coupling between nitrification and denitrification during illumination, and substantial rates of NO3
− assimilation during illumination. Nitrification during darkness could be induced by purging the bulk water with O2 gas, indicating that the stimulatory effect on nitrification by illumination was caused by algal production of O2. NH4
+ addition did not stimulate nitrification during darkness when O2 was restricted to the upper 1-mm layer, and there was thus a low nitrification potential in the permanently oxic top 1 mm of the sediment.
_ _ _ _ _ _ _ _ _~~ ~~~~The gas permeability coefficient of the cyanobacterial gas vesicle wall has been determined by comparing the concentration gradient of oxygen gas in a film of gas vesicles with the gradient in an underlying film of agar supported over an oxygen atmosphere. The gradients were determined with an oxygen microelectrode. The value of the gradient in aqueous agar was 0.81 of that in a suspension in which gas vesicles occupied 0-35 of the total volume. From this it was calculated that the notional diffusivity of oxygen through the gas vesicle was equivalent to 0.53 of the diffusivity in water. The permeability coefficient of the gas vesicle membrane is calculated to be K = 32 mm s-* , the rate coefficient for filling the gas vesicle by diffusion is a = 2.4 x 106 s-I and the folding time for equilibration of gas into a gas vesicle is t, = 0.4 ps. The permeability coefficient is about 100-fold higher than the minimum value set by previous pressure rise experiments, and confirms that gas vesicles could not store gas. The measurements also show, however, that randomly oriented gas vesicles would not provide a diffusion channel with a diffusivity higher than that in water, although a layer of gas vesicles oriented with their long axes parallel to the diffusion gradient would provide a diffusivity 3.5-fold higher. The determination of the diffusivity was made with a theory, based on diffusion equations, which can be used in the determination of the diffusivity through other cell organelles.
Microelectrodes for O2 and NO2−/NO3− and fluorescently labelled 16S rRNA-targeted oligonucleotide probes were combined to examine the activity and stratification of nitrifying bacteria in a trickling filter biofilm. Microprofiles showed that O2 consumption and NO3−/NO2− production were restricted to the upper 50-100 μm of the biofilm. The vertical distribution of the nitrifying bacteria Nitrosomonas sp. and Nitrobacter sp. was investigated by fluorescent in situ hybridisation (FISH) with specific oligonucleotides. Nitrifiers formed a dense layer of cells and cell clusters in the upper part of the biofilm. This correlates well with the measured activity profiles.
Ammonia- and nitrite-oxidisers occurred in close vicinity to each other supporting a fast sequential metabolism from ammonia to nitrate. Both species were not restricted to the oxic part of the biofilm, but also appeared -in lower numbers- in the anoxic layers on the bottom of the biofilm.
A short term decrease in the O2 concentration of the bulk water resulted in a quick decrease in O2 penetration and metabolic rates inside the biofilm. However, neither the stratification nor the cellular ribosome content of nitrifiers changed within a few hours.
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