Kassem Alef

Arginine ammonification, a simple method to estimate microbial activity potentials in soils

Abstract A simple, rapid and inexpensive method to determine microbial activity potentials, based on ammonification of arginine, was developed and tested on bacterial cultures and soil samples. The results are highly reproducible and correlate well with respiratory activities. Ammonification starts immediately after the addition of arginine and is linear for more than l h. Both properties show that physiological status and number of microorganisms remain stable during the assay.

A comparison of methods to estimate microbial biomass and N-mineralization in agricultural and grassland soils

The rate of ammonification of arginine-N, after addition to soil is proposed as a rapid index of soil microbial activity. Correlation between arginine ammonification, biomass C, heat output, soil ATP content and soil protease activity, were assessed on twenty-two agricultural and grassland soils collected from thirteen habitats in Germany and Austria. There were highly significant, positive correlations between all methods (r > 0.9). Close relationships were also found between these methods and total soil organic matter content (r = 0.862–0.957), but not with soil pH or the viable microbial count. Our results show a close relationship between the microbial biomass and N-mineralization in soils, and suggest arginine ammonification as a valid indicator of microbial activity in soil.

Regulatory aspects of inorganic nitrogen metabolism in the Rhodospirillaceae

Regulatory aspects of the assimilation of inorganic nitrogen compounds (ammonia, nitrate, nitrogen) were studied in 12 strains belonging to the Rhodospirillaceae. All strains possessed an ammonium transport system, as demonstrated by 14C-methylammonium uptake. This uptake showed saturation kinetics (Km between 50–150 μM), and was competitively inhibited by ammonium (Ki between 5–18 μM). The ammonium transport systems were repressed by ammonium in the growth medium. The nitrogenase activity of all strains was reversibly inhibited by ammonium (“switch-off”). This effect was not shown under nitrogen starvation conditions with the exception of some strains of Rhodopseudomonas capsulata, the nitrogenase of which was always susceptible to switch-off by ammonium. Assimilation of nitrate was confined to some strains of Rhodopseudomonas capsulata.

Evidence for an ammonium transport system in the N2-fixing phototrophic bacteriumRhodospirillum rubrum

An ammonium transport system in the phototrophic N2-fixing bacteriumRhodospirillum rubrum was characterized by using the uptake of14C-methylamine as a probe.Uptake showed saturation kinetics with an apparentKm=110 μM. It was competitively inhibited by ammonium (Ki=7 μM). Uptake exhibited a narrow pH maximum around pH 7.0.Up to 200-fold gradients across the membrane were formed within 40–60 min. Gradient formation was inhibited by carbon starvation, azide or cyanide. Pre-accumulated methylamine was released by ammonium pulses to more than 80%, indicating only minor metabolization.The synthesis of the transport system was repressed by ammonium in high concentrations.

Inhibitory effects of myxothiazol and 2-n-heptyl-4-hydroxyquinoline-N-oxide on the auxiliary electron transport pathways of Rhodobacter capsulatus

The effects of various electron transport inhibitors upon the rates of reduction NO3-, dimethyl sulphoxide (DMSO) and N2O in anaerobic suspensions of Rhodobacter capsulatus have been studied. A new method for the determination of the rates of reduction of these auxiliary oxidants in intact cells is presented, based on the proportionality observed between the concentration of oxidant and the duration of the electrochromic carotenoid bandshift. For NO3-and N2O good agreement was found between rates of reduction determined using electrodes and those determined by the electrochromic method.Myxothiazol and antimycin A had no effect on the rates of reduction of NO3-and DMSO suggesting that the cytochrome b/c1complex is not involved in electron transport to these oxidants. 2-n-heptyl-4-hydroxyquinoline-N-oxide (HOQNO) inhibited at two sites, one within the cytochrome b/c1complex and the other on the nitrate reducing pathay, but had no effect on electron transport to N2O or DMSO. In both intact cells and cell free extracts, HOQNO had no effect on the nitrate dependent re-oxidation of reduced methylviologen (MVH2), a direct electron donor to nitrate reductase.Our data are consistent with a branch point for the auxiliary electron transport pathways at the level of the ubiquinone pool.

Uptake of methionine sulfoximine by some N2 fixing bacteria, and its effect on ammonium transport.

The N2 fixing bacteria Klebsiella pneumoniae, Azospirillum brasilense, Rhodopseudomonas sphaeroides and Rhodospirillum rubrum, but not Azotobacter vinelandii accumulate the glutamine analogue methionine sulfoximine in the cell. In the accumulating cells methionine sulfoximine inhibits ammonium transport. Accumulation and inhibition are prevented by glutamine.

The activities of two pathways of nitrate reduction in Rhodopseudomonas capsulata

Abstract(1)The disappearance of nitrate from suspensions of intact, washed cells of Rhodopseudomonas capsulata strain N22DNAR+ was measured with an ion selective electrode. In samples taken from phototrophic cultures grown to late exponential phase, nitrate disappearance was partially inhibited by light but was not affected by the presence of ammonium. Nitrate disappearance from samples from low density cultures in the early exponential phase of growth was first inhibited and later stimulated by light. In these cells ammonium ions inhibited the light-dependent but not the dark disappearance of nitrate. It is concluded that cells in the early exponential phase of growth possess both an ammonium-sensitive, assimilatory pathway for nitrate reduction (NRI) and an ammonium-insensitive pathway for nitrate reduction (NRII) which is linked to respiratory electron flow and energy conservation. In cells harvested in late exponential phase only the respiratory pathway for pitrate reduction is detectable.(2)Nitrate reduction, as judged by the oxidation of reduced methyl viologen by anaerobic cell suspensions, was measured at high rates in those strains of R. capsulata (AD2, BK5, N22DNAR+) which are believed to possess NRII activity but not in those strains (Kbl, R3, N22) which only manifest the ammonium-sensitive NRI pathway. On this basis we have used nitrate-dependent oxidation of reduced methyl viologen as a diagnostic test for the nitrate reductase of NRII in cells harvested from cultures of R. capsulata strain AD2. The activity was readily detectable in cells from cultures grown aerobically in the dark with ammonium nitrate as source of nitrogen. When the oxygen supply to the culture was withdrawn, the level of methyl viologen-dependent nitrate reductase increased considerably and nitrite accumulated in the culture medium. Upon reconnecting the oxygen supply, methyl viologen-dependent nitrate reductase activity decreased and the reduction of nitrate to nitrite in the culture was inhibited. It is concluded that the respiratory nitrate reductase activity is regulated by the availability of electron transport pathways that are linked to the generation of a proton electrochemical gradient.

Estimation of anaerobic microbial activities in soils by arginine ammonification and glucose-dependent CO2-production

Abstract Simple and inexpensive methods for the estimation of anaerobic microbial activities in soils were developed, based on arginine ammonification and CO 2 production from glucose. Our results show that the physiological status and number of microorganisms remain stable during the assays. Statistical analysis indicated that the anaerobic arginine ammonification is significantly correlated with increased anaerobic CO 2 production after glucose amendment ( r = 0.764, n = 26). Anaerobic arginine ammonification and anaerobic CO 2 production were also correlated with the organic C content of the samples.

Correlation between the arginine ammonification, enzyme activities, microbial biomass, physical and chemical properties of different soils

Summary Significant correlations between arginine ammonification, activities of catalase, dehydrogenase, protease, alkaline phosphatase, saccharase, and microbial biomass were computed in 14 soils under different land use (agriculture, grassland, and nature reserve). Close relations were also found between arginine ammonification, organic carbon, total nitrogen content, cation exchange capacity, and bulk density. No significant correlation was observed between the arginine ammonification and total root mass or p H. Our results provide convincing evidence that the arginine ammonification can be used as an indicator for microbial biomass and activity in soils.