Control of a futile urea cycle by arginine feedback inhibition of ornithine carbamoyltransferase in Agrobacterium tumefaciens and Rhizobia

In Agrobacterium tumefaciens and Rhizobia arginine can be used as the sole nitrogenous nutrient via degradation by an inducible arginase. These microorganisms were found to exhibit arginine inhibition of ornithine carbamoyltransferase activity. This inhibition is competitive with respect to ornithine (Km for ornithine = 0.8 mM; Ki for arginine = 0.05 mM). This type of urea cycle regulation has not been observed among other microorganisms which degrade arginine via an arginase. The competitive pattern of this inhibition leads to its being inoperative in ornithine-grown cells, where the intracellular concentration of ornithine is high. In arginine-grown cells, however, the intracellular arginine and ornithine concentrations are compatible with inhibition and ornithine recycling appears to be effectively blocked in vivo.     

Relationship between phosphate stress,effluent quality,and observed cell yield in a pure-oxygen activated-sludge plant

The effect of continuous and discontinuous phosphate stress on the observed cell yield coefficient was investigated in a petrochemical pure-oxygen wastewater treatment plant. A continuous phosphate stress in the activated-sludge system resulted in a reduction of sludge production by 36%, but no improvement of the effluent quality was noticed compared with that of nonstress treatment. However, discontinuous phosphate stress produced a high-quality effluent due to the excellent setting settling properties of the sludge flocs. In addition, the observed cell yield coefficient and concomitantly the sludge production remained 45% below normal. The introduction of the discontinuous phosphate stress in the pure-oxygen activated-sludge plant at the BP Chemicals works in Antwerp resulted in a 24% reduction of the total wastewater treatment operating costs.