I. Cacciari

Phytohormone-like substances produced by single and mixed diazotrophic cultures of Azospirillum and Arthrobacter

The production of phytohormone-like substances byAzospirillum brasilense andArthrobacter giacomelloi were investigated in single and mixed batch cultures under diazotrophic conditions. The mixed culture showed higher productivity of gibberellins and cytokinins.

Effect of salinity on growth and starvation-survival of a tropical Rhizobium strain

Abstract The effect of salinity on growth, starvation-survival and recovery from salt stress of a Rhizobium sp. strain isolated from nodules of Acacia tortilis from a soil of Senegal was studied. Growth parameters of C-limited continuous cultures, grown in the presence and the absence of 342 mM NaCl, decreased in the saline medium and with increasing dilution rates. The survival capacity of starved cultures depended on the previous growth conditions: culturability of cells grown with salt was inversely related to growth rate, while culturability increased with increasing dilution rate for cultures grown without salt. Culturability of the cultures subjected to the double stress of starvation and salinity was reduced and a high percentage of cells entered the viable but nonculturable state. All the starved cultures were capable of regrowth when nutrients became available, thus showing that this strain can withstand long periods of nutrient deprivation in soil while maintaining the capacity for an active metabolism and a potential infectiousness toward an appropriate host.

Response to oxygen of diazotrophic Azospirillum brasilense — Arthrobacter giacomelloi mixed batch culture

Azospirillum brasilense and Arthrobacter giacomelloi were grown together in batch culture under different oxygen pressures. The response to oxygen of growth, nitrogenase activity and respiration rate was determined. The two microorganisms were found to be able to coexist all over the range of partial oxygen pressures examined, that is from 0.004–0.20 bar. Nitrogenase activity by mixed culture of A. brasilense and A. giacomelloi always appeared higher than that of A. brasilense pure culture. Low respiratory activity at partial oxygen pressures higher than 0.02 bar by both pure and mixed cultures seemed not to account for the high nitrogenase activity and improved oxygen tolerance of the mixed culture.

Regulation by ammonium and glucose of Arthrobacter fluorescens alanine dehydrogenase

Alanine dehydrogenase in Arthrobacter fluorescens exhibited an allosteric behaviour and two Km values for ammonium were estimated. In batch cultures at different ammonium concentrations and in continuous culture following an NH4+ pulse, the level of ADH activity seems to be regulated by the ammonium concentration, high activities being observed when extracellular ammonium was in excess. The response to the growth rate of an ammonium-limited chemostat culture of A. fluorescens seems to indicate that alanine dehydrogenase and glutamine synthetase activities were inversely related. High activities of glutamate oxaloacetate transaminase and glutamate pyruvate transaminase have been found in crude extract of ammonium-limited cultures. From the results obtained in batch cultures grown at different glucose concentrations and in carbon-limited chemostat culture it appeared that the limitation by glucose influenced alanine dehydrogenase activity negatively. No glutamate dehydrogenase activity and no glutamate synthase activity could be detected with either NADH or NADPH as coenzymes.

Ammonium assimilation in an arthrobacter sp fluorescens

Ammonium assimilation was studied in a nitrogenfixing Arthrobacter strain grown in both batch and ammonium-limited continuous cultures. Arthrobacter sp. “fluorescens” grown in nitrogen-free medium or at low ammonium levels assimilated NH4+via the glutamine synthetase/glutamate synthase pathway. When ammonium was in excess it was assimilated via the alanine dehydrogenase pathway. Very low levels of glutamate dehydrogenase were found, irrespective of growth conditions.