Environment - Mediated Changes in the Chemical Composition and Physiology of Aspergillus Nidulans

A study was made of the chemical composition of Aspergillus nidulans strain 13 mel in glucose limited chemostat culture under varying conditions of growth rate (D = 0.02 h to 0.175 h ), temperature (22°C to 50°C), pH (3*1 to 7-9) and NaCl concentration (0 to 8% w/v). Asexual differentiation occurred at a dilution rate of 0.02 h ^ (10% u ) following a progressive reduction in the glucose feed max rate, and was preceded (at D - 0.05 h ) by major variations in the levels of intracellular and wall components. Peak values for RNA and magnesium, and a minimal amino acid pool content were observed at D = 0.05 h-"', while walls isolated from mycelia grown at this rate contained double the normal content of c*-glucan. Measurements of wall thickness in association with determinations of the proportion of wall material present in the mycelium indicated that these walls were less dense than normal.

Increasing the temperature between 25°C and 50°C produced a rise in the intracellular carbohydrate content while, ever the same range, protein reached a maximal value at 38°C. The amino acid pool content was reduced at temperatures above 30°C. Increases of 47% and 139% in the protein and total mycelial carbohydrate levels, respecotively, followed a reduction in temperature from 25 C to 22 C, while the RNA content doubled between 30°G and 22°C. Magnesium concentration also rose over the latter temperature range. The wall content of protein, phosphorus, galactose and uronic acid was increased and mannose reduced at elevated temperatures. Notable changes in glucose and hexosamine levels occurred below 30°C.

Growth in the presence of increasing H+ ion concentrations was accompanied by increased synthesis of RNA. Mycelial magnesium and potassium levels were reduced below pH 4*1 when a pH differential could not be maintained. The major influence of pH, however, was on wall composition. The content of individual neutral sugar components and hexosamine increased with increasing pH, indicating an inhibitory effect of H+ ions on the extracellular polymerisation reactions concerned with wall biosynthesis. Protein, phosphorus and uronic acid contents were reduced at increased pH values.

Accumulation of intracellular carbohydrate was noted in mycelia grown in the presence of NaCl and coincided with reduced levels of magnesium, potassium, phosphorus and the amino acid pool. Wall composition was only slightly affected by changes in culture salinity. The protein synthesising activity of RNA (protein/RUA X D) was strongly dependent upon cultivation conditions, activity (1) increasing 8-fold between I) - 0.02 h ^ and 0 .17 5 h ^5 (2) doubling within the temperature rang** 20°C to 30°C; and (3) falling by 50% between 40°C and 50°G. A slight reduction in activity was noted at decreased pH values and increased NaCl concentrations.

The molar stoichiometry between HHA and magnesium, unlike that of bacteria, was also influenced by the nature of the environment but remained steady in face of changing temperature. A positive correlation between the level of mycelial potassium and the efficiency with which glucose is utilised for cell synthesis was revealed. High levels of potassium restricted the efficiency of oxygen utilisation.

Of the wall components, protein, glucose, mannose and phosphorus were the most variable, values ranging from 8.5 to 20.5% w/w (protein), 17*4 to 43*6% w/w (glucose), 1.6 to 4*0% w/w (mannose) and 0.15 to 0.78% w/w (phosphorus). Under all conditions wall phosphorus and protein contents varied in like manner, indicating the presence of a phosphoprotein component. Calcium binding to the wall is probably effected through the latter.