Growth-Associated aspects of mould physiology.

The technique of continuous-flow culture was used to study the growth Kinetics of and the effects of, growth rate on biochemical composition and anaplerotic metabolism on the mould, Aspergillus nidulans.

The validity of classical microbial growth equations, which have been well tested for unicells, was established for mould growth. Continuous cultures of A.nidulans followed Monod Kinetics when growing by simple hyphal elongation but exhibited anomalous growth Kinetics when growing by branch formation.

The Kinetics of growth in batch culture involving the production of an extracellular metabolite followed the "logistic" model more closely than the Monod model. The effect of growth rate upon respiratory parameters, glucose uptaKe and energy yield, and the Kinetics of growth in the transient state were also studied.

An inter-relationship between hyphal magnesium, polyamine and RNA concentrations was established, which held for vegetative cultures at all growth rates. A relationship between the ratio of magnesium to polyamine and ribosomal efficiency was also established. Ribosomal efficiency was itself proportional to growth rate at all values of p below 65% pmax, reaching a maximum value which was evident in the growth rate range, pmax- 65% ymax. Zero ribosomal efficiency was observed at the minimum specific growth rate Cpmin) under which conditions-differentiation into conidia tooK place. The response of ribosomal efficiency to abrupt changes in growth rate in transient states2 was immediate and positive. The effect of dilution rate upon mycelial wall content, DNA concentration and on the concentrations and energy required for synthesis of carbon-rich "storage- polymers" is also discussed.

This work describes an investigation of anaplerotic metabolism, particularly that involving CO2 fixation. The inhibitory effect of high aeration conditions upon spore germination appeared to reflect a critical requirement for bicarbonate in the growth medium. Further investigation revealed a significant phospho- enol pyruvate carboxylase (PEPC) activity in batch culture. Pyruvate carboxylase (PC) activity was also detected but at lower levels. Glucose-limited chemostat cultures showed a marked increase in both carboxylase activities. Maximum PEPC levels increased from 1.1 x 10 2 ymol protein 1 h 1 to l.Oymol mg 1 h 1 in chemostat culture and PC activity from 1.5 x 10 3 ymol mg 1 h 1 to 2.5 x 10 2 ymol mg *h 1. Both enzyme activities were growth rate dependent. Measurements of bicarbonate fixation by intact mycelium indicated that virtually all the assimilated bicarbonate was fixed by PEPC and PC. The feasibility of making more efficient use of the carbon source with respect to biomass production by maximising carboxylase activities was investigated by (a) manipulating chemostat culture conditions to provide optimum bicarbonate concentration, (b) producing step-down and step-up transitions states and (c) constructing a two-stage continuous culture system.