However, growth results in the consumption of nu- trients and the excretion of microbial products; events which influence the growth of the organism. Thus, after a certain time the growth rate of the culture decreases until growth ceases. The cessation of growth may be due to the depletion of some essential nutrient in the medium (substrate limitation), the accumulation of some autotoxic product of the organism in the medium (toxin limitation) or a combination of the two. The nature of the limitation of growth may be explored by growing the organ- ism in the presence of a range of substrate concentrations and plotting the biomass concentration at stationary phase against the initial substrate concentration, as shown in Fig. 2.3. From Fig. 2.3 it may be seen that over the zone A to B an increase in initial substrate concentration gives a proportional increase in the biomass produced at stationary phase, indicating that the substrate is limiting. The situation may be described by the equation: =− xY Ss(

where x is the concentration of biomass produced, Y is the yield factor (g biomass produced g

–1 substrate consumed), S R is the initial substrate concentration, and s is the residual substrate concentration. Over the zone A to B in Fig. 2.3, s equals zero at the point of cessation of growth.

Thus, Eq. (2.4) may be used to predict the biomass that may be produced from a certain amount of substrate. Over the zone C to D an increase in the initial substrate concentration does not give a proportional increase in biomass. This may be due to

either the exhaustion of another substrate or the accumulation of toxic products. Over the zone B to C the utilization of the substrate is deleteriously affected by either the accumulating toxins or the availability of another substrate