We establish the conditions of a bioprocess by feeding nutrients and setting pH, temperature, and the like. Anything that we do not hold constant will almost certainly be changed by the organisms. For example, if there were no thermostat, heat generated by metabolism or heat flux due to differences between the bioprocess and its surroundings will cause a temperature change.
Seldom is anything other than temperature controlled for a batch process in shake flasks or very small bioreactors. Small vessels are not amenable to the sensors, pumps, and feed lines needed for control. Pilot plant fermenters and commercial units ranging from two to hundredes of liters in size have provisions for control of concenrations, pH, and the like. Production fermenters are much more likely to have pH control, and fed-batch mode is increasingly popular. Continuous bioprocessing almost always has some form of control in addition to a thermostat; holding the feed rate constant (the chemostat) is by far the most common option.
Let's leave imposed control of bioprocesses aside and focus on the control
schemes of the organisms themselves. The context will be continuous culture
for which we provide temperature control and one of several options for
holding some additional feature constant. There are two options illustrated
by the following applet. The left figure shows that we fix the pumping
rate, and this determines the specific growth rate coefficient because
µ must equal D at steady state.
The right figure is for fixing the nutrient concentration. This requires feedback control in which the nutrient concentration is sensed somehow and controlled by a pumping system. If this is coupled to the rate of fresh feed, the system is an auxostat.