Features of microbiology at low nutrient concentrations

Biochemical engineers very seldom deal with biosystems in which the microorganisms are starving for organics nutrients; there is often, however, a shortage of dissolved oxygen. Biological waste treatment commonly deals with organisms that are desperate for nutrients. This makes good sense because the idea is to convert pollution to microbial cells that can be collected for disposal. If the organisms are poorly fed, they are more likely to use the organic pollution for maintenance and not for growth to make more cells for disposal.

If we focus on just the organic nutrients, it is obvious that gradients of concentration are not so important when the concentrations are relatively high. We can see this from a graph of the monod equation:

Graph of Monod Equation

Try a low value of ks to see a shape that is fairly typical when the graph is scaled for higher concentrations of nutrient. A small change in nutrient concentration matters little when there is plenty, but there is a tremendous difference when the nutrient concentration is low. Organisms that find a gradient toward a slightly higher concentration have a great advantage. Many microorganisms have flagella that allow them to swim. Control of their own location is termed motility. Some motile microorganisms can sense nutrients and move toward higher concentrations. Others behave similarly but find more dissolved oxygen by sensing its concentration.

Conducting research in small vessels with vigorous mixing leads to a correct assumption that the contents are homogeneous. The assumption is not very good in very large vessels even with good agitation. In natural environments where the volumes are large and the mixing is poor, the effects of gradients are crucial.

The axiom survival of the fittest holds for microorganisms. Since life appeared on Earth, organisms must compete to survive. If a microorganism is slow in ingesting nutrients, some other organisms may steal them away. Nature has favored microorganisms that can take in nutrients very quickly. An analogy is that if you sit down to eat with lumberjacks and worry about good table manners, you will probably go hungry. Lumberjacks grab the best portions fast and have notoriously bad table manners.

  • Can one bacterium hinder another ?
  • Discussion about attached organisms.
    (while on sabbatic leave at ESB in Porto, Portugal, July 1996)