Real ecosystems

In most aquatic ecosystems, there are hundreds and hundreds of microorganisms, but perhaps half a dozen constitute nearly all of the microbial biomass. The others are present in very low concentrations. This would seem to simplify the analysis greatly. All we must do is study systems that have five or six different microorganisms present and we should be able to develop a basis for unraveling a real ecosystem. The ones present in trace concentration can be ignored. In fact, this logic fails miserably. We can focus on one organism that is present in the greatest numbers and observe its behavior for days, sometimes even for weeks. Surprisingly, it may almost disappear fairly suddenly while some other organisms rises up from nowhere and predominates.

This figure taken from Cassell, et al shows pigments extracted from organisms in a complicated mixed culture. Each pigment is assumed to be an indicator for one or a group of microorganisms. Note that a pigment that is dominant for a while can completely disappear later.

Little is known about the reasons why the composition of a microbial ecosystem can change so drastically. Some possible explanations are:

  1. A dense population is more subject to disease. Perhaps a virus (phage) infects some cells and then reproduces explosively because the microorganisms are plentiful to carry the disease.
  2. Several types of microbial interactions are inherently oscillatory. We can envision that the peaks and valleys of several different interactions can reinforce each other to give some organisms a boost while surpressing others.
  3. An event such as the seasonal turnover of a lake can change the nutrients and can expose organisms that were buried in the depths or in sediments. The climate can vary temperature, salinity, pH, etc. to trigger population changes.
Activated sludge units and anaerobic digestion steps are subject to serious performance upsets due to variations in the microbial populations, and none of the above explanations seems adequate.
while on sabbatical leave at ESB in Porto, Nov. 96