Survival at low nutrient concentrations
There is little experimental support for any of the proposed relationships between µ
and S at low concentrations because even tiny traces of organic matter could serve as
nutrients and mask the effects of the compound that was the intended limiting nutrient.
Please consider the following alternatives:
We appreciate that such graphs treat microorganisms as if they were all the same. Alternative A shows
absolutely no growth rate below some concentration of S. The population would not survive for very long if the sole use of nutrients were just to stay alive because there would be no replacement of cells that died. Suppose that just a few organisms managed to outcompete the others in finding nutrients or that some used these scarce resources for growth. The result would be the graph for Alternative B. Another interpretation is that the probability of growth approaches zero as nutrient concentration drops toward zero, but some cells manage to reproduce.
In our mathematical analyses, we usually assume that the environment is homogeneous. The real world has no "perfect mixing", and there will always be gradients of nutrients.
Discussion about attached organisms.
(while on sabbatic leave at ESB in Porto, Portugal, July 1996)