Some organisms have evolved to grow best at low or high pH values, but most organisms grow best between pH 6 to pH 8. So around here is where most of these grow. Overall, independent in the environment in which they grow, the internal pH of a cell must stay relatively close to neutral. even though the external pH may be highly basic or highly acidic. So that internal pH must be neutral. And that's, again, because of the macromolecules inside, and in particular, the proteins that need to function.
Those that grow circum-neutral, so around pH 5.5 to around 7.9, are known as neutrophiles, neutral, loving, or liking. and E. coli is a great example of a neutrophile. Organisms that grow best at low pH that we see here, for example those that grow in acidic soils associated with volcanic activity, these that grow in those environments are referred to as acidophiles.
To grow in acidic environments the cytoplasmic membrane must remain stable. So these organisms have adapted so that they use high concentrations of protons. Remember, pH stands for potential hydrogen.
So they actually use those protons for membrane stability. Then down here, we have our alkali phials. These grow best at high pH values. These are found in soda lakes and high carbonate soils.
Now, Algalophiles can struggle with membrane bioenergetics, producing a proton-motive force. So some organisms in these environments, instead of using protons, they use sodium ions for energy gradients.