Before I get going, in the interest of full disclosure, organic chemistry and the specifics of microbiology aren't areas I'm particularly well-versed in. I remember enough of the general concepts to explain the basics, but I would have to research the details to address specifics. My knowledge is more along the macro scale of how nutrients, microbes, plants, and water/soil interact; not biochemistry.
To briefly answer your question, yes, if a system is not limited in labile (easily accessed) DOC/POC then addition of extra will theoretically not increase the amount or metabolism of microbes. What I forgot about in previous posts is lability. If you add a source of DOC that is more easily used than say, proteins, microbes will likely preferentially use that instead. Adding a little may also be enough to stimulate growth of bacteria to the point where, as a whole, they're more able to break down more complex molecules. This may be why one of the previous posters noticed increased clarity in his water. I can't be certain though, I'm just taking a SWAG (Scientific Wild A$$ Guess) here.
That's a basic diagram of denitrification. Carboniferous molecules are broken down for energy and the nitrogenous materials are the electron acceptor. (gross oversimplification) We do the same thing with carbohydrates and O2.
Here's a diagram of anaerobic metabolism in humans, specifically during exercise. When you "Feel the Burn" that's anaerobic and results in lactic acid.
Organic molecules are Carbon (usually more than one, but not always) atoms typically with Hydrogen, Oxygen, and Nitrogen attached. Some examples are Excel, alcohols, sugars and startches, lignins (structural carbohydrates), proteins, amino acids, Chlorophyll, and humic acids. The brown tint released by new wood is an organic molecule, but it doesn't have much in the way of things that might cause us problems like Nitrogen or Phosphorus. It's a great source of carbon for bacteria and fungi though. That's why we see slime and such growing on new wood.
Here's the structure of Methanol (NO-POx):
Notice the lack of N and P? It's a very simple organic molecule and is easily used by microbes.
Here's the structure of a protein (I have no idea which one), notice all those Ns? That's a lot of Nitrogen that will be added to the system on a molecular level when it's broken down.
Without getting into details, some of that N will be released into the water and bind with free Hydrogen atoms to form Ammonia (NH3). The rest will be used by the microbes.
Here's the structure of an Amino Acid, so named for the Amine (NH2) group. Sounds a lot like Ammonia, doesn't it? http://www.nutrientsreview.com/wp-co...-Structure.jpg
Getting back to the original topic
: In the presence of oxygen (electron acceptor) bacteria will take an electron from methanol to provide an electron for metabolism which results in the breakdown of the molecule and nets a Carbon atom. In systems lacking dissolved (DOC) or particulate (POC/POM) organic carbon/material, aerobic and anaerobic microbes are C limited, meaning they lack the carbon needed to metabolize. Because our plants are exuding complex acids, dead leaves are decomposing (source of C, N, and P), excess food and feces are present (rich source of proteins with N and P), and "mulm" is generally abundant there are tons of sources of labile carbon.
Diagram of aerobic, anaerobic, and chemical processes going on in aquatic soils, and to some extent filters.
Time to get ready for work. Hope this makes things as clear as mud. :P