Organic acids are often used as mold inhibitors in food, such as sodium benzoate. The acids are effective when neutralized as salts, because microbes absorb anions and cations separately without the homologous counterpart. Sodium benzoate would be absorbed as benzoic acid.

It works because molds usually have an extremely demanding surface which allows them to tolerate dehydration and even absorb moisture from the air. Some filamentous fungi including mushrooms gave up such demanding surface chemistry, and they are not inhibited by organic acids. So organic acids such as citric and acetic are good for inhibiting molds in mushroom cultures.

Scientists do not know what the surface chemistry consists of which allows some molds to be resistant to drying. There is very little physiology studied for filamentous fungi, because they are not easily studied in aqueous solutions as bacteria and yeasts are.

The sensitivity to organic acids indicates that the proton pump cannot be used to pump hydrogen ions out of the cells. All cell membranes (presumably) use a proton pump to send hydrogen ions across the membrane. Usually, the reason is to adjust pH by getting rid of excessive hydrogn ions. In generating ATP, the proton pump is used to push spinning proteins which move reactants into place and products out of the way.

There are some filamentous fungi in addition to mushrooms that gave up the ability to tolerate drying, and they are not sensitive to organic acids. An example is the most common foot fungus. It does not tolerate drying and it is not sensitive to organic acids. Since its environment is not dry, it it has no need to tolerate drying, which is an extremely demanding characteristic. This property is observed as wide spacing between toes. Toes have wide spaces between them to promote drying. Drying can inhibit fungal infections only for those types of fungi which gave up the demanding surface chemistry which allows dehydration.

Two Types Of Fungi

So there are two types of fungi. They need labels. Let's call the usual molds alpha-fungi and the others beta-fungi.

The alpha-fungi resist dehydration on surfaces, which requires a special surface chemistry on the mycelium. They are sensitive to organic acids, because they cannot pump hydrogen ions out of the cells.

Beta-fungi gave up their ability to tolerate dehydration, so they have normal cell surface chemistry, and they can pump hydrogen ions out of the cells, which makes them uneffected by organic acids in normal amounts.

Microbes do not absorb ions. Cations carry a hydrogen in with them leaving the anion in the exogenous solutions, and anions carry an OH in with them leaving the cation in solution.

This means, when microbes are using ammonium chloride as a nitrogen source, they leave hydrochloric acid in the exogenous solution, which lowers the exogenous pH to a point which rapidly inhibits further growth. So ammonium salts can only be used in very limited quantities as a nutrient for microbes, even though ammonia is a highly preferred source of nitrogen. Microbes absorb ammonia uninhibitedly, because it is the gold of the environment, though it is usually only found in the soil and in small quantities.

Nitrate salts leave the cation in solution when absorbed as a nutrient causing alkalinity in the exogenous media. But nitrate is not used well being low in energy and requiring two enzymes for utilization. Those two enzymes are repressed and do not de-repress until ammoniacal nitrogen is used up.

Therefore, salts of organic acids are used as mold inhibitors in food, often as sodium benzoate. Benzoic acid is absorbed resulting in acidity within the cells which cannot be corrected by a proton pump for the alpha-type fungi. Beta-type fungi are not usually in food. Yeasts and mushrooms are generally beta-type fungi, as is the primary foot fungus.

Urea functions much like ammonium salts as a nutrient for microbes. It is not a salt, so it enters the cells as a neutral nutrient without changing the exogenous or endogenous pH, until it comes apart within the cells to form ammonium hydroxide and carbon dioxide. Carbon dioxide is expelled resulting in endogenous alkalinity. Therefore, urea is an ideal nitrogen source up to about 0.2% in media, while higher concentrations becomes toxic.

This is probably why urea is found in urine, while nitrogen could be excreted in other forms. It is toxic to bacteria above 0.2%.

Soil mushrooms do not appear to expel carbon dioxide, because they can grow slowly in the absence of oxygen without fermenting or reducing exogenous pH with excess acid. What do the mushrooms do with excess carbon dioxide? No one has ever said. Maybe they combine it with calcium or magnesium to form insoluble carbonate.

At least Agaricus (the common mushroom) will metabolize energy in the absence of oxygen without fermenting and expelling acid by storing the energy as NADH (nicotinamide adenine dinucleotide hydrogenated). This molecule is a precursor to formation of ATP. By pooling NADH, rapid growth occurs when oxygen is available for ATP production, which allows a mushroom to form very rapidly as it emerges from the soil. Storing NADH is not fermentation and does not result in acid being expelled and damaging the mycelium, if carbon dioxide is not expelled.