Nitrogen Cycle

[James S. Baker]

Jason,

Please elaborate on a point from the week 5 PowerPoint lecture, slide 6, discussing filter substrates. You note, and I’ve read similar comments by Bob Goemans and others, that excessive nitrification should be avoided, as a buildup of nitrate can result. I understand the need for balance, but it seems to me that limiting nitrification would only result in a buildup of ammonia and/or nitrite, which would be worse (more toxic) than a buildup of nitrate. I can’t see how having too much oxic substrate could do any harm, other than being inefficient in the sense of providing more surface area than the bacterial population could fully utilize with a given nutrient load. I would think that we would want as much nitrification as possible, while also, of course, wanting balanced denitrification. Anyway, any further enlightenment you can provide will be appreciated.

 

[java1]

Since nitrifying bacteria are technically "saprophytic", meaning that they eat dead organic material, they actively participate in the breakdown of detritus. What this means is that bacteria can actually secrete enzymes, known as exoenzymes, which help break complex macromolecules down into easily digested components, like ammonia; it's as if they had digestive "juices" on the outside of their cell walls. In this way, they can actively get ammonia from proteinaceous organic detritus. If one considers that most life is composed of protein and nucleic acids (DNA, RNA), and that the building blocks of these are amino acids, amino groups and various forms of phosphate, then one can see that active, accelerated breakdown of these components leads to the release of amines -> ammonia. Along with higher ammonia one can usually notice a concurrent spike in phosphates for the same reason.

Providing a large surface area of substrate for nitrifying bacteria provides a place for them to actively digest detritus. In the absence of this added nitrification capacity, the ammonia stays compartmentalized in organic material, and doesn't get processed. As long as the ammonia is sequestered in an undecomposed state in more complex molecules (amino acids, peptides, proteins, nucleic acids), it isn't toxic. So in this way, the amino groups aren't processed into free ammonia just yet, and therefore ammonia is not really present in the water column in the absence of elevated nitrifying capacity.

However, if one does have elevated nitrification capacity, then the breakdown of proteins and nucleic acids into ammonia proceeds at a higher, unbalanced state. This elevated level is in comparison to the natural denitrification capacity, and one can see how a large quantity of extra surface area might be hard to balance in the absence of some auxiliary denitrification apparatus.

Hope that helped!

Jason

 

[James S. Baker]

That did, indeed, help. It seems simple enough in hindsight, but I wasn’t taking into account the processing of organic matter into ammonia (even though we’ve touched on that previously in this course). I was basically just starting with the ammonia, as if it was all being produced directly by the higher organisms in the tank. As I wrote above, I’ve seen this concept referenced before with regard to filtration setup, but never quite understood, so thank you for the explanation.