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The Nitrogen Cycle in nature: an understanding of these nutrient pathways is fundamental to success with marine aquariums

Chapter Three 83

age colonization by nitrifying bacteria. Hobbyists will be familiar with equipment ranging from undergravel filters to many types of wet/dry filters. All focus upon rapid conversion of ammonia to nitrate. In these systems, the process of "mineralization" of nutrients occurs. Nitrogen that enters the system as complex organic molecules (proteins) in food is reduced to its simpler "mineral" components by biological activity. Not all of this biological activity comes from animal metabolism. In fact, only a small part of the food energy entering the hobbyist s tank is captured by the fishes. Much more probably feeds bacteria than feeds the fishes. Even fish tecal pellets and other organic "wastes" can be utilized by some form of simple bacteria or other microorganisms residing in the aquarium. These organisms are collectively termed "heterotrophs."

The substances upon which heterotrophs feed may be, like fecal pellets, preprocessed by another organism or group of organisms before the heterotrophs get a crack at it. Large invertebrates, including annelids, mollusks, echinoderms, and arthropods, may feed on food not consumed by fishes, for example. The excretory products of these organisms are then further acted upon by microscopic life forms, such as microcrustaceans, nematodes, worms, fungi, and protozoans, and thus finally reach the heterotrophs.

All along the way, gases, minerals, trace elements, and various organic compounds are being spewed out, recycled, converted, and reconverted in a dizzying frenzy of biochemical activity. The end result consists primarily of "inorganic minerals," such as ammonia, phosphate, sulfate, and carbon dioxide gas. The ammonia is utilized by Nitro-somonas, which uses oxygen dissolved in the surrounding water to oxidize ammonia to nitrite, extracting energy in the process. Nitrobacter, in turn, oxidizes nitrite to nitrate and thereby also gains energy.

All of these processes require oxygen, and all are said to be "oxidizing" reactions. What remains of the complex food molecules at the end of this process is about what would be left after burning them in a fire: water, carbon dioxide, and minerals.

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