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Lecture 14 2 November

Lecture 14 2 November - When oxygen is not available as a...

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O 2 present, aerobic respiration O 2 absent Organic material When oxygen is not available as a terminal electron acceptor, other pathways are used to extract energy from carbon-based compounds, including other terminal electron acceptors and fermentation pathways. Where is the carbon going?
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In addition to carbon, other nutrients are central to the way that aquatic ecosystems function. Some forms of these nutrients can cause water quality problems in large quantities. In addition, primary production can be limited by one or more of these nutrients. These nutrient cycles are interrelated and interact.
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Nitrogen The most common form of nitrogen in the biosphere is N 2 gas. The atmosphere is composed of about 78% N 2. Water generally contains N 2 as a dissolved gas. Because of its molecular structure, N 2 is very difficult for most organisms to use directly. The two most important forms of dissolved inorganic N in natural waters are nitrate (NO 3 - ) and ammonium (NH 4 + ).
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Ammonium is the ionic form found in neutral to acidic waters. Under basic conditions, the ion is converted to ammonia gas (NH 3 ), which move freely between air and water. Nitrite (NO 2 - ), is also found in water, esp. when sewage is present. Organic nitrogen takes many forms – amino acids, nucleic acids, proteins, and urea (excreted by many organisms).
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Nitrogen must be assimilated in some form as it is required for many biological molecules (proteins). Multicellular heterotrophs (animals) can assimilate nitrogen only in the form of organic molecules , such as amino acids and nucleic acids. Primary producers and bacteria use nitrate, nitrite, or ammonium . However, assimilation pathways require ammonium . Thus, enzymes must catalyze the reduction (reductases) of nitrate to nitrite, and nitrite to ammonium, which can then be assimilated. In oxic environments ammonium has higher potential energy than nitrate so energy is required to make the conversion from nitrate to ammonium before assimilation. Thus, many aquatic bacteria and primary producers prefer ammonium.
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Many bacteria, including some cyanobacteria, have the capacity to assimilate N 2. This is referred to as nitrogen fixation. It is a very energetically expensive transformation – requires high activation energy. Nitrogenase (enzyme) is required. This enzyme must be protected from exposure to oxygen or used in anoxic habitats.
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