|MadSci Network: Medicine|
I notice that no one has responded to your question yet, so I'll try to give you the best answer I can.
Your ecology teacher is correct that nitrates can cause eutrophication (the movement of water bodies, by the addition of nutrients, toward a state which is more conducive to supporting life), and you are correct that this process does not directly affect the human body. You are also correct that nitrates in the water supply can affect human beings directly by causing methemoglobinemia. (You almost spelled it correctly.)
Methemoglobinemia is sometimes called blue baby syndrome, and it is most common among infants, although it is seen occasionally in adults and older children. Here is the list of symptoms provided by the University of Nebraska Cooperative Extension: "Infants suffering from methemoglobinemia may seem healthy but show intermittent signs of blueness around the mouth, hands and feet. They may have episodes of breathing trouble, some diarrhea and vomiting. In some cases, an infant with methemoglobinemia has a peculiar lavender color but shows little distress. Blood samples appear chocolate brown and don't turn pink when exposed to air. When the methemoglobin level is high, infants express a marked lethargy, excessive salivation and loss of consciousness. Convulsions and death can occur at extreme methemoglobin levels."
The direct cause of the symptoms above is a buildup of methemoglobin in the patient's blood. Methemoglobin is formed when nitrite (NO2-) reacts with hemoglobin in the blood. Hemoglobin usually absorbs oxygen and supplies it to the rest of the body. When nitrite is present, methemoglobin is formed fairly readily, but an enzyme normally present in the body reduces it back to hemoglobin, restoring the blood's oxygen-carrying capacity. Infants have less of this enzyme than adults, so they are more prone to the effects of nitrite.
The question all this begs is: Since methemoglobin is formed by the reaction of hemoglobin and nitrite, and since fertilizer runoff contains nitrates, and since I said that fertilizer runoff can cause methemoglobinemia, nitrates must be able to be turned into nitrites. How does this happen?
Certain bacteria can convert nitrate (NO3-) into nitrite. Some of these bacteria live in the human digestive tract. Normally, the digestive tract is kept at a high level of acidity (low pH) by the secretion of gastric acid. The low pH keeps the number of bacteria under control. In some individuals, however, notably young infants, the secretion of gastric acid is lower than in a normal, healthy adult. In these individuals, the nitrate-reducing bacteria are allowed to proliferate, increasing the levels of nitrite in the body. The higher levels of nitrite in turn increase the level of methemoglobin in the blood. In people who lack the enzyme which converts methemoglobin back to hemoglobin, the oxygen-carrying capacity of the blood is decreased, leading to the symptoms described above.
So to sum up, infants (and some other people) have both high gastric pH and low levels of the methemoglobin-reducing enzyme. This leaves them susceptible to methemoglobinemia when they ingest high levels of nitrates. Therefore, when one of these people drinks water which has been contaminated with fertilizer runoff containing high levels of nitrates (or food or formula made with this water), they can experience blue skin, breathing trouble, etc.
I hope this helps answer your question.
Tchobanoglous, G. and Schroeder, E., Water Quality, Addison- Wesley Publishing Company, 1985.
"Nitrate and Methemoglobinemia," http://www.ianr.unl.edu /pubs/water/g1369.htm.
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