May 3, 2004
A: FROM MENTOR JOAN LUSK IN RI
Living organisms all need nitrogen, an element found in proteins and
nucleic acids. We get ours from eating protein (and nucleic acids)
made by the plants and animals we eat, and excrete any excess as urea
(or as uric acid from the nucleic acids). The plants you tested
could reduced nitrate to ammonia and incorporate it into their own
protein and nucleic acids. That's a good thing from the plant's
point of view - the plant grows and reproduces. From a human point
of view, we might not want so much of that plant growing in our lake.
If algae grow abundantly and then die, the decaying biomass uses up a
lot of oxygen and the water may become so anoxic that fish die (and
the stench is terrible). The decay of plant material is an oxidation
process, like burning it very slowing but requiring as much oxygen as
burning it in a fire.

If there were absolutely no nitrate or other source of nitrogen in
the water, few aquatic plants could grow. Only organisms that can
fix molecular nitrogen (N2) from the air would grow. So removing
absolutely all nitrate might not be so wonderful.

But when a lot of nitrate from fertilizer runs off the land into
streams and lakes, it can lead to massive plant growth
(eutrophication) and later anoxia. If nitrate is not the limiting
nutrient, however, it can't stimulate growth - some other nutrient,
often phosphorus, may determine how much biomass can be made.
Phosphorus was removed from detergents because of this problem - it
is typically the limiting factor, and old-style phosphate-based
detergents caused a lot of eutrophication. Phosphate is another
ingredient in fertilizer, however, so run-off from excess fertilizer
can provide both nitrate and phosphate for unwanted plants as well as
for desired crops.

A neutral pH is important for most organisms, though there are
certain bacteria (acidophiles) that like a very low pH. Oxides of
nitrogen that end up as nitrate are responsible for some of the
acidity of acid rain (the other major acid being sulfuric, from
oxides of sulfur.

I'm not satisfied with any site that I found when trying to get good
links for you. The EPA site didn't say much about nitrates... This one
http://www.earthforce.org/green/nitrates/main.cfm is not very deep or
strong on science. Your best bet may be to look at ecology books in
your library.