"Algae are plants" is a controversial statement. In the days when all life
forms were classified as plants or animals, there was no problem. However,
even general biology texts these days reveal little agreement on algae.
Modifications of Whittaker's 5-kingdom scheme all place blue-green algae
(aka cyanobacteria) in Kingdom Monera with the prokaryotes. The other,
eukaryotic algae may be lumped into Kingdom Protista/Protoctista -- or
split between Protista and the Plant Kingdom. Many scientists argue that
the algae are polyphyletic - having obtained chloroplasts in independent
events - suggesting that not all are closely related to land plants.
Moral for Physiologists: Don't assume all algae are alike. Whether the
question has to do with nitrogen nutrition or whatever (especially outside the
chloroplasts), look for information on the organism of interest and its near
In general, plants/algae have enzymes available for use of ammonium. In
fact, nitrate is converted to ammonium before it can be used for amino acid
synthesis. However, ammonium (esp. converted to ammonia) is also toxic.
Therefore, in the presence of ammonium (which CAN cross membranes in
ammonia form without a specific transporter), your alga must have the
ability to detoxify (use quickly, sequester, keep intracellular concentration
low) the ammonium before it succumbs.
schmid at butleru.edu