Nitrogen is most commonly added as nitrate. However some algal species grow better on ammonium (Thompson et al, 1989). Unfortunately ammonium is frequently toxic at >100 mM so that much lower concentrations of ammonium must be used. Guillard specifies 500 mM (Guillard, 1983) but this concentration should be tested prior to widespread use. In some situations it is desirable to have ammonium available. This can be relatively easily accomplished by adding it to the nitrate stock solution to provide both nitrogen sources, for example 50 mM NH4 and standard NO3. Because algae preferentially remove NH4 they are likely to deplete all NH4 prior to using nitrate for growth.