The main goal of ammonium citrate anion is to keep Fe(III) chelated. Chelated Fe(III) is more available for cyanobacteria than mineralized Fe(III). Therefore, I would recommend to use Fe(III) EDTA ( http://pubchem.ncbi.nlm.nih.gov/rest/chemical/ferric-edta) instead of Fe(III) ammonium citrtate. However, do not "overdose" this compound because EDTA is known to damage bacterial envelops.