I have a culture algae I need an element of iron to be easily absorbed by algae, but when I began to prepare from Ferric citrate that could not absorb algae and lost the culture.
Are you using Ferric Ammonium citrate or Ferric citrate? If you're using Ferric Ammonium citrate, use it at 0.006g/L. It is a component in BG-11 medium . Make sure you check the pH of your medium too, should be around 6-7.5.
Use as Willy Yee said Ferric Ammonium citrate, in this conentration 0.006g/L it would not be toxic to any kind of algal strain. Make sure that pH of the medium should be in range of 6.5 to 7.2 for freshwater microalgae.