First, microalgae cultures are commonly co-culture of algae and bacteria. Before you worry about purifying your strains, identify key contamination hotspots (water treatment, sterilization efficiency, cultivation handling etc). Then an antibiotic and/or antifungal treatment could be applied.

You should try and isolate to a single colony first to ensure that it isnt contaminated. If needed you can add in broad spectrum antibiotics/ fungicides, but many will likely hinder the growth of the microalgae in one way or another.

Hello!

You can try some techniques to purificate microalgae strains.

First of all, before using any antibiotics/antifungic, you can try the Spread Plate Technique in a Petri dish. You can prepare solid medium by adding agarose (1%) in culture medium.

I can provide you the whole protocol by e-mail, if you want.

Kind regards,

For bacterial contamination, keep streaking your algae on agar plates (preferably inorganic media) until you get isolated colonies free from bacteria. Test these colonies on microbiological media to confirm its free from contamination. If you're streaking from liquid media, a serial dilution of the culture would before streaking would be a good idea too. You may also use antibiotics if that method fails.

For fungal contamination, here's what I do. Do a dilution of your liquid culture and Streak on agar plates (inorganic media). Observe for fungal growth daily. When fungal colonies begin to appear, use a sterile scalpel and cut out the agar section. Keep doing this to minimize fungal growth until algal colonies start to appear. Repeat streaking until you get pure algae colonies. If this method fails, use fungicides.