They can uptake ammonia directly. I never used to cycle my phyto cultures, lolIf there are enough nitrates to support algae growth then how can your tank not be cycling?
@Brandon249 just discussed this in detail in another cycling thread.
Algae, unlike plants, cannot store essential nutrients. For growth, algae require nutrients to be present in the surrounding environment. The needed nutrients are nitrogen nitrate (N), phosphate (P), and potassium (K).