Where does algea come from?

As in, where does the spark of life for algae come from? My very sciency daughter asked me that over the weekend. I really didn't know the answer. I understand that contaminated equipment/pool toys could bring it in. But assuming that you are only in your own pool and don't have contamination from another pool, where does algae come from? Does it blow in from contaminated pools?
 
Algae spores are about everywhere. Blowing in the wind. Tracked in from puddles. Carried by bugs and birds. It gets around.
 
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