Green Algae Chlorophyta are photosynthetic organisms quite a lot of in freshwaters around the earth. The Alga survives with three mainly elements: solar radiation, water and carbon dioxide. Thus it produces carbohydrates (ch2o) indispensable for its own live, oxygen and other simple gases (like hydrogen) are liberated to atmosphere by Algae metabolism process. Algae are easy to meet in rivers, lakes, fountains, etc. Green Algae collecting/harvesting are very easy to do, an Aquarium or translucent bottle could be a simple photobioreactor where Algae would be grown.
Documentation: http://algasverdes.librepensante.org/