In our oceans and in all freshwater habitats you can find little siblings of our plants. They are called microalgae and are so small that you can only see them with a microscope.
Like their big siblings, the plants, algae can use the energy of sunlight to produce sugar with the help of photosynthesis. In a simple experiment you can check whether algae are present in a water sample.
Algae need sunlight to grow and with a little patience you can establish just in a few days your own little algae culture breed.