Closterium is a Desmid, a type of one-celled algae that is the closest relative to land plants.
It has chlorophyll to make its own food from sunlight. You can see the two green chloroplasts in the photo. Between them is the nucleus, although it is not clear in this unstained sample. In the clear tips of the crescents are crystals of calcium sulfate.
This group of three closteriums was found in a fish pond at a neighbor's house.