A dinoflagellate is a one-celled aquatic protist, found in both salt and fresh water. Dinoflagellates make up a large proportion of the plankton in the ocean, and are an important part of the life cycle in many aquatic environments. The creatures have been identified and studied since the 1700s, and have probably been around for millions of years, along with several other primitive organisms - Source

Dinoflagellates - Dinoflagellates are microscopic, (usually) unicellular, flagellated, often photosynthetic protists, commonly regarded as algae (Division Dinoflagellata). They are characterized by a transverse flagellum that encircles the body (often in a groove known as the cingulum) and a longitudinal flagellum oriented perpendicular to the transverse flagellum. This imparts a distinctive spiral to their swimming motion.

Dinoflagellate - From Wikipedia - The dinoflagellates are a large group of flagellate protists. Most are marine plankton, but they are common in fresh water habitats as well. Their populations are distributed depending on temperature, salinity, or depth. About half of all dinoflagellates are photosynthetic, and these make up the largest group of eukaryotic algae aside from the diatoms.