Dr Bill Ballantine of Auckland University calls them the grass of the sea . That s a good description, because they are the energy source at the base of the marine food web, without which other life forms would not exist. Some species of microalgae produce toxins or poisons which can be harmful to other marine life, or even to humans. When conditions are right, they can grow very quickly and form visible patches in the water, called blooms. This section is all about these harmful algal blooms.
Phytoplankton is another term scientists use for microalgae that float in the sea.
One kind of phytoplankton are called Dinoflagellates. Even though they are plants, they can also swim using a long whip-like extension called a flagellum. They are very small, about 0.02 to 0.04 millimetres in diameter. A human hair is about 0.05 mm in diameter, by comparison. Dinoflagellates come in many shapes.