algae

Algae are defined as a group of predominantly aquatic, photosynthetic, and nucleus-bearing organisms that lack the true roots, stems, leaves, and specialized multicellular reproductive structures of plants. Their photosynthetic pigments are also more varied than those of plants, and their cells have features not found among plants and animals.

Algae are eukaryotic organisms and contain three types of double-membrane-bound organelles: the nucleus, the chloroplast, and the mitochondrion. In most algal cells, there is only a single nucleus, although some cells are multinucleate.

Some species of algae produce toxins that are lethal to fish, or render shellfish and finfish unsafe for consumption. The (taxonomically contentious) dinoflagellates are responsible for red tides, which not only release toxins into the water that may be lethal to aquatic life, but also wind-sprayed toxic cells that can cause health problems for air-breathing organisms.

Algae range in size from picoplankton, which are between 0.2 to 2 micrometers (0.000008 to 0.000079 inches) in diameter, to giant kelp, which can be 60 metres (200 feet) long.

Algae produce up to half of the oxygen in Earth's atmosphere, and algae help keep carbon dioxide out of the atmosphere by storing it. Algae are also the food base for almost all aquatic life, and are economically important as a source of crude oil and as sources of food and a number of pharmaceutical and industrial products for humans.