Plant
Algae comprise several different groups of organisms which produce food by photosynthesis and thus have traditionally been included in the plant kingdom. The seaweeds range from large multicellular algae to single-celled organisms and are classified into three groups, the brown, red and green algae. There is good evidence that the brown algae evolved independently from the others, from non-photosynthetic ancestors that formed endosymbiotic relationships with red algae rather than from cyanobacteria, and they are no longer classified as plants as defined here.
Harmful algal bloom
The other types of algae are diatoms and dinoflagellates, found primarily in marine environments, such as ocean coastlines or bays, where they can also form algal blooms, commonly called red tides. Red tides, however, may be a natural phenomenon, although when they form close to coastlines or in estuaries. They can occur when warmer water, salinity, and nutrients reach certain levels, which then stimulates their growth. Most red tide algae are dinoflagellates. They are visible in water at a concentration of 1,000 algae cells per milliliter, while in dense blooms they can measure over 200,000 per milliliter.