Many species are single-celled and microscopic -LRB- including phytoplankton and other microalgae -RRB- ; many others are multicellular to one degree or another , some of these growing to large size -LRB- for example , seaweeds such as kelp and Sargassum -RRB- .

Many types have one cell and are very small or microscopic -LRB- including plankton and other algae -RRB- ; many others are have many cells , some of these being very big , like seaweeds such as kelp and Sargassum .
