When I taught biology & botany during the 2nd millennium & 20th century, I used a 6-kingdom system of classification: (1) Archaebacteria (extremophiles including methanogens, halophiles & thermophiles); (2) Eubacteria (true bacteria & cyanobacteria); (3) Protista (protozoans & algae); (4) Fungi (mushrooms, molds & lichens); (5) Plantae (multicellular plants); (6) Animalia (multicellular animals). With new DNA evidence, these systems of classification are updated constantly, and there are numerous modern versions of phylogenetic trees. During the heavy rains of January 2023, I discovered minute green spheres while searching for bottleworts in the local hills east of Palomar College. To my surprise these tiny spheres are a species of yellow-green algae that in modern classification systems are now placed in the kingdom Chromista, created by the British biologist Thomas Cavalier-Smith in 1981. The following oversimplified diagram shows 7 kingdoms. Note: Thomas Cavalier-Smith also proposed an 8th kingdom Archezoa, ancient eukaryotes that diverged from other eukaryotes before mitochondria evolved; however, Archezoa have now been shown to have evolved more recently than previously believed, and mitochondria now appear to predate all presently known eukaryotes.

It may seem hard to believe that microscopic diatoms, with their delicate silica skeletons only forty millionths of a meter long, can be related to the giant kelps, which may grow as long as fifty meters, or that either one is related to the downy mildew that nearly destroyed the French wine industry. But they are related -- placed together in the great kingdom-level taxon Chromista.

The name Chromista means "colored", and although some chromists, like mildews, are colorless, most are photosynthetic. Even though they are photosynthetic, chromists are not at all closely related to plants, or even to other algae. Unlike plants, the Chromista have chlorophyll c, and do not store their energy in the form of starch. Also, photosynthetic chromists often carry various pigments in addition to chlorophyll, which are not found in plants. It is these pigments which give them their characteristic brown or golden color.

Photosynthetic chromists are some of the most important organisms in aquatic ecosystems. The cool and temperate coasts of continents are lined with kelp forests, where many commercially important fish and shellfish feed and reproduce, and diatoms are frequently the primary source of food for both marine and fresh-water organisms.

In addition to their roles as producers for marine animals, chromists provide many products for industry. Alginates are viscous chemicals extracted from kelp; these are used in paper production, toothpaste, and in ice cream, where the alginate helps to improve texture and ensure uniform freezing and melting. Ancient chromists, like coccolithophorids, are responsible for deposits of limestone and other rock formations. The skeletons of dead chromists accumulate on the floor of lakes and oceans, where they may become thick deposits of silica or calcium carbonate. These deposits are useful for interpreting ancient climate, and in searching for oil.

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