Green algae were the likely the algal ancestor of terrestrial plants. Green algae have photosynthetic apparati which are very similar to those of plants. The photosynthetic apparatuses of green algae are optimized for the capture of those photons which have not passed through large columns of water, i.e., only the atmosphere and to the bottom of shallow waters. Agar, a sulfated polymer of galactose, is harvested from red algae. Some dinoflagellates are capable bioluminescence and they are responsible for the phosphorescence of oceans waters seen at night.
Dinoflagellates are also responsible for toxic algal blooms such as red tides.
Blooms of certain kinds of algae are often a sign of pollution, especially of organic materials. Following blooms, algae death and subsequent decomposition can deplete dissolved oxygen in water thus making the habitat unsuitable for organisms requiring dissolved oxygen such as fish. No one is certain why this is occurring, but most phycologists believe that the blooms are caused by the continuous pumping of nutrients such as nitrogen and phosphorous into coastal waters. Sewage and agricultural runoff are probably major sources. With people eating more seafood, this toxic menace in the world's oceans will become increasingly more common.