Bacillariophyta are unicellular organisms that are important components of phytoplankton as primary sources of food for zooplankton in both marine and freshwater habitats. Most diatoms are planktonic, but some are bottom dwellers or grow on other algae or plants.
Except for their male gametes, diatoms lack flagella. Instead many diatoms achieve locomotion from controlled secretions in response to outside physical and chemical stimuli. Diatoms have unique shells, which serve as their cell wall. The overlapping shells, or frustules that surround the diatom protoplasm are made of polymerized, opaline silica. Identification of diatom species is based on the delicate markings on their frustules, comprising a large number of tiny, intricately-shaped depressions, pores and passageways that bring the diatomís cell membrane in contact with the environment. Diatom frustules have accumulated over millions of years to form the fine, crumbly substance known as diatomaceous earth, which has a variety of uses (e.g. for filtration and insulation). Diatom remains in both marine and freshwater sediments are also important as indicators of paleo-environmental conditions at the time the sediments were formed.
Bacillariophytes have brownish plastids containing chlorophylls a and c and fucoxanthin. The primary means of reproduction is asexual, by cell division. Most diatoms are autotrophic, but a few are obligate heterotrophs (they must absorb organic carbon) because they lack chlorophyll altogether. Some diatoms even lack their distinctive frustules and live symbiotically in large marine protozoa, providing organic carbon for their hosts.
Additional introductory information about this algal group may be found at the University of California - Museum of Paleontology.