Bacillariophyta is an important phylum of kingdom Chromista. Bacillariophytes are commonly known as diatoms. They are unicellular aquatic organisms that secrete intricate, perforated silica external skeletons. These skeletons, called frustules, are quite beautiful when viewed under a microscope. A frustule is composed of two parts--the epitheca and the hypotheca. The epitheca overlaps the hypotheca like a lid overlaps a box. Both parts consist of a flat plate, called a valve, encircled by a rim, called a cingulum.

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Diatoms can be grouped in two classes--Centrobacillariophyceae (radially symmetric) and Pinnularia (bilaterally symmetric). The centrate diatoms are usually found in marine environments, while the pennate diatoms are more often found in freshwater environments. Diatoms are photosynthetic, and thus play crucial roles in aquatic food chains and in shaping the atmosphere. A webpage given below suggests that 20-25% of Earth's carbon fixation is performed by diatoms! Some marine diatoms, such as Rhizosolenia, form mats of a few square centimeters in area. Mats that rise to the surface of the ocean can contain minerals such as nitrates found in the deeper waters, providing nutrition for surface-dwelling organisms.

Bacillariophytes can be either planktonic or non-motile. Non-motile diatoms cling to plants, molluscs, crustaceans, whales, etc. Diatoms reproduce both sexually and asexually. During asexual reproduction, both the cell and its frustule are split into two parts--the epitheca goes to one daughter cell and the hypotheca to the other. The daughter cells then secrete the other halves of their frustules. The asexual reproduction process inevitably leads to daughter cells of half the size of the parent since the frustules are inflexible. Obviously, for the sake of the phylum, diatoms can also reproduce sexually, in which case the offspring secrete an entire frustule after growth.

The oldest diatom fossils are from the Cretaceous period, but diatoms probably arose eariler than that. The richest source of diatom fossils is the accumulation of their skeletons into diatomaceous earth.

Classes of phylum Bacillariophyta


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