Bacillariophyta

Have you ever found flintstone on the beach? You probably didn't realize that the stone was once millions of one-celled diatoms! Diatoms form an important group within the marine phytoplankton. They are capable of fixing lots of energy from the sun, making them an important source of food for plankton feeders, such as worms, copepods and fish larvae. In turn, these small animals are food for larger animals, such as fish. Diatoms make up the base of the food web in the sea. Depending on the species, diatoms live in the water as well as on the bottom of the sea. They don't grow larger than several tenths of a millimeter.

Diatom identification guide & ecological resource

for water resource managers, ecologists, taxonomists, analysts, systematists, students, and the public.

We aim to provide users with accurate information about diatoms of the United States.

Expert contributors are submitting text and images for freshwater genera and North American species, including taxonomic and ecological information.  

Our project organizer is creating composite illustrations for all taxa. For many species, environmental response plots and geographic distribution maps are included.

Our Editorial Review Board is ensuring the scientific merit of each submission and directing site development.

It has been known for a long time that diatoms are abundant in aquatic habitats, forming an essential part of many food chains. However, it was not until the 1990s that their huge contribution to the global carbon economy began to be fully appreciated. A back-of-the-envelope calculation (Mann 1999) goes like this:

• total net primary production for the globe is ~ 105 Pg carbon per year (Field et al. 1998)
• of this, about 46% occurs in the oceans and 54% on land (Field et al. 1998)
• of the oceanic component, about one-quarter (11 Pg) takes place in oligotrophic (nutrient-poor) regions, one-quarter (9.1 Pg) in eutrophic (nutrient-rich) regions, and half (27.4 Pg) in the remaining mesotrophic regions (Field et al. 1998)
• diatoms account for no more than 25-30% of primary production in nutrient-poor waters, but perhaps 75% in nutrient-rich regions (Nelson et al. 1995); so, assume an intermediate value of 50% for mesotrophic waters
• the total contribution made by diatoms is then {(11 × 0.25) + (27.4 × 0.5) + (9.1 × 0.75)} = 23.275 Pg carbon per year, which is ~ 23.5% of the global total

It's probably an overestimate, but the importance of diatoms is evident nonetheless. For comparison, all the world's tropical rainforests fix 17.8 Pg, all the savannas 16.8 Pg, and all the world's cultivated area another 8 Pg. The fate of the carbon that diatoms fix is now a crucial issue in climate-change research.

Another way to appreciate diatoms is to realize that they give us every fifth breath, by the oxygen they liberate during photosynthesis.

Diatoms share several characteristics with some or all other heterokont algae, including (see also van den Hoek et al. 1995):

• plastids that are enclosed by four membranes. The inner two are homologous with the two membranes surrounding the plastids of Rhodophyta, Chlorophyta and Glaucophyta. The outer two, often referred to as 'chloroplast endoplasmic reticulum' reflect the origin of the heterokontophyte plastid as a secondary endosymbiont, related to extant Rhodophyta.
• between the outer and inner chloroplast membranes, there is often a network of anastomosing tubules called the periplastidial reticulum.
• grouping of the thylakoids into stacks of three (lamellae) within the plastid.
• presence of a girdle lamella beneath the plastid membranes, surrounding all the other lamellae.
• chlorophylls a and c and fucoxanthin as the major light-harvesting pigments for photosynthesis.
• chloroplast DNA usually concentrated within a ring-shaped nucleoid at the periphery of the plastid (but there are exceptions in some diatoms!)
• a β-1,3-linked glucan as the main reserve polysaccharide.
• possession of special tripartite stiff hairs ('mastigonemes') on a flagellum.
• mitochondrial inner membrane developed into tubular invaginations.

Diatoms share with the bolidophytes a unique 2 amino-acid insertion in the large subunit of Rubisco.

The characteristics of diatoms are that:

• all species are unicellular or colonial coccoid algae. None are free-living flagellates.
• the only flagellate cells produced are the male gametes (= sperm, spermatozoids) of 'centric' diatoms. These have a single forward-pointing flagellum, which bears mastigonemes.
• the relative proportions of the chlorophylls and fucoxanthin produce a yellow-brown or greenish-brown colour in the plastids.
• most have a large central vacuole or pair of vacuoles.
• cells (especially during stationary-phase) often accumulate large quantities of lipids and fatty acids; polyphosphate bodies are also present and sometimes take the form of discrete spherical or complex 'volutin' granules, one per vacuole.
• secretion of extracellular polymeric material (usually polysaccharides) is common, as stalks, pads, capsules, tubes, chitin fibres, or trail material from locomotion.
• all cells (except the gametes and endosymbiotic diatoms) possess a bipartite cell wall comprising two overlapping halves.
• each half-wall itself consists of a large end-piece, the 'valve', and several or many narrow bands or segments, which together form the 'girdle'.
• the cell wall is almost always heavily silicified.
• cell wall elements (valves, girdle bands, and auxospore scales and bands) are formed intracellularly, in special membrane-bound 'silica deposition vesicles' associated very closely with the cell membrane; they are not secreted from the cell until they are complete.
• new wall elements are always produced within the confines of an existing cell wall. As a result, average cell size usually decreases with successive mitotic divisions during the life cycle.
• size is restored via the formation and expansion of a special cell, the auxospore, which is usually a zygote. The basic shape of each diatom species is largely created during the expansion of the auxospore, but is often modified during subsequent mitotic cell divisions.
• during vegetative mitoses, the nucleus always lies to one side of the cell immediately beneath the girdle, at the edge of the hypotheca.
• mitosis is open, the nuclear envelope breaking down before metaphase; the spindle is a narrow cylinder, persistent at telophase, consisting of two interdigitating half-spindles, each associated with a polar plate.
• the chromosomes bunch closely around the cylindrical spindle at metaphase, becoming impossible to separate and count.
• cytokinesis occurs through cleavage.
• the life cycle is strictly diplontic: as far as is known, all vegetative cells of all species are diploid, and all mitoses take place in the diploid phase. However, haploids have occasionally been grown in culture in a few species.
• they occur just about everywhere in aquatic and damp terrestrial habitats, providing that photosynthesis is possible!
• they are amazingly diverse, with hundreds of genera and perhaps 200,000 species (Mann & Droop 1996), of which only a tenth have been described so far.