Bacillariophyta (ba-sil-a-re-O-fa-ta) is made of two Greek roots meaning stick (bakillos -βάκιλλος); and plant (futo -φυτό).  The reference is to the stick or rod-like nature of many of the members of this photosynthetic (plant-like) phylum.  Also, it is a formal phylum (division) name derived from a common genus, Bacillaria.  Haeckel (1878) described the class Bacillariophyceae within the Chrysophyta.  Engler and Gilg (1924) then raised the class name to the rank of Division (Phylum).


Diatoms are united in having an elaborate frustule made of silicaceous overlapping halves (valves).  Indeed, the common name, diatom, is derived from two Greek roots that mean cut in two as a reference to the structure of the frustule.  The valves are usually highly ornamented with punctae, striae, costae, and other variations in the wall (Figure 1).    All are are diploid in the vegetative state with gametic meiosis. Thus, it is a diplontic life cycle. The centrics seem to have oogamous sexual reproduction (Figure 2).  The pennates, however, reproduce by isogamous conjugation (Figure 3).  Unlike most microbial eukaryotes, the diatoms seem to be triggered to undergo sexual reproduction when the mean cell size (also variance in cell size is highest) is lowest.  The zygospore (called an auxospore) returns the population to the maximum cell size.



FIGURE 2. The stylized life cycle of a centric diatom.  Meiosis is triggered by cell size.  The large cells produce eggs and small cells generate monoflagellate sperm.  The zygote begins to enlarge and forms an auxospore, within which the initial cell, the largest possible cell for that population, forms.

From Hasle and Syvertsen (1997)

FIGURE 3. Stylized life history of a pennate diatom.  As in the centrics (Figure 2), meiosis is triggered by cell size.  However, following meiosis and gametogenesis, cell associate (conjugate) and amoeboid gametes are exchanged.  As in the centrics, the zygote develops into an auxospore within which the initial cell forms.

From Hasle and Syvertsen (1997)




Diatoms are among the most common microbial eukaryotes and dominate in many different aquatic and marine habitats. In the oceans, they make up much of the plankton in the open ocean (Figure 4).  As such, their primary production and oxygen output is significant from a global perspective.  Together with dinoflagellates and other marine primary producers, diatoms likely are sources of half the available molecular oxygen.  Furthermore, diatoms are very common members of the attached algal (periphyton, Figure 5) and plankton communities of lakes, ponds, and reservoirs.  In addition, they make up a major component of the attached algal community in streams, where they often form the basis of the aquatic food web.  They often make a dark brown film that is very slick on stones in creek beds.  In a local stream, I have found as many as 64 species of diatoms growing on a single stone.  Diatom taxa, because their frustules are made of glass and very highly ornamented, are relatively easy to identify.  Furthermore, many taxa have been "calibrated" with regard to a spectrum of environmental conditions (e.g. temperature, pH, phosphate, etc.) such that their occurrences can reflect the conditions of the stream or lake over the time that the community has been in place (usually integrates conditions over the previous three weeks).  Thus, they have been used as indicator organisms for most aquatic environments.  Typically, the frustules are cleaned with acid to remove all extraneous organic matter.  Then, the cleaned frustules are concentrated and enumerated.  Such samples can be mounted on permanent slides or dried and preserved for many years in museum collections where they provide a basis against which changes in aquatic systems can be documented and measured.  In many lake sediments conditions favor the maintenance of the silicaceous frustule.  If the layers of sediment are undisturbed, they can yield information about changed in the immediate environment over thousands of years.

Diatom frustules were so abundant in former marine and freshwater environments, that fossil deposits of their frustules (called diatomaceous earth) can be hundreds of feet thick.  Also called diatomite, diatomaceous earth is used for many industrial and domestic applications such as water filters, dynamite, metal polish, glass, pesticides, etc. 





FIGURE 6. A cladogram of the Bacillariophyta (taxa in the shaded box) and its sister groups (taxa in bold) after the analyses of Medlin and Kaczmarska (2004).  The clades in the shaded box are numbered according to the following:








All of these taxa are diplontic and have overlapping silicaceous walls, as described in the introduction.  The sister group to the diatoms is Bolidophyceae, a taxon discovered only recently by Guillou et al. (1999).  The relationship between the diatoms and Bolidophyceae was confirmed by Richards et al. (2005), Medlin and Kaczmarska (2004), and Alverson et al. (2006).  The surprise is that Bolidomonas, the single known genus in the group, is nothing like vegetative diatoms.  The genus does not have frustules, and the form of the motile cell, although very much like the archetypal heterokont motile cell, is very different from the centric diatom sperm (the only motile cell in the Bacillariophyta).  However, the photosynthetic pigment system is dominated by fucoxanthin in Bolidomonas and the diatoms.


This group is Coscinodiscophytina, a subphylum with a single class in the system of Medlin and Kaczmarska (2004).  However, it is a polytomy and future revision likely will resolve the group (Alverson et al. 2006).  Cells in this clade are distinguished by the following characters from Medlin and Kaczmarska (2004).

  • radial centrics
  • G-ER-M golgi (each golgi associated with a mitochondrion) dispersed through the cytoplasm
  • one pyrenoid per plastid
  • isometric auxospores
  • no raphe
  • processes marginal

Although members of this group are the radial centrics, they vary quite a lot in vegetative form.  Also, they can range from unicells to filaments.  `The whole group really is not a clade, but a large polytomy (Medlin and Kaczmarska 2004).

The sexual life history generally follows that of Figure 2.  However, the auxospores are isometric (expand equally in all directions) and covered with scales.  The sperm are formed by a merogenous process (Werner 1977) in which the haploid nuclei following meiosis form buds on the mother cell and leave the plastids behind.  The sperm cell body is elongated with concomitantly elongate nuclei and mitochondria.

Examples of this clade include:

Paralia (Figure 8) is filamentous and centric.  It has a thin outer wall that interdigitates with the walls on either side (top image Figure 8).  However, the inner walls are quite thick, and those walls are attached by a ring of marginal processes.  Paralia is Greek for beach, where they are often found associated with bottom sand.  Some of the analyses of the Bacillariophyta (e.g. Medlin and Kaczmarska 2004) have the Paraliales as the most basal of all diatoms.

Melosira (Figure 9) also is filamentous and centric, but its cells are much more elongate than those of Paralia.  In addition, the wall does not separate as it does in Paralia.  The cells of Melosira are maintained in a filament by mucilagenous pads secreted at their faces (Round et al. 1990), which also are covered with small spines (Figure 8).  They occur commonly in freshwater (streams, lakes, and ponds) and in near shore marine habitats.  Sexual reproduction has been well-studied in this genus (e.g. Crawford 1974 and 1975), and served as the model for Figure 2.  Note the isometric expansion of the Melosira auxospore in Figure 10.

Rhizosolenia (Figure 11) is a marine genus in which most of the length of the cell is taken by overlapping girdle bands.  The valve faces are asymmetrical and pulled into a point.  Though chain-forming species do occur, most are solitary.  The thin girdle band that occupies most of the cell exterior makes it almost invisible.






Taxa in this subphylum are quite variable and range from centric to pennate in form.  They also tend to range from oogamous to isogamous sexual reproduction.  The auxospores, unlike those of the Radial Centrics, expand in one direction with scales and a perizonium, stacked ring structures that are added as the auxospore elongates (Figure 12).  Golgi are associated with and surround the nucleus.


This group, made up of the bipolar centrics and the radially symmetrical Thalassiosirales, was created by Medlin and Kaczmarska (2004) to account for the separation of the centric taxa into two major groups that they interpreted as subphyla.  The 18S and 16S ribosomal RNA gene comparisons showed the clear separation.  Medlin and Kaczmarska (2004) then identified the following structural characters that correlated with the group. 

  • perinuclear golgi
  • 1-many pyrenoids per plastid
  • nonisometric properizonia
  • oogamy (generally hologenous - the sperm mother cell divides without leaving a residual cellular component)
  • sperm nuclei rounded
  • processes mainly central, but marginal in Thallasiosirales

Examples of this group include:

Cyclotella (Figure 13) is a member of the Thalassiosirales and its cells are fully radially symmetrical.  The cell has the appearance of a wagon wheel with striae arrayed radially to a central disk.  The processes are on the outer edge of the cell like those of the Coscinodiscophytina.

Chaetoceros (Figure 14) is a filamentous genus with long spines on the margins of the valve faces. Cells form filaments by the intertwining of the spines. The valves are elongate or oval and clearly biradial.  

Biddulphia (Figure 15), from valve view, appears pennate in form.  However, the terminal poles of the oval valve face are elevated making it biradial,a hybrid between a centric and a pennate form.





This is the only major clade of the diatoms that clearly is monophyletic.  Cells in this class are elongate and decidedly bipolar.  The valve face has a sternum and generally bilaterally symmetrical. with striae more or less perpendicular to the primary axis.  Many of the taxa have a raphe, usually a slit that runs from the center of the cell to the poles and allows for mobility of the cell.

Medlin and Kaczmarska (2004) identified the following morphological features of the pennates.

  • Golgi are perinuclear
  • one pyrenoid per plastid
  • auxospores enlarge by means of a perizonium
  • sexual reproduction by conjugation of non-flagellated isogametes.
  • the raphe and processes tend to be centrally located.

Examples of this group include:

Fragilaria (Figure 16) is a genus that may be filamentous.  The cells are regular and tapered to the poles.  Though it does not have a raphe, the striae produce a longitudinal gap forming a pseudoraphe.  Fragilaria is a member of the araphide pinnates, which, according to Medlin and Kaczmarska (2004), are polyphyletic.

Cocconeis (Figure 17) is one of the most important colonizing diatoms on stones and other hard substrates in streams.  They are flattened and form an almost scale-like covering onto which other members of the diatom community, as well as other members of the periphyton, can attach.  This taxon has a true raphe on one valve and a pseudoraphe on the other valve.  

Navicula (Figure 18) means "little boat" in Latin.  This genus is very speciose within the diatoms even though it has been split into a number of other genera (Round et al. 1990).  The cell is symmetrical about the primary and secondary axes and both valves have raphes.  A closer look at the striae shows that the punctae look like slits.

Nitzschia (Figure 19) has a raphe that is part of a keel system.  These cells, with the arch-like fibulae, are quite distinctive.  Some members of this genus are very large. 

Didymosphenia geminata (Figure 20), also called didymo or river snot, has begun to become a nuisance in freshwater streams of the temperate zones of the earth.  Didymo has become especially troublesome in New Zealand, North America, Europe, and Asia.  The alga can grow quickly and cover stream beds with stalks made of extracellular polysaccharides that have the consistency of hair.  The beds can develop to 20cm thick, effectively smothering almost all benthic invertebrates, and algal communities.  Thus, the affected vertebrate communities necessarily become simplified.






The system that we follow is from Medlin and Kaczmarska (2004), who recognize two major groups, which they call subphyla.  In this system, there are two groups of centrics: the radial centrics and the biradial centrics, both of which are polytomies.  The pennates, however, are monophyletic.  This current taxonomy grew from Medlin et al. (1997) who present two different clades of diatom taxa.  Clade 1 contains centric diatoms that have a process called a rimoportula.  Clade 2 contains all of the pennate diatoms and the remaining centric diatoms which have a process called a fultoportula.  This would suggest a 2 class system for the phylum based on the presence of the processes as defining synapomorphies.  Medlin and Kaczmarska (2004) supplanted the system of Round et al. (1990) who recognized 3 classes: Coscinodiscophyceae (the centrics); Fragillariophyceae (the pennates without raphes); and the Bacillariophyceae (the pennates with 1 or 2 raphes).  They followed the lead of others [e.g. Margulis and Schwartz (1988, Pr-11 and 1998, Pr-16), Sleigh et al. (1985), Patrick and Reimer (1966)] in which the diatoms were given phylum-level status. 

Earlier taxonomic treatments recognized two fundamental groups, usually considered to be orders, the centrics (Centrales) and the pennates (Pennales) [e.g. Smith (1950), Bold and Wynne (1985) and Patrick and Reimer (1966) which are modifications of the Hustedt system (cited in Werner 1977)]. Patrick and Reimer (1966) make no taxonomic distinction between the pennate and centric diatoms and lump all nine of the diatom orders into a single class. Bold and Wynne (1985), Sze (1986) and Lee (1980) present the diatoms as a class within the chrysophyte complex of phyla (the Ochtophytes of Cavalier-Smith and Chao 1996; and Graham and Wilcox 2000). The analyses of Dodge (1973) and Taylor (1976) seem to support the chrysophyte taxonomy.

Mann and Marchant (1989) propose that the diatoms arose from a chrysophyte which produced a cyst bearing several silicaceous plates. Diatoms then evolved as a reduction of the number of plates into two large overlapping structures. Then, the diploid cyst became the vegetative stage. In my view, such changes are great enough to require phylum status for the diatoms, even if they are retained within the chrysophyte complex.  I follow Cavalier-Smith (1989), Patterson (1999), Sogin and Patterson (Tree of Life Project), and Baldauf (2003) in uniting this group with other Heterokont taxa.  However, the synthesis of Baldauf (2003) suggests that the diatoms are primitive within the heterokont supergroup.






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By Jack R. Holt.  Last revised: 02/24/2014