HETEROKONTAE

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.

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. All are are diploid in the vegetative state with gametic meiosis. The centrics (Figures A-D) seem to have oogamous sexual reproduction. The pennates (Figures E-I), however, reproduce by isogamous conjugation. 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.

Some pennate diatoms have the ability to creep across a substrate, but how they move is still a major question in science. What we do know is that only those pennate taxa that have a raphe, a longitudinal slit in the valve, have the ability to move. Furthermore, they move only if the raphe is in contact with a substrate. The best explanation so far is that the cell lays down a mucopolysaccharide through the raphe and allows the cell to move. Such movement by means other that flagella we call mobility.

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. As such, their primary production and oxygen output is significant from a global perspective. They are important as members of the Furthermore, diatoms are very common members of the attached algal (periphyton) 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.

A diatom has even been identified as an invasive species. Didymosphenia geminata (see Figure J), 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.

A. An SEM micrograph of Cyclotella, a centric diatom that is common in the phytoplankton. In valve view.

B. (left)An SEM micrograph of Melosira, filamentous taxon made of centric cells. In girdle view. (right) An SEM micrograph of the auxospore of Melosira.

C. A DIC micrograph of Rhizosolenia, a centric diatom with many girdle bands between the two valves which are lightly silicified. In girdle view.

D. A DIC micrograph of Chaetoceros, a taxon that makes filaments and has long radiating projections from the edges of the valve. In girdle view.

E. A Light micrograph of Fragilaria, a pinnate taxon with a pseudoraphe on each valve face.

F. A DIC micrograph of Eunotia, is usually curved with a short raphe at each end of the cell.

G. An SEM micrograph of Cocconeis has a true raphe on one valve (this valve). The other valve has a pseudoraphe. Note the striae made of many punctae.

H. An SEM micrograph of Navicula that illustrates the raphe, striae, and punctae.

I. An SEM micrograph of Nitzschia which is characterized by having raphes within keels.

J. An SEM micrograph of the invasive diatom, Didymosphenia.

Images taken from:
A-B;F-J: The Systematic Biology Biodiversity Collection.
C: http://www.chbr.noaa.gov/PMN/PhytoplanktonPictures/SpeciesListWebPhotos/Rhizosolenia400x01.jpg
D: http://www.ifremer.fr/envlit/photos/Archive/200409/img/photo39_2.jpg
E: http://biodidac.bio.uottawa.ca/thumbnails/filedet.htm?File_name=BRIE014P&File_type=jpg
J: EPA White Paper; Spaulding & Elwell (2007)

This taxonomy is based on that 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 follow 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 are 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.

Alternative relationships within the diatoms seem to be supported by molecular sequence evidence. For example, Medlin, et al. (1997) 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.

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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