Bacillariophyta

Foodplant / open feeder
Acmaea tessulata grazes on Bacilliariophyta

Foodplant / open feeder
Acmaea virginea grazes on Bacilliariophyta

In Great Britain and/or Ireland:
Foodplant / parasite
Chytridium versatile parasitises Bacilliariophyta

View Diatoms Tree

Just as it is something of a mystery at present as to how and why the diatoms arose from among the other heterokont algae, so also we have little idea about relationships among the major lineages within the diatoms. At the generic level and, to a lesser extent, within families of diatoms, considerable progress has been made during the last 40 years, as a result of huge injections of new information and analysis, first from electron microscopy (particularly scanning electron microscopy), then from examination of cell structure and sexual reproduction – which are characteristics previously ignored by most diatomists – and most recently from molecular phylogenetics. However, these data have not yet provided a clear idea of higher-level relationships in diatoms, among families, orders and classes. Round et al. (1990) described many new genera and resurrected others from obscurity, on the basis of morphological and cytological surveys; these have on the whole been supported by subsequent investigations, including studies based on molecular data. Round et al. also provided a new framework of classes, orders and families to 'hold' the revised genera. These have not been so successful. Gene sequence data have shown over and over again that informal analysis of relationships, based on morphology, often fails to reveal the true pattern of evolution. The problem seems to be that parallel or convergent evolution of shape and wall structure has been rampant within the diatoms. But in many cases, molecular analyses have also failed us thus far, partly because of convergent evolution and partly because of analytical difficulties, such as establishing homology in rDNA sequences and hence developing a correct alignment matrix.

Round et al. divided the diatoms into three classes: Coscinodiscophyceae, Fragilariophyceae and Bacillariophyceae, which correspond to three of the main types of valve organization. The Coscinodiscophyceae were to be recognized by having valves in which the pattern of ribs and striae (lines of pores) radiates out from a ring (the annulus). In the Fragilariophyceae, the pattern was feather-like, with the ribs and striae being arranged either side of one or two longitudinal ribs or strips (sterna). And in the Bacillariophyceae, the pattern was similar to that in in the Fragilariophyceae, except that the central strip contained a raphe system. Informally, these three structural variants can be referred to as 'centrics' (Coscinodiscophyceae), 'araphid pennates' (Fragilariophyceae) and 'raphid pennates' (Bacillariophyceae).

Although analyses of molecular sequence data have not yet provided us with a clear picture of the early evolution of the diatoms, one thing has become obvious: the Round et al. three-class system is wrong, if the aim is to reflect phylogeny. The phylogenetic trees that have been published during the last 15 years disagree in many respects, but all show the primary radiation to have occurred among diatoms with a centric valve structure, the pennates having evolved later, from ancestors with centric structure. Furthermore, the Fragilariophyceae are not monophyletic, being paraphyletic with respect to the Bacillariophyceae. So, of the three Round et al. classes, only one – the Bacillariophyceae – is satisfactory.

The main features that appear in several phylogenetic trees (once the topologies of the trees have been simplified to include only those relationships that have good statistical support) are:

1. a basal polytomy of clades, all but one of which have circular valves (very rare exceptions) with a centric valve pattern and monopolar or radial symmetry; the clades with circular valves and centric organization are often referred to as the 'radial centrics'. The 'radial centrics' may be a monophyletic group and appear so in some published trees; on the whole, however, it appears that they are not.
2. a further clade in the basal polytomy that comprises the remainder of the centric diatoms and all of the pennate diatoms. The centric diatoms in this clade usually have a polar organization. The pattern still radiates from an annulus, but the valves are usually elliptical or elongate, triangular or triradiate, etc, and the structure of the valve shows bi- to multipolar symmetry; circular valves are uncommon and possible secondarily derived. Centric diatoms of this kind are often referred to as 'polar centrics'.
3. the polar centrics rarely appear in molecular phylogenies as a monophyletic group; instead, they are usually paraphyletic with respect to the pennate diatoms.
4. the pennate diatoms are monophyletic.

Medlin & Kaczmarska (2004) proposed a replacement for the Round et al. scheme, based on interpretations of molecular and some nonmolecular data, in which the diatoms were divided into two subphyla, Coscinodiscophytina (= 'radial centrics') and Bacillariophytina ('polar centrics' + pennates), and the Bacillariophytina in turn into two classes, the Mediophyceae ('polar centrics') and Bacillariophyceae (pennates). Note that Medlin & Kaczmarska's use of 'Bacillariophyceae' (all pennate diatoms) differs from that of Round et al (1990) (only raphid pennate diatoms). It is very likely that the Mediophyceae are paraphyletic and quite likely that the Coscinodiscophytina are also paraphyletic. Because of this, and because some published molecular analyses are said not to be repeatable, the Medlin–Kaczmarska scheme has been heavily criticized (e.g. Williams & Kociolek 2007). However, the new classification does have the virtue, relative to the Round et al. (1990) classification, that it reflects better the finding, now generally agreed and consistent with the fossil record, that diversification occurred first among diatoms with a centric valve structure, and that pennates evolved later from within one out of several or many centric clades. Although this is not a new idea (e.g. Simonsen 1979), it was only one of many possibilities aired before the advent of molecular systematics (see Mann & Evans 2007).

Adl et al. (2005) adopted Medlin & Kaczmarska's names but noted that the Coscinodiscophytina and Mediophyceae might be paraphyletic. Here, Medlin & Kaczmarska's formal taxa are used only where there seems to be good support for monophyly (i.e. the Bacillariophytina and Bacillariophyceae). Nevertheless, it is often useful to distinguish Medlin & Kaczmarska's Coscinodiscophytina and Mediophyceae informally, as 'radial centric diatoms' and 'polar centric diatoms', to highlight what seems to be a well-established feature of diatom evolution, that complex shapes and bi- or multipolar structure developed in one clade of centric diatoms (within which the pennates evolved later), but not in several others.

Heb je wel eens vuursteen gevonden op het strand? Wist je dat de kiezel van de vuursteen afkomstig is van miljoenen eencellige kiezelwiertjes? Diatomeeën of kiezelwieren zijn belangrijk in zee. Ze zetten heel veel zonne-energie om. Ze zijn hoofdvoedsel voor allerlei kleine planktoneters, zoals roeipootkreeftjes en vislarven, die op hun beurt weer op het menu staan van grotere vissen. Zo staan de kiezelwieren aan de basis van het voedselweb in zee. Kiezelwieren leven in het water en op de zeebodem. Ze worden tot een paar tienden van een millimeter groot.

Diatom identification guide & ecological resource

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