What is a diatom?



 Fig. 1 Example of diatoms from the freshwaterbenthos at the Ebro river. a) Achnanthidium pyrenaicum,  b) Cymbella excisa, c) Navicula cryptocephala, d)Diatoma tenuis, e) Navicula cryptotenella, f) Aulacoseira granulata, g) Achnanthidium minutissimum, h) Nitzschia inconspicua, i) Cocconeis lineata, j)Cyclotella cf. ocellata, k) Denticula tenuis, l) Gomphonema olivaceum, m) Gomphonema stoermeri, n) Amphora pediculus, o) Encyonema minusculum, p) Reimeria sinuata, q) Encyonopsis minuta, r) Nitzschia supralitorea. Image taken with a Zeiss AxioImager Z.2 with Metafer 4 automatic slide scanner system.

Diatoms are unicellular algae in a glass case (or frustule). They pertain to the Heterokontophyta phylum, only due to an extinct ancestor, although sperm from centric diatoms can have a flagellum. Diatoms occur in every kind of water and can even survive on relatively dry substrate. They have been divided classically into two groups: pennate (bilateral symmetry) and centric (radial symmetry). In the advent of phylogenetics, it was established that these artificial groups were paraphyletic (Williams & Kociolek, 2007, Medlin 2011). Another group was added, Mediophyceae (Medlin & Sato, 2009), to establish the intermediate forms. In these phylogenetic studies, the interior subdivision of pennate diatoms was also found to have been paraphyletically subdivided into 'araphid' and 'raphid' (Medlin & Kaczmarska, 2004). All of these categories can still be used, but only as a descriptor, not in a taxonomical sense (Medlin, 2011). Descriptively, raphid diatoms are those that have a raphe, a slit-like opening that leads the cell to move autonomously. 
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References
MEDLIN, L. K. 2011 The Use of the Terms Centric and pennate, Diatom Research, 24:2, 499-501.
MEDLIN, L. K. and KACZMARSKA, I. 2004. Evolution of the diatoms, V. Morphological and cytological support for the major clades and a taxonomic revision. Phycologia, 43: 245–70.
MEDLIN, LINDA., JUNG, INES., BAHULIKAR, RAHUL., MENDGEN, KURT., KROTH, PETER and KOOISTRA, WIEBE H. C. F. 2008. Evolution of the Diatoms. VI. Assessment of the new genera in the araphids using molecular data. Nova Hedwigia, Beihefte, 133: 81–100.
MEDLIN, L and SATO, S. 2009. Opinion. The biological reality of the core and basal group of araphid diatoms. Diatom Research, : 503–508.
SATO, S. 2008. Phylogeny of araphid diatoms, inferred from morphological and molecular data, PhD Dissertation University of Bremen. WILLIAMS, D. M. and KOCIOLEK, J. P. 2007. The rejection of paraphyletic taxa. European Journal of Phycology, 42: 313–319.

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