An introduction to diatoms.

Diatoms are almost ubiquitous. They live in aquatic environments, soils, ice, attached to trees or anywhere with humidity and their remains accumulates forming diatomite, a type of soft sedimentary rock.
They are the dominant marine primary producers in the oceans and play a key role in the carbon cycle and in the removal of biogenic silica from surface waters.

Diatoms are unicellular algae with golden-brown photosynthetic pigments. They have a siliceous skeleton known as frustule that comprise two valves, one overlaps the other like the two pieces of a petri dish. The frustule is ornamented with pores, processes, spines, hyaline areas and other features The size range is between 1 to 2000 μm in length.

Diagrammatic sections of a frustule. From UCL.

They belong to the Division Chrysophyta, Class Bacillariophyceae and are divided in two Orders: The Centrales or Biddulphiales and the Pennales or Bacillariales. The Pennales or pennate diatoms have frustules that are elliptical or rectangular in valve view, with sculpture that is bilaterally symmetrical about a central line while the Centrales are characterized by frustules which are circular, triangular or quadrate in valve view and rectangular or ovate in girdle view.

Diatoms could be single or could form colonies. The cell has two or more golden-brown photosynthetic chloroplasts, a central vacuole, a large central diploid nucleus. Diatoms also store oils rather than starch and lacks of flagella or pseudopodia.

The first record of diatom frustules are centric forms from the Early Jurassic although very few remains are known before the Late Cretaceous, they were moderately affected by the massive extinction at the end of the Cretaceous.

Crossophialus gyroscolus and C. glabrus from the Upper Cretaceous of Antarctica. Image from the Ocean Drilling Program.

Centric diatoms had a major radiation in the Paleocene. Pennate diatoms also appeared during the Paleocene. The provincialism among diatoms increased in the latest Miocene and during the Pleistocene, diatom assemblages start to closely resemble modern ones.

Azpeitia tabularis (Miocene to Recent) and Fragilariopsis ritscheri (Pliocene to Recent). From UCL

During the 19th century, Ehrenberg, Grunow, Schmid and others studied and made great illustrations of diatoms, but was E. Hackel who created the most beautiful and artistic representations of diatoms in his work “Kunstformen der Natur”.

Living diatoms are very sensitive to parameters like salinity, oxigenation and other physical and chemical conditions, so they provide a valuable tool for studies of modern water quality and for the reconstruction of past environments, particularly for evidence of climatic cooling and changing sedimentation rates in the Arctic and Antarctic oceans. Also the evolutionary history of diatoms has been punctuated by several floristic turnovers which makes diatoms great tools for biostratigraphic correlations.

References:

Armstrong, H. A., Brasier, M. D., 2005. Microfossils (2nd Ed). Blackwell, Oxford.

Barron, J.A. (2003). Appearance and extinction of planktonic diatoms during the past 18 m.y. in the Pacific and Southern oceans. “Diatom Research” 18, 203-224