Diatom teratological forms and environmental alterations: a review

The foremost feature of a diatom is the species-specific ornamentation of the silicon cell wall, which is preserved and faithfully reproduced through the generations. If exposed to different kinds of stress during reproductive processes, the diatom cell outline and striation pattern can change in different ways, producing teratological forms. These modifications can be slight, leading to difficulties in establishing a threshold between normal and teratological cells, or so marked that it is very difficult to recognize whether an unknown form is teratological or whether it belongs to a new species or variety. Teratological forms appear as an accidental effect of environmental stresses, which can be both physical and chemical. Artificial conditions also often lead to the development of teratological forms. Most frequently, diatoms present abnormal valve outline (lack of symmetry, bent, incised, swollen, or notched profile), unusual raphe system (fragmented, displaced, and bifurcated), abnormal striation pattern (irregular, altered, fragmented, and branched), and unusual raphe channel system (distorted, curved, and occasionally doubled back). In this review we analyzed 222 articles, published from 1890 up to 2008, with the aim to correlate the abnormal diatom cell morphology to environmental alterations, in a perspective which can greatly enhance the evaluation of river environmental quality for biomonitoring purposes.