Morphology and Phyletic Relationships of the Silicified Algae and the Archetypal Diatom — Monophyly or Polyphyly

Abstract

Considering the abundance of silicon in nature it is surprising how few groups of organisms have utilized this element. Its use is most likely to be in the formation of resistant wall material, since carbon has achieved an overwhelming role in metabolic pathways. We should not, however, neglect to speculate on and investigate the possibility of the participation of silicon in other metabolic processes. The reason so few groups utilize silicon may be that they already produce basic organic compounds via their photosynthetic pathway and these can be used for wall/skeletal formation. However, to incorporate silicon rather than to convert a polysaccharide produced by photosynthesis might be much more economical of energy—possibly by a factor of 5–10 (Werner, 1977). Of the 14 divisions of the algae, only three, plus one small family make any notable use of silicon in construction and one question we should perhaps pose is, “How is it that most plants avoid depositing silica?” A few algae, e.g., Pediastrum, Tetraedron and Hydrodictyon, are reported to incorporate silica into organic walls, but no algae use it simply as an additional strengthening element as do some macrophytic plants such as Equisetum, many Gramineae and Cyperaceae, or in the hairs as do Commelinaceae, Ulnus, Canabis, and the Zingiberaceae (Chaps. 14, 15).