(^) Vrieling et al. (1999) used vital dyes that fluoresce at low pH to show that
precipitation of opal in the silica deposition vesicles occurs under acid conditions. At
acid pH, both the hydroxyl groups and, particularly, the repeated N-methyl-
propylamine chains catalyze silicic acid polymerization. The resulting opal frustules
are well protected during the life of the diatom by organic coatings on the outer
surfaces. Bacteria colonize and enzymatically degrade the organic matrix (Bidle &
Azam 2001). Upon decay of the coating after cell death, weakly basic seawater slowly
redissolves the opal. Thus, accumulation of diatomaceous sediments requires rapid
burial rates, and often only species with the very thickest shells remain in the
geological record.
(^) In many, but not all, diatom species, cell size diminishes progressively as division
proceeds (MacDonald 1869). Since the hypotheca becomes the epitheca for one of the
daughter cells, that cell is smaller than its sister. This process terminates by formation
ff
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