Chemistry Reference
In-Depth Information
Figure 2
. Molecular phylogeny of eukaryotic organisms, showing the phylogenetic distribution of
mineralized skeletons. C = calcium carbonate minerals; P = calcium phosphate minerals; S = opaline
silica. Letters in parentheses indicate minor occurrences. Phylogeny based principally on Mishler et al.
(1994) for the Plantae; Giribet (2001) for opisthokonts; and Baldauf (2003) for the Eucarya as a whole.
Data on skeletons principally from Lowenstam and Weiner (1989).
Phylogeny thus points toward repeated innovations in the evolution of carbonate
skeletons, but this leaves open the question of homology in underlying molecular process.
As pointed out by Westbroek and Marin (1998), skeleton formation requires more than
the ability to precipitate minerals; precipitation must be carried out in a controlled fashion
in specific biological environments. Skeletal biomineralization requires directed transport
of calcium and carbonate, molecular templates to guide mineral nucleation and growth,
and inhibitors that can effectively stop crystal growth. All cells share the ability to bind
Ca
2+
ions and regulate calcium concentrations (e.g., MacLennan et al. 1997; Sanders et
al. 1999), and both photoautotrophic and heterotrophic eukaryotes regulate their internal
inorganic carbon chemistry using carbonic anhydrase and other enzymes (e.g., Aizawa
and Miyachi 1986). Thus, the biochemical supply of ions required for calcite or aragonite
precipitation appears to be an ancient feature of eukaryotes. Organisms in which
carbonate formation has been well studied also synthesize acidic proteins and
glycoproteins that provide templates for mineralization (Weiner and Addadi 1997),
