Chalcedony (a crystalline variety of silica): Biogenic origin in electric organs from living Psammobatis extenta (family Rajidae)

PRADO FIGUEROA M, BARRERA F, CESARETTI NN.

MICRON

PERGAMON-ELSEVIER SCIENCE LTD

Lugar: Amsterdam; Año: 2008 vol. 39 p. 1027 - 1035

0968-4328

The electricorgans of electric fish have been used extensively for the study of peripheral cholinergic synapses. Aluminum and silicon have been observed in the electrocytes of Psammobatisextenta, a fish belonging to the familyRajidae, using a combination of scanning electron microscopy and X-ray spectrometry. Based on this evidence, the presence of silica minerals has been documented by means of mineralogical techniques. Electricorgan cryostat sections and subcellular fractions were observed using a Leica DMLP mineralogical microscope. The shape, size and color, among other properties, were analyzed in plane-polarized light, while birefringence and the extinction angle, which allow for mineral identification, were observed through crossed-polarized illumination. The distribution of chalcedony, an oxide silicon mineral, in the sections and all the fractions of the electricorgan was recorded. X-ray diffraction analysis of the electricorgan segments showed a similar result, with a low-quartz variety. Chalcedony precipitation occurred at a specific pH (7–8) and oxidation potential (Eh; 0.0 to −0.2). This observation supports the important role played by pH and Eh conditions in silica precipitation in electrocytes, as has been reported in geological environments. It is possible that silica formation and silica degradation in electricorgans are also related to the enzymes, silicatein and silicase, that direct the polymerization and depolymerization of amorphous silica in sponges. Carbonic anhydrases (silicase) are involved in physiological pH regulation. Crystallization of chalcedony via spiral growth from a partially polymerized fluid is consistent with processes known to occur in organic systems. This is the first time that a biogenically produced crystalline mineral phase (i.e., chalcedony) has been observed in the electrocytes and cholinergic nerves from livingelectric fish.