Ko. Konhauser et al., METAL SORPTION AND MINERAL PRECIPITATION BY BACTERIA IN 2 AMAZONIAN RIVER SYSTEMS - RIO-SOLIMOES AND RIO-NEGRO, BRAZIL, Geology, 21(12), 1993, pp. 1103-1106
The surfaces of epiphytic and episammic bacteria from the solute-rich
waters of the Rio Solimoes, Brazil, interacted with available cations
in solution and provided major sites for iron deposition. Once bound t
o the bacteria, the iron served as nucleation sites for the formation
and growth of fine-grained authigenic mineral phases. Because of progr
essive mineralization, the precipitates exhibited a wide range of morp
hologies, from amorphous ''gel-like'' to crystalline structures. Compl
ete encrustation of some bacterial cells was observed. The most abunda
nt mineral phase associated with the bacteria was a complex (Fe,Al) si
licate with a variable composition. Energy-dispersive X-ray spectrosco
py suggested that the gel-like structures were similar in composition
to a chamositic clay, whereas the crystalline phases were increasingly
kaolinitic. Bacteria collected from the solute-deficient waters of th
e Rio Negro remained conspicuously unmineralized, suggesting that meta
l sorption and biomineralization largely reflect the availability of d
issolved metals in the water column. In a solute-rich river system, th
e fate of the metal-loaded bacteria has profound implications for the
transfer of metals from the hydrosphere to the sediment. Through diage
nesis, the bound metals may either be recycled to the overlying water
column or become immobilized as stable mineral phases. If the latter o
ccurs, microorganisms will have played an important role in metal depo
sition, low-temperature clay formation, and, invariably, mudstone diag
enesis. This implies that in the geologic record, microorganisms may h
ave been instrumental in the genesis of many sedimentary rocks of fluv
ial origin.