Geology Reference
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However, 200-300 mm coccoid and short-rod
shaped bacteria frequently occurred in close proxi-
mity to the micropeloids and larger micropeloids
typically contain EPS-filled cavities many of
which also host the same coccoid bacteria. If
these bacteria were chemo-organic (heterotrophs)
feeding on the EPS (Arp et al. 2001; Turner &
Jones 2005) their metabolic activity should inhibit
precipitation (Visscher & Stolz 2005) via CO 2 pro-
duction. If they were responsible for the precipi-
tation, as seems likely, this would suggest they
were either anaerobes involved in reducing sulphate
or iron (Visscher & Stolz 2005), or EPS digestion
was releasing sufficient calcium to supersaturate
the system despite the presence of abundant respired
CO 2 , or that organisms are manipulating the micro-
environment of biofilm interstitial fluid in a manner
that promotes precipitation for example via pro-
duction of extracellular carbonic anhydrases (Li
et al. 2005; Kupriyanova et al. 2007).
The basal calcite layer also developed from dis-
crete nanospherulite nucleation points within the
EPS. Initially, unrestricted but close stacking of
EPS-supported nanospherulites occurred. Each
developed progressively into anhedral crystals
which eventually became attached to the flume sub-
strate (see Fig. 4b). SEM studies show that crystal
growth involved the progressive occlusion of EPS
from between and within the growing nanospheru-
lite aggregates and anhedral crystals, though the
process rarely reached completion. Consequently,
EPS frequently was still present as strands
between and within the microspar throughout the
biofilm (see Fig. 3a) and must have played a funda-
mental role in the epitaxial growth of the crystals.
The associated crystal perforations (best seen after
removal of the EPS) are so common that Pedley
et al. (2009) considered this feature was diagnostic
of biomediated calcite wherever
it occurred
naturally.
Significantly, the development from nanospher-
ulite to euhedral calcite was not by random chance
but rather by a systematic addition of nanospheru-
lites onto a well ordered calcite lattice. Ultimately,
Fig. 10. (a) Nanospherulite crystal building blocks within the EPS. Although apparently randomly arranged these
200-500 nm diameter calcite nanospherulites are pre-arranged into sheets associated with a well ordered calcite atomic
lattice. This is illustrated at the right side of the view by the presence of a well developed tabular crystal face. Summer,
fast-flow experiment. Air dried SEM sample. (b) Enlarged view of the crystal face showing the nanospherulites arranged
into sheets parallel to the rhombic face. Summer, fast-flow experiment. Air dried SEM sample. (c) Basal calcite layer
(base is at top) showing the tightly interlocking nature of calcite crystals and the striated compromise boundaries on the
central crystal. Autumn, fast-flow experiment. Air dried SEM sample. (d) Close-up of the striated compromise boundary
in Figure 10c (same orientation). This reveals that the striations coincide with rows of nanospherulites. The larger
spheres scattered on the striated surface are coccoid heterotrophic bacteria.
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