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this EPS will liberate chelated ions resulting in high
[Ca
2รพ
(all orientated with filaments normal to substrate);
heterotrophic bacteria (coccoids and short rods)
and diatoms. Although temperature strongly influ-
ences the composition of the biofilm community it
is flow rate that influences calcite precipitation
rate. Overall, however, there is a fine balance
between maximized precipitation and biofilm
erosion. Under very high flow (and ion supply)
regimes unusual and apparently physico-chemical
precipitate morphologies may develop beyond the
surface of the biofilm. Consequently, not only
does flow rate-regulated ion supply account for the
development of barrages, it can also provide a
cause for the occurrence of additional, laminated,
and coarsely crystalline carbonates at barrage
spillover points.
Biomediated calcite precipitates commence as
micro-peloidal aggregates of nanospherulites sus-
pended within the EPS. These grow rapidly into
calcite crystals, by a process involving occlusion
of EPS and the addition of further sheets of nano-
spherulites, to a well defined calcite lattice. The ear-
liest EPS precipitated fabrics form a basal calcite
layer within the EPS which becomes attached to
the flume substrate. Subsequently, calcite precipi-
tation within the EPS generally is towards the base
of the EPS sheet. However, some is localised
along linear, vertical polygonal zones. Here, bio-
mediated precipitates (microspar) grow closely
associated with bacteria. Typically, the microspar
crystals show c-axis orientations normal to the
biofilm base. Internally, in the upper part of longer
established EPS sheets a new zone of calcite crystals
can form and provide a vaulted 'roof' to cap the
polygon walls.
The trigger for the initiation of a new basal layer
is not clear though over an 8-month period sub-
sequent to the initial experiments 5 such repeated
basal calcite layers were developed, each separated
by varying thicknesses of green EPS containing
ill-defined polygonal fabrics (Fig. 13a, b). As
these basal calcite layers became buried by succes-
sively younger biofilm layers they coarsened in
grain-size and became coherently cemented into
thrombolitic tufa (Fig. 13c).
In this way, the development of a multilayered
fabric composed of alternations of calcite laminae
separated by EPS leads directly to the development
of a well laminated freshwater stromatolite. Pro-
gressively, as the earlier laminae became more
deeply buried the green colour (phototrophs) disap-
peared and the EPS became darker. These darker
EPS zones are often the sites of further calcite
precipitates which progressively occlude the
EPS. Complete occlusion appears rarely to occur
and the older EPS disappears from the fabric
leaving open cavities which contribute to the
characteristic cellular microfabric seen in ancient
(aq)
], and therefore enhanced supersaturation,
within sump biofilm interstitial water. The result is
that the degraded EPS molecules act as colloids
that filter metal ions from the water within the
flume and transport these ions down into the
sump, where they are released back into solution.
If sufficient ions are transported, some of them
will eventually be incorporated into precipitates.
This colloid filtration mechanism can be expected
to be active within lake systems (transporting ions
from surface water to the lake bottom) and in
caves (transporting ions from the vadose zone into
drips and groundwater) and would be a fruitful
topic for further investigation.
Precipitate products
The absence of vaterite in the mesocosm is perplex-
ing as it is a common microbial associated precipi-
tate in other experiments which have yielded
nanospherulites (e.g. Braissant et al. 2003; Nehrke
& Van Cappellen 2006). However, in nature
though vaterite can occur in micro-oncoids ('bis-
cuits' of Giralt et al. 2001) it is more typically
recorded within the 'opalescent' milky precipitates
(whitings) developed within the water column of
some tufa pools (e.g. Rolands & Webster 1971; Lu
et al. 2000). It is important to note that these data
are all effectively from still waters. Consequently,
the actively flowing regime, and specifically the
high calcium ion supply rate within the mesocosm
experiments, may be critical factors controlling
mineral species growth. Significantly, vaterite nano-
spherulites were also absent from fast-flowing
natural site samples at Big Hill Springs Provincial
Park, Canada (Turner & Jones 2005). Consequently,
it is possible that the ion supply rate is a limiting
factor in determining the presence or absence
of vaterite.
Conclusions
The laboratory mesocosm provides easier access to
living biofilms and their precipitation products than
do natural field sites and allows for the manipulation
of environment parameters independently and in
incremental stages. The mesocosm experiments
described herein successfully simulated natural
tufa precipitating conditions and enabled a close
study of biofilm development and the initial stages
of freshwater stromatolite development. The
flowing water experiments reported in this paper
provide the following conclusions. The Mesocosm
freshwater biofilm community closely parallels the
natural community with phototrophs including fila-
mentous green algae and filamentous cyanobacteria
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