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Fig. 5. (a) Details of the polygonal structures (septae are here composed of dehydrated EPS) which develop within the
EPS (the basal calcite layer uniformly covers the glass sample slide). These septae are the sites of further calcite
precipitation and the development of polygonal walls of crystals which attach to the previously established basal calcite
layer. Autumn, fast-flow experiment. Air dried SEM sample. (b) Details of the area at the base of a microspar septum
wall. The filamentous bacteria are Oscillitoriacea. Note the abundant 200-500 nm diameter coccoid and short-rod
heterotrophic bacteria (pale raised patches now apparently lying on the glass slide in the lower part of the view but
originally suspended within the EPS and intimately associated with the precipitates). Autumn, slow-flow experiment.
Air dried SEM sample. (c) Details of calcite microspar suspended within EPS in a septum. Note how the anhedral
microspar becomes larger towards the base of the septal zone (bottom of the view). Also note the subparallel orientation
of the crystal long-axes in the centre and top (these are parallel to the cyanobacterial filaments which are in approximate
life position). Summer, slow-flow experiment. Air dried SEM sample.
EPS. The precipitates continued to grow without
morphological modification under both summer
and autumn conditions.
flumes 1 (slow-flow) and 2 (fast-flow) during the
summer experiments. The higher flow rates in
flume 2 however, created trailing filamentous cyano-
bacterial EPS with unusual precipitate mor-
phologies developed on to downstream-pointing
biofilm 'fingers'. SEM studies revealed the calcite
crystals to have accelerated epitaxial growth in
areas where water flow was fastest (Fig. 7a, b).
Flow Rates and precipitates
There was no observable difference in morphology
between biofilm microstructure developed in
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