In vitro investigations of the impact of different temperature and flow velocity conditions on tufa microfabric - Tufas and Speleothems: Unravelling the Microbial and Physical Controls - page 197

Fig. 6. (a) Microspar crystals developed within thick biofilm which coated the submerged wall of the water sump.
This sealed sump received no light therefore the biofilmmust have been generated exclusively by heterotrophic bacteria
(note the total absence of filamentous cyanobacteria and algae). Summer, slow-flow experiment. SEM view. (b) Detail
of a similar sump-grown calcite rhombs without EPS. Note the abundant cavities within these crystals which are
occupied by EPS in (a). Summer, slow-flow experiment. Air dried SEM sample.

However, crystal growth along the c-axis (normal to
the water surface) was minimal (Fig. 7b) but fast
lateral growth resulted in a dense interlocking
mosaic. Away from the fastest flow the precipitates
were unremarkable in their morphologies. However,
there was significantly more observable calcite
precipitate associated with the flume 2 (fast-flow)
biofilm compared to flume 1 (cf. Fig. 8a, b).
During the autumn experiments similar biofilm
volumes developed in flumes 1 and 2. In both
flumes the biofilm initially established attachment
to the flume substrate. However, the hemispherical
biofilm topography best developed where the
biofilm became locally detached from the substrate.
In flume 2 (fast-flow) this partial detachment
encouraged areas to tear and become carried away
in the flow. Again, at the end of the experiments

much more calcite precipitation was observed
associated with the flume 2 (fast-flow) biofilm.

Discussion

The experimental runs show that temperature rather
than flow rate controlled the composition of the
microbial community. Nevertheless, biofilm coloni-
zation rates were about the same in both summer and
autumn biofilm experiments. It may be that biofilm
adhesion and consequent colonization rates were
also influenced by other factors such as calcium
ion availability (Fletcher & Floodgate 1976) and
nutrient availability but these are yet to be investi-
gated. The experiments also supported the findings
of Bissett et al. (2008) that although increased
biological activity, ion diffusion rates and daylight

Fig. 7. (a) Undulose-topped calcite crystals coating trailing 'fingers' of biofilm at the flume outflow. The topography
of these crystalline surfaces has been limited by the thin film of fast flowing water which covered them during
growth. The resulting crystals show rapid lateral growth but almost no growth parallel to c-axis. Summer, fast-flow
experiment. Air dried SEM sample. (b) Details of the broad lateral growth increments which appear to have grown
above the EPS sheet. Summer, fast-flow experiment. Air dried SEM sample.