Geology Reference
In-Depth Information
experimental runs the sump was connected to a pair
of 'Korallin C501' calcium reactors. These supplied
calcium ions derived by dissolution of aragonitic
coral debris within the reactor towers. The supply
rate of calcium ions was adjusted by controlling
the flow of carbon dioxide gas delivered to each of
the reactor towers. The flume waters were main-
tained at a conductivity level of 300-400 micro-
siemans (ms), this conductivity principally being
generated from calcium and bicarbonate ions in
solution, and a pH was typically c. 8.4. Calibration
of conductivity to calcium hardness (established
via titration with EDTA) was undertaken during
a previous experiment (Pedley et al. 2009) and
is shown as Figure 2a. Consequently, it can be
assumed the system ranged within calcium hard-
nesses of 250-350 mg L
21
. The Corallin system
typically yields magnesium hardnesses one order
of magnitude lower than calcium hardness.
Rapid biofilm colonization of the mesocosm was
achieved by seeding the fast flow flume with part of
a living Crataneuton moss colony and associated
biofilms freshly taken from a tufa producing
stream (River Lathkill, N. Derbyshire). The biofilms
contained an abundant diverse range of Oscillitoria-
cea and other cyanobacteria, diatoms and filamen-
tous green algae and heterotrophic bacteria (see
accounts in Pedley 1994). The reader is referred
to Figure 1 in Pedley et al. (2009) for other details
of the mesocosm construction and operational
procedures.
Building on experimental work reported in
Pedley et al. (2009); Rogerson et al. (2008) a
sequence of medium term experiments were set up
to better assess the role of the biofilm in calcium
carbonate precipitation under controlled conditions.
Each experiment commenced after 3-4 weeks, once
extensive biofilm colonization of the flume surface
had been established. During the experimental
period the biofilm community was also encouraged
to colonize 1 cm
2
glass sampling slides which were
inserted behind small transverse barriers within
the flumes (see Pedley et al. 2009 for details).
The glass slides (Fig. 2b) provided a convenient
means of obtaining representative surface colonized
samples of known age which could be air dried and
examined by means of scanning electron micros-
copy (SEM) using a Zeiss SMG EVO 60 SEM
with an Oxford Instruments INCA energy 350
energy dispersive x-ray (EDX) spectrometer probe.
Additional crystalline calcite analyses were carried
out on a Seimens D5000 X-ray Diffractometer
(XRD) using CuK alpha radiation.
Most specimens were either gold or carbon
coated prior to study, or remained uncoated for
backscatter examination. In addition, living biofilm
was examined by stereoscopic optical microscopy.
This enabled many of the 3D interrelationships
within the biofilm to be resolved. Acradine Orange
which stains all DNA luminous green but selec-
tively also stains the DNA of heterotrophs orange.
However, focus at high magnifications made record-
ing details difficult in comparison to SEM photo-
micrography, consequently, the latter is used for
all figures in this account. Some biofilm was also
examined using a wet stage attachment to the
SEM which helped in interpreting EPS distributions
and morphologies prior to desiccation shrinkage.
Water in the system was changed at the end of
each experiment and replaced with fresh spring
water in order to avoid build up of magnesium and
trace elements derived from the calcium reactors
and any toxic metabolic byproducts. The established
biofilms, however, were kept alive and used for
re-seeding the flumes. Nutrients were supplied at 3
week intervals by the addition of 200 ml of dilute
slurry extracted from submerged, decayed leaves
taken from the Welton catchment.
Experimental procedures
The first experiment simulated conditions compar-
able to 'summer' in central England (18 hours of
'daylight' and a water temperature of 18 degrees
maximum during the daylight period). The parallel
flume arrangement permitted high-flow and low-
flow conditions to be run simultaneously. Once set
up the experiment continued for 14 weeks.
In the second experiment flow conditions were
maintained identically to those established for
the summer experiment, however, daylight was
reduced to 6 hours and temperatures were reduced
to 10 degrees. This equates to 'autumn' conditions
in Central England. Once set up the experiment was
left to run for 17 weeks.
Results
Biofilm structure
Once established, the summer biofilm was pale
green in colour tending towards yellow-green.
The colour generally darkened slightly after nutri-
ents were added. Biofilm EPS coated all available
surfaces and was dominated by diatoms and fila-
mentous algae, though cyanobacteria and hetero-
trophic bacteria were also ubiquitous (Fig. 2b).
The filamentous algae showed a tendency to
develop into trailing filament blanket weed blooms
beneath which an additional diatom, cyanobacterial
and heterotrophic community developed. Similar
developments occurred under both fast and slow-
flow conditions.
At the start of the autumn experiment the sump
water was refilled with fresh Welton spring water
Search WWH ::
Custom Search