Microbial influence on macroenvironment chemical conditions in alkaline (tufa) streams: perspectives from in vitro experiments - Tufas and Speleothems: Unravelling the Microbial and Physical Controls - page 77

data, but we argue that the two different datasets can
be unified by the concept that photosynthetically
induced precipitation dominantly occurs in both set-
tings during periods of rising pH independent of its
absolute value. The period of maximum precipi-
tation (i.e. ion flux out of the Ambient Water inven-
tory, Fig. 10) reflected in a period of minimum
conductivity, is succeeded by a period of return to
background conditions, reflected by slowly rising
conductivity in the Ambient Water inventory. The
length of this recovery period will be dependent
on the buffering capacity of the system (i.e. the
capacity for microbiologically-induced fluxes to
significantly alter the state of the Ambient Water
inventory) and the variability of the light source
(i.e. occurrence of period of increasing photosyn-
thetic activity), both of which tend to lead to a
longer recovery period for the Chinese system.
A further consequence of precipitation under
rising pH conditions is that the pH of water actively
precipitating calcite will be biased towards values
lower than at midday. This may be significant for
studies of d 18 O calcite , as fractionation of oxygen iso-
topes between HCO 3(aq)

2008b). It seems that biofilms are more effective
at modifying the streams they live within than the
streams are at modifying them. In combination,
these experiments therefore imply that externally
forced changes in photosynthesis are not a likely
explanation for reduced winter precipitation. For
biological effects to be a significant factor in devel-
opment of seasonal laminae, a change in the
microbial assemblage may be necessary. It has pre-
viously been reported that the ecological compo-
sition of tufa biofilm is seasonally variable (Arp
et al. 2001), and recent evidence for microbial par-
titioning of encrusting/non encrusting surfaces
within a single river system indicates that this may
prove to be a critical factor (Ledger et al. 2008) in
determining the occurrence of laminae.

Conclusions

Physical modelling of photosynthetic effects on tufa
stream water reveals that precipitation of calcite
occurs preferentially under conditions of rising pH,
and consequently at the night-day transition. The
amplitude of diurnal changes are regulated by the
buffering capacity (i.e. alkalinity) of the ambient
water and by the daytime balance of photosynthesis
and respiration. Respiration is shown to be strongly
affected by temperature, whereas photosynthesis is
found to be limited by nutrient and/or DIC avail-
ability making temperature impacts minor. Conse-
quently, macroenvironment pH during both day
and night-time tend to be higher under lower
temperatures, in contrast to expectation.
These observations have potential implications
for the isotopic geochemistry of tufa carbonate, pro-
moting slightly lower d 18 O, due to the carbonate ion
effect (Zeebe 1999), and more significantly negative
d 13 C, due to incorporation of respired CO 2 accumu-
lated during the night. The observation that long
periods of daylight are not necessarily needed for
photosynthetically induced precipitation to occur
confirm that seasonal lamination requires either
strong variability in ambient physicochemical
activity or an ecological change in the microbial
assemblage, and cannot be ascribed to reduced
temperature and light

22 means that
the equilibrium position for d 18 O calcite is expected
to change approximately in proportion with the
change in pH (Zeebe 1999). Consequently, incor-
poration of significant photosynthetically induced
precipitate will tend to cause a reduction in tufa
d 18 O calcite . The apparent reliability of the tufa
d 18 O calcite archive (Andrews et al. 1997) may there-
fore require some reinvestigation using interstitial
water from the biofilm rather than ambient water
from the river. Impact on tufa d 13 C calcite would be
expected to be even more significant, as much of
the carbon incorporated is likely to be that liberated
by respiration in the preceding night period. As
with d 18 O, this will tend to cause photosynthe-
tically induced precipitate to have relatively low
d 13 C values.

2

and CO 3(aq)

Implications for seasonal lamination

On initial inspection, the extended period of low pH
in the colonized systems under short daylight
periods is consistent with the expectation of
impeded precipitation during the winter (Arp et al.
2001). However, this apparent agreement is under-
mined if precipitation occurs during the parabolic
part of the pH curve and by the fact that the
maximum pH reached in the diurnal cycle is actu-
ally higher under low temperature conditions.
These results indicate that changing conditions
within the microenvironment of the biofilm are
capable of overcoming physicochemical changes
in the macroenvironment of the entire flume
system, and thus build on previous work indicating
that the reverse does not happen (Bissett et al.

intensity (Bissett et al.

2008b).

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