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 63

Microbial influence on macroenvironment chemical conditions in
alkaline (tufa) streams: perspectives from in vitro experiments

M. ROGERSON*, H. M. PEDLEY & R. MIDDLETON
Department of Geography, University of Hull, Cottingham Road, Hull, HU6 7RX, UK
*Corresponding author (e-mail: [e-mail])

Abstract: Tufas represent a palaeoclimatic archive of potentially global significance. However,
uncertainty remains over the exact process of calcite precipitation from these systems, inhibiting
our ability to decipher the precise meaning of geochemical records. For example, field studies
of alkaline stream systems are unable to disentangle the influence of temperature and photosyn-
thesis on ambient hydrochemistry on diurnal and annual timescales. This report describes a
series of flume experiments in which temperature and light conditions are manipulated separately.
These experiments reveal that precipitation of calcite occurs preferentially under conditions of
rising pH, and consequently at the night-day transition. The amplitude of diurnal changes is regu-
lated 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 availability making tempera-
ture impacts minor. Consequently, macroenvironment pH during both day and night-time tend to
be higher under lower temperatures, in contrast to expectation. These observations may have
potential implications for the isotopic geochemistry of tufa carbonate, promoting slightly lower
d 18 O, due to the carbonate ion effect, and more significantly negative d 13 C, due to incorporation
of respired CO 2 accumulated during the night. The observation that long periods of daylight are
not necessarily needed for photosynthetically induced precipitation to occur confirm previous
arguments 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 intensity.

The freshwater systems fromwhich tufa precipitates
are subject to a wide range of physical and biologi-
cal influences that alter the state of ambient (macro-
environment) water chemistry (Pentecost 1992;
Drysdale et al. 1997, 2003; Kawai et al. 2006;
Shiraishi et al. 2008a, b). In the field, these influ-
ences can be difficult to disentangle, leading to
ambiguity in our understanding of whether physico-
chemical or biological processes are controlling
temporal or spatial variability (Kawai et al. 2006).
In this regard, several investigations in which single
sites have been monitored over diurnal cycles are
of particular importance (Spiro & Pentecost 1991;
Bayari & Kurttas 1995; Drysdale et al. 2003; Liu
et al. 2008; Takashima & Kano 2008), and these
studies collectively highlight the role played by the
relative influences of photosynthesis and respira-
tion. Despite some agreement over the importance
of photosynthesis in these studies, the major photo-
trophic agent seems to be regionally variable with
cyanobacteria dominant in the UK (Spiro & Pente-
cost 1991), algae in Turkey (Bayari & Kurttas 1995)
and a combination of micro- and macrophytes in
China (Liu et al. 2008) identified as dominant. It
should also be noted that biological control of
diurnal chemistry changes is not universally agreed

upon, as studies of hydrochemistry in an Australian
creek are argued to reflect air temperature as the
major influence on calcite supersaturation (Drysdale
et al. 2003). However, as light intensity will closely
correlate with water temperature on diurnal time-
scales, the impact of biotic and abiotic influences in
isolation remain poorly constrained (Liu et al. 2008).
As with diurnal hydrochemistry changes, annual
lamination in tufa stromatolites have frequently
been linked to biological influences via diurnal or
seasonal cycles in photosynthesis (Irion & M¨ ller
1968; Hevesi 1970; Pentecost 1987), biofilm
ecology (Arp et al. 2001) or both (Janssen et al.
1999). Though a significant literature has developed
concerning laminated tufas, which present a very
exciting potential archive of sub-annual climate
(Andrews & Brasier 2005), few studies have been
performed in which tufa systems have been moni-
tored over several years, so the forcing mechanisms
behind lamina formation remain largely the subject
of conjecture. One study (Kano et al. 2003) revealed
that at sites proximal to carbonate springs, hydro-
chemical processes within the aquifer provide the
dominant control on precipitation rate and suggests
little biotic influence. However, this gives little
insight into more distal locations where biological