Geology Reference
In-Depth Information
nocturnal pH within the EPS. However, at the start
of each day the biofilm switches to a dominantly
phototrophic metabolism and much of this CO 2(aq)
is consumed, raising pH and driving a simultaneous
conversion of HCO 3 2 (aq) to CO 2 (aq) . The result of this
is an adjustment of the chemical system towards
precipitation, effectively 'mopping up' additional
heterotroph-derived carbon and also the Ca 2รพ ions,
liberated from chelation within the EPS by hetero-
troph digestion (see Chekroun et al. 2004; Dupraz
& Visscher 2005 for
further consideration of
precipitation processes).
The mesocosm experiments show that fresh-
water biofilms have many similarities with the
marine stromatolite model of Reid et al. (2000)
and Decho et al. (2005). In both scenarios, in
addition to defending the community from physical
attack, EPS provides a medium within which ion
transportation takes place allowing movement of
excess dissolved solids away from individuals to
stabilized 'waste' sites. In the marine systems, the
'waste disposal' occurs at the base of the photo-
trophic layer, and is enhanced by the presence of
anaerobes. Ions are therefore mainly transported
vertically, and the biofilm system as a whole
shows significant vertical and little horizontal varia-
bility. However, in freshwater communities the
vertically arranged septae caused calcite to form a
mechanically rigid scaffold (Fig. 12) with 'walls'
enclosing polygonal spaces above a carbonate
'floor' developed in close analogy to the marine
system. To take the analogy further, the occurrence
of multiple layers of EPS implies that some 'walls'
have 'roofs' which are also the 'floor' for the next
layer. Though the spatial components of elements
of the microbial consortium are not fully resolved,
it is likely that 'rooms' are dominantly occupied
by the phototrophs which generate much of the
EPS, with boundaries between 'rooms' being occu-
pied by aerobic and anaerobic heterotroph commu-
nities. Consequently, disposal of EPS waste occurs
throughout the biofilm system in freshwater settings
providing both vertical and horizontal transpor-
tation of ions and, therefore, both vertical and hori-
zontal variability in structure.
Fig. 12. Block diagram of the freshwater (tufa) biofilm
constructed from optical binocular microscope images.
Note the basal calcite layer (black layer at the figure
base) initially grows within the EPS and does not always
attach to the substrate. Extending vertically away from
the basal calcite layer are narrow zones (septae)
containing suspended calcite microspar crystals which
decrease in size away from the basal calcite layer. At
their base these septae extend laterally into a less densely
packed crystal layer lying close above the basal calcite
layer (unconnected black shapes above the basal calcite
layer). When viewed from above the septal zones appear
as polygonal zones within the transparent EPS. Inset
lower right side: Calcite crystals growing anywhere
within the biofilm EPS all grow perfect rhombic
terminations and crystal c-axes appear dominantly
orientated parallel to the septae (i.e. normal to underlying
substrate) despite their lack of direct attachment with a
solid substrate.
consideration. Classic considerations of physical
degassing and photosynthesis driven enhancement
of pH as a precursor to precipitation are probably
not sufficient to explain precipitation, as the sump
did not have the same potential to degass as the
flume does and was unlit. Even the slight diurnal
changes imposed on the entire system by photosyn-
thesis external to the sump (Rogerson et al. 2010) is
likely to be exceeded by respiration in the sump
water. Carbonate buffering of certain anaerobic
metabolisms (Visscher & Stolz 2005) may provide
a partial explanation for the sump precipitates,
with the EPS providing the required nucleation
sites. However, it should be noted that nutrition
for sump organisms can only be provided by the
constant supply of degraded EPS in suspension
derived from the photic parts of the flumes. This is
an important characteristic to note as digestion of
Significance of sump grown calcite
The original heterotroph colonizers within the
sump undoubtedly were derived by biofilm erosion
(Rittmann 1989) and re-adhesion (Stolzenbach
1989; Turner & Jones 2005), especially in flume 2
where the high flow velocity created fluid shear
at the biofilm-water interface (Characklis 1990).
Precipitation in the absence of light has conse-
quences for both tufa and speleothem systems, in
addition to precipitation in lakes and soil. Conse-
quently, this precipitate deserves some additional
Search WWH ::




Custom Search