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derived from the Northern Limestone Alps (e.g.
Rott 1991; H¨gele et al. 2006; Arp 2008).
Indeed, tufa-forming biofilms are one of the few
natural biofilm systems in Europe where processes
involved in biofilm calcification and present-day
stromatolite formation can be studied. Also, tufa
deposits form important climate archives because
of high depositional rates and annual lamination
(e.g. Andrews et al. 2000; Matsuoka et al. 2001;
Garnett et al. 2004; Kano et al. 2004; Andrews &
Brasier 2005; Pentecost 2005).
Tufa deposits in Germany were first recognized
by scientists in the 18th century (Baier 1708:
'tophus'; Sch¨tte 1761: 'Tuff'; Walch 1773: 'Toph-
steine') and were described as porous friable lime
deposits of freshwater streams encrusting moss,
leaves and plant stems. Later, four different pro-
cesses have been suggested as the primary cause
of tufa (i.e. cool water travertine) formation:
(1) Trapping of calcite particles, either derived
from older limestone formations (e.g. von Buch
1809: 24), or precipitated within the water column
(La Touche 1913: 326), (2) physicochemical CO
2
degassing (e.g. Unger 1861: 509; Burger 1911;
Sch¨rmann 1918), (3) photosynthetic CO
2
assimila-
tion by cyanobacteria, mosses and other aquatic
plants (e.g. Cohn 1864: 592; Pokorny 1865: 35),
and
In parallel to the studies mentioned above, there
was an increasing interest in the species compo-
sition of the algal community involved in tufa for-
mation. However, while some descriptions
mention only a limited number of 'key species'
(e.g. Phormidium incrustatum (Naegeli) Gomont
to form major parts of the biofilms (Fritsch 1949;
Arp et al. 2001b), other authors have demonstrated
a much larger biodiversity (Freytet & Verrecchia
1998; Rott 1994; Reichardt 1994; Pentecost &
Whitton 2000). So far, non-phototrophic prokar-
yotes were investigated only rarely (Caudwell
1987; Pentecost & Therry 1988; Szulc & Smyk
1994; Ng et al. 2006).
Biofilm composition may change with seasons,
and corresponding tufa stromatolites (Riding
1991) display a clear lamination. However, the
assignment of laminae to seasons, important for
palaeoclimate reconstructions, differs between
study sites (e.g. Stirn 1964: 13; Golubi´ 1969;
Geurts 1976; Monty 1976; Freytet & Plet 1996;
Janssen et al. 1999; Pentecost & Whitton 2000;
Arp et al. 2001b; Kano et al. 2003), either because
of
differing
ecological
parameters,
or
different
methods of investigation applied.
Consequently, key questions in the study of bio-
films and tufa formation presented here were as
follows:
(1) What is the identity and diversity of the most
abundant phototrophic and non-phototrophic micro-
organisms within the tufa-forming biofilms? Which
non-phototrophic prokaryotes are present and what
kind of metabolic properties can be derived from
phylogenetic assessment?
(2) Is calcification linked to specific clusters of
extracellular polymeric substances (EPS) within
the biofilms? Are these EPS cell-surface associated
or detached from cells and are they related to
specific species?
(3) Does tufa stromatolite formation result from
passive calcite impregnation of biofilm due to high
calcite supersaturation by physicochemical CO
2
degassing, or does cyanobacterial photosynthesis
drive
Ca
2þ
]
(4)
adsoption
[of
to
plant
surfaces
(Kl¨hn 1923: 303).
Subsequently, authors either emphasized the
impact of photosynthesis on the carbonate equili-
brium (e.g. Wallner 1934: 12), or that of inorganic
CO
2
degassing (e.g. Usdowski et al. 1979;
Herman & Lorah 1987; see also von Pia 1933:
18). In fact, conclusions summarized by Gr¨ninger
(1965) do not differ too much from the present-day
view (e.g. Golubi
´
1973; Pentecost 1978, 1993,
2005; Ford 1989; Merz-Preiß & Riding 1999;
Pedley 2000; Arp et al. 2001b; Zhang et al. 2001):
tufa formation is driven by evasion of carbon
dioxide, independent from day - night cycles of
light and temperatures. Aquatic cryptogams (cyano-
bacteria) provide the site of calcite crystal growth
within their mucus (Gr ¨ninger 1965: 92). In addi-
tion, moss plants enhance calcite deposition by baf-
fling of tiny calcite crystals ('fyke nets') from the
water column, and by forming steps in the water
course with increased turbulence and hence CO
2
evasion (Gr¨ninger 1965: 93).
More recent investigations focussed on the
quantification of the relative impact of the different
processes by stable isotope analysis (Spiro & Pente-
cost 1991), the potential effect of exopolymers
on calcite nucleation (Borowitzka 1982; Pentecost
1985) and the possible impact of non-phototrophic
prokaryotes on calcite precipitation (Caudwell
1987; Pentecost & Therry 1988; Szulc & Smyk
1994).
or
assist
calcite
precipitation
within
the
biofilms?
(4) Do non-phototrophic prokaryotes support or
inhibit calcite precipitation within the biofilms by
their
physiological
(heterotrophic)
activity,
e.g.
exopolymer degradation.
(5) Do geochemical signatures (especially d
13
C)
directly reflect macroenvironmental changes only
(climate), or are they modified by microbial activity
within the biofilms?
This paper summarizes published (Table 1) and
unpublished results of tufa biofilm studies carried
out by members of the DFG (German Research
Foundation)
research
unit
571
'Geobiology
of
Organo- and Biofilms'.
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