Tufa-forming biofilms of German karstwater streams: microorganisms, exopolymers, hydrochemistry and calcification - Tufas and Speleothems: Unravelling the Microbial and Physical Controls - page 104

500 mm of the tufa stromatolites, thereby forming a
phototrophic biofilm (Fig. 13a, b). These photo-
trophic microorganisms are associated with numer-
ous non-phototrophic bacteria (Fig. 13c), which
occur throughout the biofilm as well as below
into several mm of depth. Living cyanobacteria
have been observed, scattered in carbonate tubes,
in 1.8 mm depth below the biofilm surface
(Fig. 13d). Composition and calcification pattern
in

study sites. Internally calcified empty filaments,
which locally occur in porous (winter - spring)
laminae below the living phototrophic biofilm
(e.g. Arp et al. 2001b), may in turn reflect calcite
precipitation in empty cells still surrounded by an
inhibiting exopolymer matrix.

Annual lamination

the investigated tufa stromatolite-forming

Seasonal changes in the biofilm community go
along with the formation of annual lamination in
the tufa stromatolites: A single year of deposition,
represented by a porous and a dense lamina, varies
between 1.6 - 5.4 mm thickness at the lower
section of the Westerh ¨fer Bach (site WB5) and
3.9 - 7.6 mm thickness at the lower section of the
Deinschwanger northern side stream (site DB2). In
both streams, the porous winter-spring layers,
which include organic and quartz detritus, are less
thick (WB5: 1.0 - 1.5 mm; DB2: 1.3 - 3.6 mm) than
the dense summer - autumn layers (WB5: 0.5 -
2.0 mm; DB2: 1.4 - 4.0 mm). In fact, the transition
from porous winter-spring to dense summer -
autumn laminae is gradual, while the bases of the
porous

biofilm change seasonally:
Winter-spring biofilms (approximately
November - May) are brownish to pale green, with
abundant diatoms upon a summer-autumn lamina
with scattered cyanobacteria. The diatom colonies
remain free of calcite, but commonly show attached
detrital quartz and organic debris. During spring
times, the diatoms are successively replaced
by moderately dense arranged erect Phormidium
incrustatum filaments, which locally form bush-like
arrays. These filamentous cyanobacteria are
enclosed within dominantly microsparitic tubes,
with the diatoms remaining within pore spaces of
porous microspar layers.
Summer-autumn biofilms (approximately
June - October) are finally intensively green, with
erect P. incrustatum within microcrystalline tubes.
These are locally overgrown by a dense microcrys-
talline layer of closely arranged erect Leptolyngbya
aff. foveolarum filaments.
Limestone cobbles at the basis of tufa stromato-
lites, as well as growth interruptions within the tufa
stromatolites, commonly show endolithic cyano-
bacteria of the Hyella fontana morphotype, which
also occurs scattered throughout the biofilm. Non-
phototrophic prokaryotes occur thoughout biofilm
as well as below, i.e. within subfossil tufa stro-
matolite layers. While a vertical distribution gradi-
ent of major groups is evident from mm-scale
sampling of single lamina-pairs (see chapter Non-
phototrophic Prokaryotes), detailed microscale dis-
tribution pattern of phylogenetic groups and single
phylotypes within the phototrophic biofilm (top
500 mm) remains to be investigated. First investi-
gations using DAPI staining and CARD-FISH
using general probes and tape-stabilized cryosec-
tions (Shiraishi et al. 2008c) demonstrate the pres-
ence of coccoid, rod-shaped (some in chain-like
arrangement) and filamentous Bacteria (Fig. 13c).
Their function, e.g. in exopolymer degradation,
however remains to be elucidated.
Tufa stromatolite surfaces free of biofilms, e.g.
due to freezing and chipping off of top laminae in
winter, are characterized by further, inorganic
crystal growth. This aggrading neomorphism at
the absence of biofilms and (less intensive) within
deeper, porous tufa crust parts is the likely cause
of

winter-spring laminae are mechanically

instable, i.e. commonly break during sampling

(Fig. 13a). In addition, a minute sub-layering

within the annual cycles is evident.
This seasonal pattern observed in Deinschwan-
ger and Westerh¨fer Bach tufa stromatolites
coincides with the stable isotope composition
of the carbonate (Fig. 14): Porous winter-spring
laminae show less negative d 18 O values than
dense summer-autumn laminae. Based on that, cal-
culated palaeo-water temperatures for the Wester-
h ¨fer Bach are approximately 7 to 9 8C for porous,
and 9 to 10.5 8C for dense laminae (Shiraishi et al.
2008b). Seasonal changes in d 13 C of tufa stromato-
lite calcite, however, reflect a number of different
factors, with lower values in warmer seasons due
to stronger supply of 12 C from respiration in soils
of the catchment area (see e.g. Hori et al. 2008).

Discussion: implications for the fossil record
Seasonal lamination and Quaternary
palaeoclimate signals

Results of investigating calcifying biofilms and tufa
stromatolites in karstwater streams are of interest for
the interpretation of palaeoclimate signals in subfos-
sil and Quaternary tufa deposits (e.g. Andrews &
Brasier 2005). The interpretation of palaeoclimate
signals recorded in tufa stromatolites indeed relies
on the correct assignment of laminae to specific
seasons. However, seasonal lamination pattern
described in some tufa stromatolite studies (e.g. palisade crystal laminae in the investigated