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and re-seeded with biofilm coated moss. Once
established the resulting autumn biofilm was much
darker green in colour and contained few filamen-
tous green algae. A significantly thicker amount of
viscous EPS in the biofilm (up to 6 mm thick,
Fig. 2c, d) developed in association with filamentous
cyanobacteria colonization. Irregular, centimetre-
scale hemispherical undulations developed on the
exposed EPS surface on both fast-flow and slow-
flow biofilms (Fig. 2c).
The detailed interrelationships between the
microbes and the EPS are difficult to demonstrate
in the thin summer biofilm and this was exacerbated
by shrinkage during dehydration, in preparation for
SEM work. More detail was seen when wet samples
were viewed using stereoscopic optical microscopy.
In the summer biofilm the EPS consisted of a
,100 mm thick sheet attached to the substrate
from which filamentous algae extended. In contrast,
the autumn biofilm was up to 6 mm thick. Algae
were relatively infrequent in the autumn biofilm
whereas cyanobacteria were dominant, diatoms
were common and coccoid and short-rod varieties of
heterotrophic bacteria were widespread especially
within and in close proximity to the associated
calcite precipitates. This microbial association con-
forms most closely to the 'Group 1 aquatic commu-
nity of (Freytet & Verrecchia 1998). Importantly,
the autumn EPS (especially when living samples
were viewed normal to substrate under light
microscopy) had an internal structure of closely
aligned cyanobacterial filaments (mainly Phormi-
dium but also with abundant Nostoc) all arranged
normal to substrate which were anchored on to the
flume base. Many filaments extended beyond the
outer EPS surface into the overlying water. In
addition, the outermost few hundred mm of the
EPS also contained a surficial zone of coiled cyano-
bacterial filaments giving a 'woolly' appearance.
When the autumn biofilm was viewed parallel
to substrate the microbial community was also
seen to be partitioned within the EPS into clumps
of oscillitoriacean filaments associated with rela-
tively few coccoid heterotrophs. These clumps
were separated from each other by narrow, vertical
zones arranged into polygons which were frequently
dominated by larger filamentous cyanobacteria cf.
Nostoc (best seen under light microscopy but well
shown in Fig. 5b). Small (200-500 nm) coccoid
and short rod bacteria (see Fig. 5b) were also
abundant in the polygonal zones and were identified,
using Acridine Orange stain, predominantly as
heterotrophs.
Significantly, during both summer and autumn
experiments a sparse, aphotic biofilm community
also developed on the submerged walls of the unlit
water sump and on the pump. This community
developed in total darkness and did not contain
phototrophs. Rather, it appeared as a sparsely dis-
tributed heterotroph community associated with
very little visible EPS but abundant calcite
precipitates.
Calcite precipitates within the photic biofilm
Considerable quantities of calcite were precipitated
within the biofilm during both summer and autumn
experiments. These precipitates, exclusively devel-
oped within the EPS, add further support to the
conclusions of Ercole et al. (2007); Pedley et al.
(2009); Shiraishi et al. (2008) that EPS plays a sig-
nificant role calcite precipitation in association with
microbial metabolic activities. Five XRD analyses
showed that the experimental precipitates were all
composed of pure calcite. There was no evidence
either of aragonite or vaterite being present even
when analyses were carried out within 5 hours of
sampling the living biofilm. Unlike the poikilotopic
lamellar calcite crystals associated with obligate cal-
cifying Rivulariacea these precipitates were com-
posed of 200-500 nm diameter, apparently solid,
calcite nanospherulites that are morphologically
similar to vaterite spherules in experiments by
Braissant et al. (2003) and Nehrke & Van Cappellen
(2005). These were arranged into packed groupings
to form micropeloids and small imperfectly-faced
(anhedral) microspar crystals (Fig. 3a). Typically,
all precipitates grew with extensive perforations
(Fig. 3b-d) which were occupied during their devel-
opment by EPS strands and frequently (Fig. 3d)
by small coccoid (possibly heterotrophic) bacteria
as previously illustrated and reported in Pedley
et al. (2008, see fig. 10b), cf. figure 2b of Freytet &
Verrecchia (1998).
Significantly, these micropeloid precursors and
subsequent microspar were not randomly distribu-
ted within the biofilm EPS. Initially, they developed
within the basal part of the biofilm forming a basal
calcite layer (Fig. 4a) which was associated with
heterotrophic bacteria, cyanobacteria and algae.
Fig. 2. (Continued) filamentous algae downstream of the barrage. Pale coloured areas are calcite. (c) Typical
EPS-dominated biofilm developed under autumn slow-flow conditions after 17 weeks. These biofilms show a tendency
to develop a mammilated upper surface as a result of partial detachment of the EPS from the substrate. (d) EPS-
dominated biofilm developed under autumn fast-flow flume conditions after 17 weeks. The biofilm is partly detached
from the substrate and has become torn under the fast-flow conditions revealing the basal calcite layer (1). The paler
biofilm colour compared to (b) is caused by the greater quantities of calcite precipitates within the fast-flow EPS. N.B.
both flumes were drained before photographing.
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