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of the biofilm morphology, the remaining cells, though less
numerous, could still perform photosynthesis to a certain ex-
tent. In this way, chl
a values would be more affected than
photon yield, which is independent of the algal biomass. Ef-
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at concentrations higher than 5
The effect of Cu on the photosynthesis of the Meuse bio-
films manifested as a linear decrease of in time, showing
M Cu.
II
a steady progress of the effect of Cu on photosynthesis. This
could be explained if the protective feature of the biofilms
matrix lost efficiency against metal contamination during the
long-term exposure. The loss of a part of the biofilm biomass
would also imply a loss of binding sites to immobilize Cu.
This would make the biofilm more and more sensitive to Cu,
as can be seen by the decrease of apparent EC50 values with
exposure time from 18 to 5
week exposure.
M Cu from the first- to the second-
A progressive decrease of photosynthetic efficiency with
time, implying a decrease of the biofilm capacity to lower Cu
toxicity, can also be seen when comparing the present results
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M Cu decreased photosynthetic activity
only by 20%. We then concluded that structure and density
rather than specific composition contributed to Cu resilience
in biofilms. However, in the present work, we see how the
biofilm structural change and Cu penetration into the biofilm
are time dependent, resulting in a resilience of the consortium
decreasing with time.
Our results lead to the conclusion that the River Meuse
biofilms acted, independently of their species composition, as
a dynamic unit under Cu exposure with effective feedback
mechanisms. Changes in physiognomy and biomass degra-
dation occurred as a result of the attack of the biofilm super-
ficial layer of cells, followed eventually by a decrease in pho-
tosynthesis. The doses of Cu needed to operate such changes
were very close to those to which the biofilm could success-
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Acknowledgement—This work is part of the European Project Mi-
crobenthic Communities in European Rivers Used to Assess Effects
of Land-Derived Toxicants, Contract PL 95 01 07 and BIOLFILMS,
Contract EVK1-CT-1999-00001. We thank Ronald Gylstra for his help
in the statistical analysis.
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