inverse correlations between exposure concentrations and BCFs along a gradient of
contamination concentrations in the Seine River. This divergence could also be the
consequence of a saturation of cellular binding sites at high exposure concentrations,
with a possible influence of competition between contaminants in mixtures. Indeed,
competitive sorption is likely to occur in the environment due to the co-occurrence
of multiple contaminants in surface waters. For instance, the accumulation rate of
the organosulfur fungicide isoprothiolane in two microalgae (Scenedesmus
quadricauda, Aulacoseira granulata) and one cyanobacterium (Microcystis
aeruginosa) decreased in presence of other pesticides (the herbicide p-nitrophenyl
2,4,6-trichlorophenyl ether and the insecticide O,O-dimethyl O-(3-methyl-4nitrophenyl) phosphorothioate) in the mixture (Guanzon et al. 1996). This phenomenon is more likely to occur in laboratory experiments in which biofilms are usually
exposed to higher concentrations than those found in the environment. Indeed, in our
dataset BCF calculated from laboratory experiments were generally lower than those
from field experiment (Figs. 5, 6 and S1).
The influence of environmental factors on metal accumulation in periphytic
biofilm has been reviewed by Guasch et al. (2010). In particular, metal speciation
is influenced by a range of physicochemical factors (including pH, salinity, and
nutrients), affecting their bioavailability (Meylan et al. 2003) and subsequent accumulation and toxicity for microbes. Biofilm characteristics (community composition, biomass, organic matter content, EPS content) can also influence the
bioavailability and therefore the accumulation and toxicity of contaminants
(Berglund 2003; Berglund et al. 2005; Lambert et al. 2016; Pesce et al. 2018). In
river biofilms, the sorption of certain contaminants such as triazines or metals has
been attributed to specific bacterial colonies producing an EPS matrix with a unique
composition (Lünsdorf et al. 1997; Lawrence et al. 2001). A change in community
composition can modify lipid content and therefore influence the accumulation of
organic compounds such as PCBs according to their high log K ow value (Wang et al.
1999). Finally, toluene accumulation in a bacterial biofilm has been shown to
increase negatively charged carboxyl groups in EPS and might thus enhance biofilm
ability to accumulate cations such as metal ions (Schmitt et al. 1995).
Through their capacity to uptake contaminants from the surface water, periphytic
biofilms can also be viewed as passive samplers of contaminants in surface waters.
This has prompted the idea that identifying and quantifying the contaminants
accumulated within the biofilm could be a monitoring strategy for surveillance of
aquatic ecosystem contamination by both organic contaminants (e.g., PAHs;
Froehner et al. 2012) and metals (Leguay et al. 2016). However, following equilibrium partitioning theory, contaminants accumulated in biofilms are also likely to
diffuse back in the water when their dissolved concentration has dropped. Sorption
and desorption kinetics have been studied for some contaminants such as PCBs and
PAHs (Bertini 2016) and for a handful of pesticides (Headley et al. 1998) and
antibiotics (Wunder et al. 2011). Therefore, due to the dynamic processes involved
in contaminant accumulation in biofilms and the potential influence of many abiotic
and biological parameters, the use of bioaccumulation in biofilms as an indicator
of contamination has been challenged, in particular for organics (Bertini 2016).
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C. Bonnineau et al.
contamination concentrations in the Seine River. This divergence could also be the
consequence of a saturation of cellular binding sites at high exposure concentrations,
with a possible influence of competition between contaminants in mixtures. Indeed,
competitive sorption is likely to occur in the environment due to the co-occurrence
of multiple contaminants in surface waters. For instance, the accumulation rate of
the organosulfur fungicide isoprothiolane in two microalgae (Scenedesmus
quadricauda, Aulacoseira granulata) and one cyanobacterium (Microcystis
aeruginosa) decreased in presence of other pesticides (the herbicide p-nitrophenyl
2,4,6-trichlorophenyl ether and the insecticide O,O-dimethyl O-(3-methyl-4nitrophenyl) phosphorothioate) in the mixture (Guanzon et al. 1996). This phenomenon is more likely to occur in laboratory experiments in which biofilms are usually
exposed to higher concentrations than those found in the environment. Indeed, in our
dataset BCF calculated from laboratory experiments were generally lower than those
from field experiment (Figs. 5, 6 and S1).
The influence of environmental factors on metal accumulation in periphytic
biofilm has been reviewed by Guasch et al. (2010). In particular, metal speciation
is influenced by a range of physicochemical factors (including pH, salinity, and
nutrients), affecting their bioavailability (Meylan et al. 2003) and subsequent accumulation and toxicity for microbes. Biofilm characteristics (community composition, biomass, organic matter content, EPS content) can also influence the
bioavailability and therefore the accumulation and toxicity of contaminants
(Berglund 2003; Berglund et al. 2005; Lambert et al. 2016; Pesce et al. 2018). In
river biofilms, the sorption of certain contaminants such as triazines or metals has
been attributed to specific bacterial colonies producing an EPS matrix with a unique
composition (Lünsdorf et al. 1997; Lawrence et al. 2001). A change in community
composition can modify lipid content and therefore influence the accumulation of
organic compounds such as PCBs according to their high log K ow value (Wang et al.
1999). Finally, toluene accumulation in a bacterial biofilm has been shown to
increase negatively charged carboxyl groups in EPS and might thus enhance biofilm
ability to accumulate cations such as metal ions (Schmitt et al. 1995).
Through their capacity to uptake contaminants from the surface water, periphytic
biofilms can also be viewed as passive samplers of contaminants in surface waters.
This has prompted the idea that identifying and quantifying the contaminants
accumulated within the biofilm could be a monitoring strategy for surveillance of
aquatic ecosystem contamination by both organic contaminants (e.g., PAHs;
Froehner et al. 2012) and metals (Leguay et al. 2016). However, following equilibrium partitioning theory, contaminants accumulated in biofilms are also likely to
diffuse back in the water when their dissolved concentration has dropped. Sorption
and desorption kinetics have been studied for some contaminants such as PCBs and
PAHs (Bertini 2016) and for a handful of pesticides (Headley et al. 1998) and
antibiotics (Wunder et al. 2011). Therefore, due to the dynamic processes involved
in contaminant accumulation in biofilms and the potential influence of many abiotic
and biological parameters, the use of bioaccumulation in biofilms as an indicator
of contamination has been challenged, in particular for organics (Bertini 2016).
132
C. Bonnineau et al.