Effects of Organic Herbicides on Phototrophic Microbial Communities in Freshwater Ecosystems - Reviews of Environmental Contamination and Toxicology - page 114

4.1 Improving Exposure Assessments

4.1.1 The Question of Mixtures

Despite the fact that pesticides frequently occur in mixtures, our review clearly
shows a dearth of studies that evaluate pesticide mixture effects on autotrophic
microbial communities. Even fewer mixture studies have been performed under
environmentally relevant conditions. This point has recently been emphasized by
Van den Brink et al. ( 2009 ), who noted the scarcity of data on community-level
effects of pesticide mixtures. Several authors have emphasized the need to consider
mixtures, when assessing the ecological effects of pesticides (Chèvre et al. 2006 ;
Knauer et al. 2010 ). However, there is still debate over the best way to address this
issue (Knauert et al. 2008, 2009 ), and it can be argued that the assessment of mix-
ture effects is in its infancy (Belden et al. 2007 ). DeLorenzo et al. ( 2001 ) empha-
sized the need to address the toxicity of pesticide degradation products to aquatic
microorganisms. The integration into risk assessments of the effects shown by pes-
ticide metabolites, both alone and in pesticide mixtures, would signiicantly improve
ecological risk assessment processes (Sinclair and Boxall 2003 ). Unfortunately,
there is still too little research of this type being performed. Another progressive
step will be to take into account the mode of action of pesticides, since mechanistic
insights may help explain the toxicity of mixtures (additivity or independence of
action; Chèvre et al. 2006 ; Ricart 2011 ).

4.1.2 Beneiting from the Development of Chemical Tools

Another area in which progress is needed is development of better chemical
sensing and ield sampling devices. The recent development of passive sampling
techniques for monitoring organic pesticides in freshwaters (e.g., polar organic
chemical integrative samplers, diffusive gradients in thin ilms, semipermeable
membrane devices, silicon rods, etc.) opens new avenues to screen for a large
variety of organic and inorganic contaminants. Such improvements would facilitate
the assessment of the relationship between community-level tolerance induction
and mean contaminant exposure (Pesce et al. 2010b ; Rotter et al. 2011 ). Moreover,
recently, some authors have proposed combining passive samplers with bioas-
says to assess the toxicity of toxicant mixtures extracted directly from the envi-
ronment. This combination method may constitute a simple and cost-effective
way to determine potential acute effects of contaminant mixtures in various
aquatic environments (e.g., Muller et al. 2007 ; Liscio et al. 2009 ; Shaw et al.
2009 ). Recently, polar organic samplers have been combined with photosynthe-
sis bioassays (using microalgae cultures) to assess phytotoxicity of various mixtures
of organic toxicants (Escher et al. 2006 ; Muller et al. 2007 ; Shaw et al. 2009 ).
The use of natural phototrophic microbial communities in such an approach
could improve outcomes and usefulness of previous results, which combined