A multiphasic characterization of the impact of the herbicide acetochlor on freshwater bacterial communities

Abstract

Acetochlor is the third most frequently detected herbicide in natural waters; however, it is unknown if exposure to environmentally relevant concentrations of acetochlor will impact bacterial community structure and function. This study examined the impact of acetochlor on freshwater heterotrophic bacteria number, and community structure and function using direct counting, community level physiological profiling (CLPP) and denaturing gradient gel electrophoresis (DGGE) analysis. Acetochlor concentration did not appear to correlate with the number of total (P ¼ 0.69) and viable (P ¼ 0.80) bacteria, even at concentrations up to 500 lg l À1 . However, CLPP indicated that acetochlor increased functional diversity as shown by (i) an increase in the number of carbon sources utilized by the microbial community, relative to nonexposed controls and (ii) increased functional evenness within the heterotrophic bacterial community. Conversely, DGGE fingerprints suggested that exposure to acetochlor generally decreased the community complexity, as the average number of DGGE bands in most treatments was significantly less than in the control treatment. Cluster analysis of DGGE fingerprints revealed three distinct, dose-dependent clusters (i) communities exposed to 0, 1 and 5 lg l À1 ; (ii) 50 and 100 lg l À1 and (iii) 500 lg l À1, indicating a relationship between acetochlor concentration bacterial community changes. This study indicated that while exposure to environmentally relevant concentrations of acetochlor resulted in no significant impact to the number of freshwater bacteria, impacts to the function and structure of the community were revealed by adopting a multiphasic approach.

Herbicide Contamination of Freshwater Ecosystems: Impact on Microbial Communities

“…In addition to photosynthetic microorganisms, which are directly impacted by herbicides because they share some physiological metabolisms with terrestrial plants, these compounds can also have direct impacts on bacteria. For example, in a study of Foley et al (2008), DGGE fingerprinting showed that freshwater bacterial communities exposed to acetochlor, an inhibitor of cell growth and division, displayed reduced diversity. However, contradictory results have been obtained, and, for example Tadonleke et al (2009) did not find any significant impact of diuron, a phenylurea herbicide, on bacteria, whereas previous studies had seemed to do so.…”

Comparative responses of river biofilms at the community level to common organic solvent and herbicide exposure

“…Spawn et al (1997) stated that alachlor exposure for 21 days led to a reduction of the algal biomass as measured by chlorophyll a up to 10 μg L −1 and changes in algal community structure and density up to 30 μg L −1 . Foley et al (2008) underlined that acetochlor exposure (96 h), which has similar toxicological mechanisms to alachlor, did not result in significant impact on bacterial community structure analyzed by DGGE at environmental concentrations (1 and 5 μg L −1 ).…”

“…The number of carbon sources utilized by the bacteria communities from untreated and treated biofilms was calculated as the sum of the number of wells exhibiting absorbance greater than 0.25 OD (Foley et al 2008) and was run using an analysis of similarity (ANOSIM) on Bray-Curtis similarity matrices generated from the absorbance data with PRIMERr v6 software (Clarke 1993).…”

“…The majority of studies on chloroacetanilide impacts focused on acetolachlor and metolachlor at the scale of bacterial, diatom, and green algal communities (e.g., Foley et al 2008;Vallotton et al 2008;Roubeix et al 2010Roubeix et al , 2011Roubeix et al , 2012Thakkar et al 2013;Coquillé et al 2015). The literature mainly provides EC 50 values from single-species acute toxicity testing of alachlor on green algae (Fairchild et al 1997), cyanobacteria (Singh and Datta 2005), and bacteria and protozoa (Bonnet et al 2007).…”

“…Unlike herbicides which inhibit photosynthesis (e.g., triazines or phenylureases) (e.g., Dorigo et al 2004;Villeneuve et al 2011a), the most commonly studied chloroacetanilide herbicides can also have direct effects on bacteria (Foley et al 2008). The majority of studies on chloroacetanilide impacts focused on acetolachlor and metolachlor at the scale of bacterial, diatom, and green algal communities (e.g., Foley et al 2008;Vallotton et al 2008;Roubeix et al 2010Roubeix et al , 2011Roubeix et al , 2012Thakkar et al 2013;Coquillé et al 2015).…”