The Use of Attached Microbial Communities to Assess Ecological Risks of Pollutants in River Ecosystems: The Role of Heterotrophs

Twenty Years of Research in Ecosystem Functions in Aquatic Microbial Ecotoxicology

One of the major threats to freshwater biodiversity is water pollution including excessive loads of nutrients, pesticides, industrial chemicals, and/or emerging contaminants. The widespread use of organic pesticides for agricultural and nonagricultural (industry, gardening, etc.) purposes has resulted in the presence of their residues in various environments, including surface waters. However, the contribution of pesticides to the deterioration of freshwater ecosystems (i.e., biodiversity decline and ecosystem functions impairment) remains uncertain. Once in the aquatic environment, pesticides and their metabolites can interact with microbial communities, causing undesirable effects. The existing legislation on ecological quality assessment of water bodies in Europe is based on water chemical quality and biological indicator species (Water Framework Directive, Pesticides Directive), while biological functions are not yet included in monitoring programs. In the present literature review, we analyze 20 years (2000-2020) of research on ecological functions provided by microorganisms in aquatic ecosystems. We describe the set of ecosystem functions investigated in these studies and the range of endpoints used to establish causal relationships between pesticide exposure and microbial responses. We focus on studies addressing the effects of pesticides at environmentally realistic concentrations and at the microbial community level to inform the ecological relevance of the ecotoxicological assessment. Our literature review highlights that most studies were performed using benthic freshwater organisms and that autotrophic and heterotrophic communities are most often studied separately, usually testing the pesticides that target the main microbial component (i.e., herbicides for autotrophs and fungicides for heterotrophs). Overall, most studies demonstrate deleterious impacts on the functions studied, but our review points to the following shortcomings: (1) the nonsystematic analysis of microbial functions supporting aquatic ecosystems functioning, (2) the study of ecosystem functions (i.e., nutrient cycling) via proxies (i.e., potential extracellular enzymatic activity measurements) which are sometimes disconnected from the current ecosystem functions, and (3) the lack of consideration of chronic exposures to assess the impact of, adaptations to, or recovery of aquatic microbial communities from pesticides. Environ Toxicol Chem 2023;42:1867-1888. © 2023 SETAC.

Biotic and Abiotic Factors Influencing Arsenic Biogeochemistry and Toxicity in Fluvial Ecosystems: A Review

This review is focused on the biogeochemistry of arsenic in freshwaters and, especially, on the key role that benthic microalgae and prokaryotic communities from biofilms play together in through speciation, distribution, and cycling. These microorganisms incorporate the dominant iAs (inorganic arsenic) form and may transform it to other arsenic forms through metabolic or detoxifying processes. These transformations have a big impact on the environmental behavior of arsenic because different chemical forms exhibit differences in mobility and toxicity. Moreover, exposure to toxicants may alter the physiology and structure of biofilms, leading to changes in ecosystem function and trophic relations. In this review we also explain how microorganisms (i.e., biofilms) can influence the effects of arsenic exposure on other key constituents of aquatic ecosystems such as fish. At the end, we present two real cases of fluvial systems with different origins of arsenic exposure (natural vs. anthropogenic) that have improved our comprehension of arsenic biogeochemistry and toxicity in freshwaters, the Pampean streams (Argentina) and the Anllóns River (Galicia, Spain). We finish with a briefly discussion of what we consider as future research needs on this topic. This work especially contributes to the general understanding of biofilms influencing arsenic biogeochemistry and highlights the strong impact of nutrient availability on arsenic toxicity for freshwater (micro) organisms.

Light and temperature influence on diuron bioaccumulation and toxicity in biofilms

Variations of temperature and photoperiod throughout different seasons can affect aquatic communities such as biofilms. Biofilms, generally present at the base of trophic chains in freshwaters, are also subject to organic contamination, and are especially affected by herbicides. Many studies have investigated the effect and interactions of herbicides and environmental factors on biofilms, but never with a toxicokinetic point of view. The objective of this study was to assess structural and functional changes in biofilms exposed to diuron, and to link them with contaminant accumulation, under the influence of temperature and light variations. To this aim, biofilms were exposed to all possible combinations of three concentrations (0, 5 and 50 µg L^-1) of diuron, two temperatures (10 and 26 °C), and two light/dark photoperiods (16/8, 10/14), for durations of 0, 1 and 3 days. Diuron accumulation in biofilms was quantified and structural descriptors (protein and polysaccharide contents, dry weight) and functional endpoints (photosynthetic and enzymatic activities) were analyzed. The results obtained mainly highlighted the influence of temperature on diuron bioaccumulation and the associated toxic impact on biofilms. Bioaccumulation in biofilms exposed during three days at 10 °C, at the highest diuron concentration, was in average 1.4 times higher than bioaccumulation on biofilms exposed to 26 °C. Accordingly, the photosynthetic yield was more inhibited at lower than at higher temperatures. Temperature was also the highest impacting factor for metabolism regulation; for example, at 26 °C after three days of exposure, polysaccharide production was boosted under both photoperiods tested.

N,N-Diethyl-m-Toluamide Exposure at an Environmentally Relevant Concentration Influences River Microbial Community Development

Studies of the South Saskatchewan River confirmed that N,N-diethyl-m-toluamide (DEET) is ubiquitous at 10 to 20 ng/L, whereas in effluent-dominated Wascana Creek, levels of 100 to 450 ng/L were observed. Effects of DEET exposure were assessed in microbial communities using a wide variety of measures. Communities developed in rotating annular reactors with either 100 or 500 ng/L DEET, verified using gas chromatography-mass spectrometry analyses. Microscale analyses indicated that both DEET concentrations resulted in significant (p < 0.05) declines in photosynthetic biomass, whereas bacterial biomass was unaffected. There was no detectable effect of DEET on the levels of chlorophyll a. However, pigment analyses indicated substantial shifts in algal-cyanobacterial community structure, with reductions of green algae and some cyanobacterial groups at 500 ng/L DEET. Protozoan/micrometazoan grazers increased in communities exposed to 500 ng/L, but not 100 ng/L, DEET. Based on thymidine incorporation or utilization of carbon sources, DEET had no significant effects on metabolic activities. Fluorescent lectin-binding analyses showed significant (p < 0.05) changes in glycoconjugate composition at both DEET concentrations, consistent with altered community structure. Principal component cluster analyses of denaturing gradient gel electrophoresis indicated that DEET exposure at either concentration significantly changed the bacterial community (p < 0.05). Analyses based on 16S ribosomal RNA of community composition confirmed changes with DEET exposure, increasing detectable beta-proteobacteria, whereas actinobacteria and acidimicrobia became undetectable. Further, cyanobacteria in the subclass Oscillatoriophycideae were similarly not detected. Thus, DEET can alter microbial community structure and function, supporting the need for further evaluation of its effects in aquatic habitats. Environ Toxicol Chem 2019;38:2414-2425. © 2019 The Authors. Environmental Toxicology and Chemistry published by Wiley Periodicals, Inc. on behalf of SETAC.

Biofilm phosphorus uptake capacity as a tool for the assessment of pollutant effects in river ecosystems

Biofilms are a key component in the nutrient removal from the water column. However, nutrient uptake by biofilms may be hampered by the occurrence of pollutants or other stressors. This study aimed: (i) to investigate the biofilm phosphorus (P) uptake capacity as a relevant process for the maintenance of fluvial water quality and (ii) to explore the sensitivity of this process to different chemical and environmental stressors. We conducted chamber experiments to test for the relevance of biofilm P uptake capacity (PUC) as a tool to detect effects of pollutants on river self-depuration. PUC was calculated by measuring P temporal decay after performing controlled P-spikes in chambers with biofilm-colonized tiles. Four different experiments were conducted to evaluate the response of PUC to: (a) several river waters from increasing polluted sites; (b) the effect of the bactericide triclosan (TCS); (c) the combined effect of TCS and grazers; and (d) the effect of TCS after a drought episode that affected the biofilms. These experiments showed that biofilms decreased their PUC along the pollution gradient. The biofilm PUC was significantly reduced after receiving high TCS concentrations, though lower TCS concentrations also affected the biofilm when this was submitted to grazing pressure. PUC decrease was induced by flow interruption which further enhanced the TCS negative effects. Overall, PUC was sensitive to the effects of pollutants like TCS as well as to the action of biological (grazing) and environmental (drought) factors. The study also showed that multiple stressors enhance the negative effects of pollutants on the PUC of biofilms. Our study values the use of biofilms' PUC as a sensitive ecological-based tool to assess the effects of chemicals on freshwater communities and their derived functioning in river ecosystems.

Community structure and nutrient level control the tolerance of autotrophic biofilm to silver contamination

Autotrophic biofilms are complex and fundamental biological compartments of many aquatic ecosystems. Since microbial species differ in their sensitivity to stressors, biofilms have long been proposed for assessing the quality of aquatic ecosystems. Among the many stressors impacting aquatic ecosystems, eutrophication and metal pollution are certainly the most common. Despite that these stressors often occur together, their effects on biofilms have been far much studied separately than interactively. In this study, we evaluated the interactive effects of silver (Ag), a reemerging contaminant, and phosphorus (P), a nutrient often associated with freshwater eutrophication, on the structure and functioning of two types of autotrophic biofilms, one dominated by diatoms and another one dominated by cyanobacteria. We hypothesized that P would alleviate the toxic effects of Ag, either directly, through the contribution of P in metal detoxification processes, or indirectly, through P-mediated shifts in biofilm community compositions and associated divergences in metal tolerance. Results showed that Ag impacted biofilm community structure and functioning but only at unrealistic concentrations (50 μg/L). P availability led to significant shifts in biofilm community composition, these changes being more pronounced in diatom- than those in cyanobacteria-dominated biofilm. In addition, P tended to reduce the impact of Ag but only for the cyanobacteria-dominated biofilm. More generally, our results highlight the preponderant role of the initial community structure and nutrient level on biofilm response to metallic pollutants.

Drought episode modulates the response of river biofilms to triclosan

The consequences of global change on rivers include altered flow regime, and entrance of compounds that may be toxic to biota. When water is scarce, a reduced dilution capacity may amplify the effects of chemical pollution. Therefore, studying the response of natural communities to compromised water flow and to toxicants is critical for assessing how global change may affect river ecosystems. This work aims to investigate how an episode of drought might influence the response of river biofilms to pulses of triclosan (TCS). The objectives were to assess the separate and combined effects of simulated drought (achieved through drastic flow alteration) and of TCS exposure on biofilms growing in artificial channels. Thus, three-week-old biofilms were studied under four conditions: Control (normal water flow); Simulated Drought (1 week reduced flow+2 days interrupted flow); TCS only (normal water flow plus a 48-h pulse of TCS); and Simulated Drought+TCS. All channels were then left for 2 weeks under steady flow conditions, and their responses and recovery were studied. Several descriptors of biofilms were analyzed before and after each step. Flow reduction and subsequent interruption were found to provoke an increase in extracellular phosphatase activity, bacterial mortality and green algae biomass. The TCS pulses severely affected biofilms: they drastically reduced photosynthetic efficiency, the viability of bacteria and diatoms, and phosphate uptake. Latent consequences evidenced significant combined effects caused by the two stressors. The biofilms exposed only to TCS recovered far better than those subjected to both altered flow and subsequent TCS exposure: the latter suffered more persistent consequences, indicating that simulated drought amplified the toxicity of this compound. This finding has implications for river ecosystems, as it suggests that the toxicity of pollutants to biofilms may be exacerbated following a drought.