Pollution-induced community tolerance of freshwater biofilms: measuring heterotrophic tolerance to Pb using an enzymatic toxicity test

Artificial Light at Night Alleviates the Negative Effect of Pb on Freshwater Ecosystems

Artificial light at night (ALAN) is an increasing phenomenon worldwide that can cause a series of biological and ecological effects, yet little is known about its potential interaction with other stressors in aquatic ecosystems. Here, we tested whether the impact of lead (Pb) on litter decomposition was altered by ALAN exposure using an indoor microcosm experiment. The results showed that ALAN exposure alone significantly increased leaf litter decomposition, decreased the lignin content of leaf litter, and altered fungal community composition and structure. The decomposition rate was 51% higher in Pb with ALAN exposure treatments than in Pb without ALAN treatments, resulting in increased microbial biomass, β-glucosidase (β-G) activity, and the enhanced correlation between β-G and litter decomposition rate. These results indicate that the negative effect of Pb on leaf litter decomposition in aquatic ecosystems may be alleviated by ALAN. In addition, ALAN exposure also alters the correlation among fungi associated with leaf litter decomposition. In summary, this study expands our understanding of Pb toxicity on litter decomposition in freshwater ecosystems and highlights the importance of considering ALAN when assessing environmental metal pollutions.

How do physicochemical properties influence the toxicity of silver nanoparticles on freshwater decomposers of plant litter in streams?

AgNP physicochemical properties may affect AgNP toxicity, but their effects on plant litter decomposition and the species driving this key ecosystem process in freshwaters have been poorly investigated. We assessed the impacts of AgNPs with different size and surface coating (100nm PVP (polyvinylpyrrolidone)-dispersant, 50-60nm and 35nm uncoated) on freshwater decomposers of leaf litter by exposing leaf associated microbial assemblages to increasing concentrations of AgNPs (up to 200mgL^-1) and of AgNO₃ (up to 25mgL^-1). We further conducted a feeding preference experiment with a common invertebrate shredder, Limnephilus sp., which was allowed to feed on microbially-colonized leaves previously exposed to AgNPs and AgNO₃. Leaf decomposition and microbial activity and diversity were inhibited when exposed to increased concentrations of 100nm AgNPs (≥25mgL^-1), while microbial activity was stimulated by exposure to 35nm AgNPs (≥100mgL^-1). Invertebrate shredders preferred leaves exposed to 35nm AgNPs (25mgL^-1) and avoided leaves exposed to AgNO₃ (≥2mgL^-1). Results from the characterization of AgNPs by dynamic light scattering revealed that AgNps with PVP-dispersant were more stable than the uncoated AgNPs. Our results highlight the importance of considering the physicochemical properties of NPs when assessing their toxicity to litter decomposers in freshwaters.

Improved short-term toxicity test protocol to assess metal tolerance in phototrophic periphyton: toward standardization of PICT approaches

Pollution-induced community tolerance (PICT) approaches involve comparing tolerance levels of natural communities to a particular contaminant or a contaminant mixture using short-term toxicity tests performed under controlled conditions. However, results from toxicity tests can be modulated by various environmental and experimental conditions, raising questions about their reproducibility and comparability. In this context, the present study aimed to determine the influence of exposure duration, periphyton suspension concentration, and periphyton maturation stage on the measurement of short-term effects of copper on phototrophic periphyton communities. Our results showed the very weak influence of exposure duration in the tested range (2-6 h) on toxicity level, whereas periphyton biomass in the tested suspension (in terms of both chlorophyll a concentrations and dry weight), proved a crucial determinant in toxicity assessment. Results also highlighted the potential tolerance increase with the periphyton maturation stage. This parameter conditioned the positive linear relationship between tolerance level and periphyton suspension concentration, leading to an increase in the linear regression slope with the maturation stage. This suggests that such a relationship is probably highly periphyton-dependent. Consequently, to enable data toxicity comparisons, an a priori normalization of the periphyton suspension biomass is necessary, and PICT approaches require the use, as much of possible, of periphyton with similar maturation stage. Finally, the present study clearly shows that a better standardization of PICT approaches could help to improve reproducibility. It could thus facilitate the comparison of tolerance levels measured in the same study (e.g., spatial and/or temporal and/or inter-treatment comparison) as well as the comparison obtained from different experimental and in situ research.

Impact of an urban multi-metal contamination gradient: metal bioaccumulation and tolerance of river biofilms collected in different seasons

The aim of this study was to investigate the repeatability and seasonal variability of the biological response of river biofilms chronically exposed to a multi-metal pressure in an urban contamination gradient. Biofilms were grown on immersed plastic membranes at three sites on the Seine river upstream (site 1) and downstream (sites 2 and 3) from Paris (France). They were collected in four different seasons (autumn, spring, summer and winter). Biofilm tolerance to Cu, Ni, Pb and Zn was measured using a PICT (Pollution-Induced Community Tolerance) approach with a previously developed short-term toxicity test based on β-glucosidase (heterotrophic) activity. Metal concentrations in the river and also in the biofilm samples (total and non-exchangeable bioaccumulated metals) were also monitored. Biofilm-accumulated metal concentrations reflected the increase of the multi-metal exposure along the urban gradient. These concentrations were strongly correlated with dissolved and particulate organic carbon and with the total metal fraction in the river water, which recalls the significant influence of the environmental parameters on metal uptake processes in river biofilms. Overall, natural biofilms allow monitoring water quality by integrating the variations of a diffuse metal contamination overtime. Tolerance levels globally increased from site 1 to site 3 reflecting the metal pollution gradient measured in the river water collected at the three sites. Cu tolerance tended to increase during warm seasons but no clear seasonal tendency could be found for Ni, Pb and Zn. Furthermore, principal component analysis clearly discriminated samples collected upstream (site 1) from samples collected downstream (sites 2 and 3) along the first principal component which was correlated to the metal gradient. Samples collected in winter were also separated from the others along the second principal component correlated to parameters like water temperature and Total Suspended Solids concentration. This study shows that chronic in situ exposure to environmental metal concentrations has a significant impact on natural biofilms. Biofilm tolerance to metals and biofilm metal bioaccumulation both reflect metal exposure levels although they remain low when compared to Environmental Quality Standards from the European Water Framework Directive. Yet temperature appears as an important environmental variable shaping community structure and response to toxic exposure which shows that the sampling date is an important parameter to consider when using natural river biofilms to assess the impacts of urban pressure.

Linking community tolerance and structure with low metallic contamination: a field study on 13 biofilms sampled across the Seine river basin

It is difficult to assess the biological consequences of diffuse water contamination by micropollutants which are present in rivers at low, even sublethal levels. River biofilms, which respond quickly to changes of environmental parameters, are good candidates to acquire knowledge on the response of aquatic organisms to diffuse chemical contamination in the field. The study was designed as an attempt to link biofilm metal tolerance and metallic contamination in a field survey covering 13 different sampling sites in the Seine river basin (north of France) with low contamination levels. Cd and Zn tolerance of heterotrophic communities was assessed using a short-term toxicity test based on β-glucosidase activity. Metal tolerance levels varied between sites but there was no obvious correlation between tolerance and corresponding water contamination levels for Cd and Zn. Indeed, metallic contamination at the sampling sites remained subtle when compared to water quality standards (only two sampling sites had either Zn or both Cu and Zn concentrations exceeding the Environmental Quality Standards set by the EU Water Framework Directive). Yet, multivariate analysis of the data using Partial Least Squares Regression revealed that both metallic and environmental parameters were important variables explaining the variability of metal tolerance levels. Automated Ribosomal Intergenic Spacer Analysis (ARISA) was also performed on both bacterial and eukaryotic biofilm communities from the 13 sampling sites. Multivariate analysis of ARISA fingerprints revealed that biofilms with similar tolerance levels have similar ARISA profiles. Those results confirm that river biofilms are potential indicators of low, diffuse contamination levels of aquatic systems.