Arsenic and fluvial biofilmsbiogeochemistry, toxicity and biotic interactions

Resumo

Arsenic (As) contamination of natural waters is a worldwide problem due to its important impacts for human and ecosystem health. Natural (geological processes, mainly) and anthropogenic activities, including mining, are the sources of arsenic pollution in the environment. High concentrations have been reported for water samples in several parts of the world, becoming an environmental concern because of its harmful effects on organisms. Arsenic toxicity depends on numerous interacting factors which makes effects difficult to estimate. In freshwaters, arsenate (AsV) can be taken up by microorganisms (especially those forming biofilms) due to its similarity with phosphate (PO43-) molecules, resulting its toxicity be dependent on environmental phosphate conditions. Microorganisms play a key role on the arsenic biogeochemistry (speciation, distribution and cycling) in aquatic systems, since they incorporate the dominant iAs (inorganic arsenic) form and may convert it to other arsenic forms. These transformation reactions have a big impact on the environmental behavior of arsenic, since the different chemical forms of this element exhibit different mobility and toxicity. Fish are another key constituent of aquatic ecosystems, and their effects due to arsenic exposure could be influenced by their interaction with microorganisms (i.e biofilms). Based on the current knowledge about biofilms ecotoxicology and arsenic biogeochemistry in freshwater ecosystems, this thesis is aiming to study, under realistic environmental arsenic concentrations, i) the role of benthic biofilms on As-bioavailability and As-detoxification in a freshwater system, ii) the toxic effects of arsenic on the structure and function of benthic fluvial biofilms, with especial attention to diatom responses, and iii) the interaction between these As-exposed primary producers and As-exposed higher organisms (fish). In Chapter 1, an experiment combining ecological and ecotoxicological descriptors was conducted to investigate the effects of AsV (130 µg L-1 over 13 days) on the structure and function of fluvial biofilm under phosphate-limiting conditions. We further incorporated fish (Gambusia holbrooki) into our experimental system, expecting fish to provide more available phosphate for algae and, consequently, protecting algae against arsenic toxicity. However, this protective role was not fully achieved. Arsenic inhibited algal growth and productivity but not that of bacteria. The diatom community was clearly affected, showing a strong reduction in cell biovolume; selection for tolerant species, in particular Achnanthidium minutissimum, and a reduction in species richness. Our results have important implications for risk assessment, as the experimental arsenic concentration used was lower than the acute toxicity criteria established by the United States Environmental Protection Agency (US EPA), 340 µg As L-1. In Chapter 2, we examined the effects of arsenic exposure (130 µg L-1 over 9 days) in the invasive mosquitofish G. holbrooki, in the same laboratory experiment as Chapter 1, incorporating some of the complexity of natural systems by including the interacting effects with the microbial community (the biofilm). Our aims were to quantify the effects of arsenic on some complex behaviors and physical parameters in mosquitofish, and to assess whether the detoxifying mechanisms of algae would ameliorate any effects of arsenic exposure. Aggression increased significantly with arsenic whereas neither food capture efficiency nor consumption was notably affected. Bioaccumulation increased with arsenic and unexpectedly so did fish biomass. Possibly increased aggression facilitated food resource defense allowing bigger fish to gain weight. The presence of algae aggravated the effects of arsenic exposure. For increase in fish biomass, algae acted antagonistically with arsenic, resulting in a disadvantageous reduction in weight gained. For bioaccumulation, the effects were even more severe, as algae operated additively with arsenic to increase arsenic uptake and/or assimilation. Aggression was also highest in the presence of both algae and arsenic. We highlight that multidisciplinary, cross-taxon research, particularly integrating behavioral and other effects, is crucial for understanding the impacts of arsenic toxicity and thus restoration of aquatic ecosystems. In Chapter 3, a biofilm translocation experiment was carried out during 51 days in a mining-impacted river, the Anllóns River (Galicia, NW Spain), where concentrations up to 270 mg AsV kg-1 are found in sediments. The translocation was performed moving biofilm-colonized substrata from upstream (less As-polluted) to downstream the mine area (more As-polluted site with also more easily extractable As), to explore the effect of arsenic on benthic biofilms and the role of these biofilms on arsenic retention and speciation in the water-sediment interface. Eutrophic conditions (high total dissolved phosphorus and total nitrogen) were detected in water at both sites, while sediments were not considered P-polluted. Translocated biofilms accumulated more arsenic and showed higher potential toxicity (higher As/P ratio). In concordance, their growth was reduced to half that observed in those non-translocated. Moreover, they became less nutritive (less N content) and with higher bacteria and dead diatom densities than the non-translocated biofilms. Besides the higher arsenic exposure, other environmental conditions such as the higher amount of DOC (dissolved organic carbon) and riparian cover in the more As-polluted site could contribute to those effects. Methylated As-species (DMAV) were found in the intracellular biofilm compartment and also in the river water, suggesting a detoxification process by biofilm (methylation) and a contribution to arsenic speciation in the water-benthic biofilm interface. Since most arsenic in sediments and water was arsenate (AsV), the high amount of arsenite (AsIII) detected in the biofilm extracellular compartment may also confirm AsV reduction by biofilms. This study provides new arguments to understand microorganism contribution to arsenic biogeochemistry in freshwater environments. The results obtained in this thesis provide valuable information to understand the contribution of benthic biofilms to the arsenic biogeochemistry in freshwater environments, and specifically in the water-biofilm interface. Also, it was demonstrated once again the importance of using biofilms and a multi-endpoint approach to measure effects of toxicants in freshwater ecosystems, as well as study the toxicity to different trophic organisms, such as biofilm and fish, since aggravated effects resulted in their interaction. Finally, environmental factors such as nutrients or light may influence and modulate arsenic toxicity. Therefore, it is crucial to take them into account for the measurement of real toxic effects in the ecosystems.