Hierarchical responses to organic contaminants in aquatic ecotoxicological bioassays: from microcystins to biodegradation
In this thesis I explore the ecotoxicological responses of aquatic organisms at different hierarchical levels to organic contaminants by means of bioassays. The bioassays use novel endpoints or approaches to elucidate the effects of exposure to contaminants and attempt to give mechanistic explanations that could be used to interpret effects at higher hierarchical scales. The sensitivity of population growth rate in the cyanobacteria species Microcystis aeruginosa to the herbicide glyphosate and the insecticide metabolite pp'-DDE was compared with a novel endpoint based on microcystin production. Growth was equally or less sensitivity than microcystin production indicating a decoupling between population growth and microcystin production. Low concentrations of glyphosate increased the production of the most toxic microcystin analog MC–LR and its relative intracellular contribution to the microcystins pool. Glyphosate triggered a trade-off between growth and toxin production at photoperiods of 12:12 and 16:8 but not at dimmer conditions (8:16), favouring growth during the early exponential growth phase and microcystin production during the late. The metabolic and ecological costs involved in microcystin production were traced in terms of lower cell quotas of ATP, growth rates and tolerance to the herbicide in the wild type strain compared to its mutant unable to produce the toxins. Growth seems to be a conservative trait in M. aeruginosa, making it an all-round population endpoint to detect effects of contaminants on cyanobacteria. The sensitivity of grazing efficiency as an ecotoxicological endpoint was assayed in a simple system of zooplankton-algae. Daphnia pulex grazed less efficiently on Scenedesmus spp. when pre-exposed to pp'-DDE via water and to glyphosate via food. The decreased grazing rates were associated with the treatments that gave the highest body burden of pp'-DDE (direct effect on animals), and the lowest of glyphosate (indirect effects mediated through increased algal biomass). The bioassays helped to elucidate the importance of secondary effects of contaminants on species interactions. The impact of glyphosate and the insecticide fenthion on the diversity–productivity relationship in Chlorophyta communities was examined on a bivariate plane. The resulting relationship was unimodal (concave) at low diversities, asymptotic at higher, and independent of species composition when communities were competing for acquisition of limiting resources. Productivity was maximized at a diversity of four species, and pesticides did not greatly change the trajectory of the relationship but its amplitude, which implies an impact on the magnitude of the effect of diversity on productivity. For example, glyphosate supported the relationship acting as a nutrient source and subsidizing productivity at almost all assayed concentrations. Algae-bacteria associations and dominance did change the trajectory of the relationship (convex), highlighting their potential role for the magnitude and complexity of the community response to changing diversity. In an attempt to estimate the contribution of residence time of groundwater to the persistence and effects of organic contaminants in aquifers of a selected area (León-Chinandega, Nicaragua), I collected ground waters to provide natural inoculates for the biodegradation, and nitrogen cycling assays, and to determine recharge sources and apparent ages by stable isotopes and chlorofluorocarbons (CFC). The ground waters are mixtures of water infiltrated at the plains and at the volcanic slopes, affecting the resolution of the CFC age dating so that the deep and shallow aquifers (average 23 years) were not distinguished. Both, a pristine deep aquifer and a shallow contaminated aquifer have biodegradation potential for DDE and glyphosate, and the most contaminated site may have strains adapted for faster degradation. The moderately contaminated sites had the highest potential for nitrification and denitrification, and the most heavily contaminated the lowest for nitrification, most probably due to the inhibitory presence of toxaphene. Denitrification was less sensitive to contamination than nitrification. The persistence of the in situ contaminants is not explained by the provenience or apparent age of groundwater.
Source Type:Doctoral Dissertation
Keywords:NATURAL SCIENCES; potential trade-off; grazing efficiency; glyphosate mineralization; DDE; potential denitrification; microcystin production; potential nitrification; DDE biodegradation; water stable isotopes; groundwater apparent age; fenthion; Chlorophyta; Daphnia pulex; diversity productivity relationship; glyphosate
Date of Publication:01/01/2008