Time- and temperature-dependent relationships in ammonia tolerance after feeding and/or pre-exposure in freshwater and marine fish
Supervisors: Prof. Gudrun De Boeck
Co-Supervisor: Jyotsna Schrivastava
Ammonia is an important emerging pollutant due to eutrophication, but it is also produced endogenously as a waste product of protein metabolism, which makes it a risk factor in fish maintenance and transport, and in aquaculture. Before transport, fish are usually starved for several days as a standard practice in aquaculture. This reduces energy metabolism and thus oxygen demand and eliminates excessive ammonia excretion during transport. However, recent research in our lab indicates that it also reduces ammonia excreting capacities and ammonia resistance of fish (which is up-regulated after feeding or pre-exposure, but down-regulated during fasting). This will increase the risk of fish mortality events if ammonia does build up during handling or transport. Expression of Rh-glycoprotein seems to be involved in the change in ammonia handling and tolerance. As a result, the time dependent expression of these genes can be a key is determining the ideal fasting period prior to fish handling or transport. Also temperature is a key factor determining energy metabolism, including oxygen consumption and ammonia excretion, and is likely to fluctuate during transport. Therefore, the interaction between ammonia sensitivity during fasting and temperature will be assessed.
The main physiological effects of ammonia and temperature changes include behavioural effects, changes in swimming capacity, changes in iono-regulation and changes in energy metabolism. This study will elucidate physiological and biochemical characterization of fish subjected to environmental ammonia, temperature and a combination of both stressors after fasting or pre-exposure by investigating responses at metabolic, ion-regulatory and hormonal level. The results of present work will extend our insights on the strategies used by fish species to handle different ecological stressors at a time, and will also help to identify underlying mechanisms involved in determining sensitivity to ammonia exposure.
Three-spined stickleback thrive in historically polluted sites: adaptation or acclimation?
Supervisor: Prof. Gudrun De Boeck
Despite chronic exposure to contaminants at levels known to elicit toxicities in naive-animal models, fish populations may thrive at highly polluted sites. Populations may continue to persist, even under drastic or rapid environmental changes, if either they adjust through phenotypic plasticity (intragenerational physiological acclimation), adapt through rapid changes of the (epi)genome or modify standing genetic variation (intergenerational heritability). While there is a large body of research on acute and chronic toxicant responses in terms of acclimation within generations, the effects of multi-generational exposure on adaptive responses is less well understood and among others requires relevant exposed wildlife populations (evolutionary ecotoxicology). Within Flanders, historically Mercury contaminated sites with populations of three-spined stickleback provide good opportunities to study the underlying mechanisms of adaptation/acclimation. The focus of our proposal involves the identification of the genetic, molecular and physiological mechanisms that underlie evolutionary processes following multi-generational exposure to biologically relevant chemical pollutants. Responses of interest include behavior (as Hg is a neurotoxicant), swimming capacity and respiration (as Hg interferes with aerobic mitochondrial functioning and cardiovascular performance), and induction of defensive responses such as metal-binding protein and defence systems against oxidative stress. We will compare populations (F0) from historically contaminated sites with pristine sites, and continue to see if differences persist in their offspring (F1 & F2).
The influence of temperature and exercise on the toxicity of metal mixtures in common carp
Supervisor: Prof. Gudrun De Boeck
Co-Supervisor: Vyshal Delahaut
Environmental quality standards are developed for individual compounds and are generally based on toxicological assays with model species that are reared and tested under optimal culturing conditions where environmental conditions, including temperature, pH, oxygen content and water hardness, are kept constant and optimal for the tested species. Natural environments receive numerous different compounds and are much more diverse and variable than the conditions typically created in an experimental laboratory set up. Prediction of mixture toxicity on the basis of single compound toxicity data is difficult and has still a large degree of uncertainty associated to it. Especially the effects of mixtures of pollutants and the effect of temperature on toxicological processes remains very poorly documented and understood. This is surprising given the fact that temperature is one of the most important physiological and ecological factors with a large impact on many key processes. Ongoing work of our group indicates that strong interactive effects are observed in metal mixture scenarios that are not expected on the basis of single metal exposures. Likewise the effect of temperature on species sensitivity does not appear to be straightforward and the assumption that toxicity increases with temperature does not appear to be correct or at least generally valid.
Besides temperature, water velocity is also a changing factor in natural streams and aquatic ecosystems. Fish will adjust their swimming velocity to maintain their position in the stream, leading to different levels of exercise. Also foraging, escape reactions and seasonal migrations can affect swimming performance in fish. Exercise leads to increases in metabolic demand, and thus respiration efforts. Swimming induces increased ventilation rates, branchial blood perfusion and lamellar recruitment. Consequently, the enhanced functional surface area of gills promotes permeability for gas exchange but also leads to increasing demands concerning iono- and osmoregulation (sometimes called the osmo-respiratory compromise). This will not only affect ion transport mechanism, but also exposure to and uptake of pollutants.
The goal of this proposal is to provide a better molecular- and physiology-based understanding of the impact of mixture stress on the structural and functional organization fish. We will focus on single and mixed exposures for 3 metals that were chosen because of their environmental relevance: copper, cadmium and zinc. Within the framework of the project we specifically want to evaluate the effect of swimming and optimal and suboptimal temperatures on metal uptake and toxicity. This information and mechanistic understanding of shifting sensitivities is needed to further improve environmental impact and risk assessment for the aquatic environment.