Thermal tolerance and acclimation capacity of the ragworm Hediste diversicolor under global change scenarios

• Autor(a) principal: Fernandes, Joana Filipa da Cunha
• Data de Publicação: 2019
• Tipo de documento: Dissertação
• Idioma: eng
• Título da fonte: Repositório Científico de Acesso Aberto de Portugal (Repositórios Cientìficos)
• Texto Completo: http://hdl.handle.net/10773/28486
• topic: Multiple stressors; Polychaete; Thermal limits; Physiology; Biomarkers; Energy allocation

Climate projections predict significant changes in ocean chemistry, salinity variations and the increase in ocean temperature by 2100. This has led to a substantial interest in the study of thermal ecophysiology, as temperature is a major factor shaping marine communities. Considering that most marine species are ectotherms, and thus cannot regulate body temperature, biochemical changes can occur with temperature variations. Moreover, salinity fluctuations can also induce physiological and metabolic changes in marine organisms. In this study, we experimentally evaluated the physiological and molecular responses of the ragworm Hediste diversicolor under predicted global change scenarios. Organisms were collected from the intertidal zone in Ria de Aveiro (Portugal) and subjected to an experimental trial under control (24 °C), and two treatment scenarios (ocean warming +3 ºC - 27°C and heat wave +6 ºC - 30°C) predicted by the Intergovernmental Panel on Climate Change, combined with salinity variations (20 and 30) in full factorial design. Environmental data (temperature, salinity and pH were collected during 60 days in the summer. Organisms were subjected an experimental acclimation trial during 29 days, and after 14 and 28 days of acclimation (D14 and D28 respectively), individuals were sampled (per time point) for molecular biomarkers analyses (heat shock protein 70kDa, ubiquitin, catalase, glutathione-S-transferase, superoxide dismutase activity, total antioxidant capacity, lipid peroxidation and glucose), fatty acid profiles and energy reserves (total protein, total lipid and glycogen content). At day 30, upper thermal limits (Critical Thermal Maximum - CTMax), thermal safety margins (TSM) and acclimation capacity were measured. In situ data collection, showed that H. diversicolor experiences wide temperature variations, but salinity showed no considerable variations during the sampling period. Higher acclimation temperatures led to higher thermal tolerance limits, confirming that H. diversicolor has some physiological plasticity, acclimation capacity and a positive thermal safety margin (CTMax > maximum habitat temperature – MHT). For biomarkers, the significant interaction of the three factors (temperature, salinity and day) indicates that the effect of one factor depends on the levels of the others factors. Particularly, significant temperature-salinity-time interactions were recorded for Hsp70, Ub and CAT, and all biomarkers except Ub were significant in factor day. The biomarkers that contributed most to the separation of the experimental treatments were glucose, glutathione S-transferase, ubiquitin, Hsp70 and catalase, and the differences between temperatures were more evident and prolonged when combined with low salinity. This suggests that glucose was mobilized to produce energy for cellular defenses, and antioxidant enzymes, ubiquitin and heat shock protein played an important role during hypo-osmotic and thermal stress, protecting cells from the toxic effects of reactive oxygen species (ROS) and managing the integrity of the protein pool, thereby allowing the proper management of cellular functioning. For the fatty acids (FA), the major saturated fatty acid (SFA) was palmitic acid (16:0), the dominant monounsaturated fatty acid (MUFA) was vaccenic acid (18:1n-7), in the polyunsaturated fatty acids (PUFA) the most abundant FA was linoleic acid (18:2n-6) and eicosapentaenoic acid (EPA) (20:5n-3) was the FA with the highest value between all of the highly unsaturated fatty acids (HUFA). An increase in the different fatty acid classes was observed, as well as in Σn-3 PUFA, Σn-6 PUFA and in essential fatty acids over time in polychaetes exposed to salinity 30, contrary to what was observed in salinity 20. These results suggest that salinity 30 is an optimal condition for H. diversicolor, favoring its physiological condition. Temperature also significantly influenced fatty acids, interacting with salinity. At 14 days of exposure, under optimal salinity conditions (30), there was an increase in HUFA from 24 °C to 27 °C, followed by a decrease at 30 °C. Omega 3 and 6 fatty acids remained stable at 24 ºC and 27 ºC, with a decrease detected at 30 ºC. These decreases may relate to changes in the lipid composition of cell membranes in order to maintain homeostasis. At 28 days there were no differences, suggesting that polychaetes can acclimate to elevated temperatures when they are in optimal salinity conditions. If the temperature increase occurs under low salinity conditions (20), changes in fatty acids are more pronounced, with an increase in all fatty acid classes after 14 days of exposure to 27 °C and 30 °C, comparing with the control (24 °C). At 28 days, the difference remains only in HUFA. In general, when polychaetes are grown at low salinity, a moderate increase in temperature causes Σn-3 PUFA, Σn-6 PUFA, ARA, EPA and DHA fatty acids to increase. However, if the temperature increase is more extreme (30 °C), the concentrations of these fatty acids decrease, following the expected pattern. These results indicate that fatty acids may be important in providing energy (especially saturated ones) and may play an important role in osmotic balance and membrane fluidity (especially unsaturated ones). Energy reserves, namely lipids and glycogen were not significantly affected by the time of exposure to treatments. Since the organisms had glucose available for energy, they did not need to degrade the carbohydrate, keeping their energetic reserves. However, there was an increase of total protein over time in the treatments at 27 ºC, in both salinities, suggesting some growth of the polychaetes under this temperature during the month of the experimental trial. There was also a decrease in total lipids at 27 °C, potentially due to lipid mobilization to provide energy for protein synthesis. The main conclusion is that H. diversicolor can easily acclimate to increased water temperature and salinity fluctuations. Even when organisms are adapted to tolerate these extreme environments and populations are able to persist, the environmental variability characteristic of estuaries is potentially stressful to animals inhabiting these conditions, leading to certain molecular mechanisms being activated to maintain homeostasis. Future studies on the current topic are therefore needed in order to evaluate the physiological plasticity of organisms to new global change conditions. Moreover, longer experimental trials are necessary, including all life-cycle stages, as well as trans-generational experiments and the integration of physiology into ecological modelling, in order to identify vulnerable life stages, and detect changes at higher levels of biological complexity and on an evolutionary scale.