Brown algae are an important group of organisms inhabiting coastal ecosystems worldwide. Because of their sessile nature, they are exposed to natural abiotic stresses such as high and low irradiances, desiccation, thermal fluctuations and mechanical stress, as well as anthropogenic-derived stresses such as chemical pollution. While the impacts of metal pollution affect brown algae on a local scale, there is growing concern on the potential interactions between pollutants and abiotic pressures resulting from global climate change. The main objective of this study was to determine the nature of the interactions (synergistic, additive or antagonistic) of different concentrations of copper in combination with increased temperatures in controlled laboratory experiments using the model brown alga Ectocarpus siliculosus as a proxy for brown seaweeds, which are globally important primary producers and bioengineers of near-shore waters. The responses in E. siliculosus were evaluated at different levels of biological organisation. At the whole organism level Cu or temperature affected growth but no interactions occurred. Antagonistic interactions occurred between stressors in the photosynthetic efficiency response (measured as chlorophyll a fluorescence), being less affected by Cu at higher temperatures. The bioaccumulation of Cu ions showed and antagonistic response to temperature as less Cu ions were accumulated at elevated temperature. The concentrations of H2O2 and lipid peroxides (TBARS), which are indicators of oxidative stress, were synergistically affected by interactions of stressors. In contrast, the concentrations of antioxidants ascorbate and glutathione reflected both additive and antagonistic interactions respectively. This also occurred in the activity of antioxidant 8 enzymes (superoxide dismutase, ascorbate peroxidase, catalase and glutathione reductase) and the expression of related genes. Finally, the results of the biochemical and physiological tests were integrated with the whole transcriptome response to temperature and Cu stress. These results showed that interactions between temperatures and Cu stress could be highly complex, but also lead to the discovery of potential stress markers such as light harvesting complex proteins and several transporters. This research provides new insights into the responses of brown macroalgae to metal and thermal stress. Those responses indicate that synergistic or antagonistic interactions can occur at different levels of organisation, being the regulation of antioxidant metabolism, photosynthetic physiology and related gene expression, the most important mechanisms involved. This information will aid to understand potential effects of climate change on the toxicity of metals for macroalgae in estuaries and coasts affected by pollution.

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