Mechanisms of Tolerance in Metal-exposed Brown Algae and the Importance of Monitoring Studies: A Review

Abstract

Marine habitats are under threat from the continued release of contaminants into waterways, and monitoring becomes crucialto future management decisions and regulations to protect these environments. A key aspect of this monitoring is the choiceof a marine species that is able to cope with this pollution, and the tolerant species exhibit more relevant ecological responses.These species-responses are called biomarkers of tolerance and are tested using a variety of measurements that reflect exposureto chemicals. Marine brown macroalgae can be found in a wide range of marine environments, including those with high metalpollution and they are widely used in monitoring programs. Mechanisms of tolerance are vital to the survival of this group ofmacroalgae in polluted habitats, but these mechanisms have not yet been reviewed. The aim of this critical review article isto summarize the historical information available regarding metal tolerance in marine brown macroalgae. We review studiesabout tolerance mechanisms in marine brown macroalgae exposed to metals, conducted both under experimental conditionsand in the field. Also, bibliometric analyses are used to identify the most relevant investigations, country contributions andgaps in knowledge on the topic. Results showed that the principal extracellular tolerance mechanisms described for marinebrown macroalgae are physical exclusion, exudated ligands, and metal-binding to the cell wall. Epiphytic microorganismswere also found to be capable of accumulating a given metal. Internally, metallothioneins and phytochelatins were importantchelators; furthermore, antioxidant responses may increase metal tolerance. Omics analyses were used to reveal the internalmechanisms used by these macroalgae. Biomarkers of tolerance have occasionally been used in monitoring studies, providingadditional information regarding tolerance to complex chemical mixtures found in the field. Future studies should make use ofthese biomarkers to improve our understanding of metal-tolerance strategies.