Towards adverse outcome pathways for metals in saltmarsh ecosystems – A review

Anthropogenic metal pollution remains a substantial threat to remnant saltmarsh ecosystems as they continue to decline globally. Metal pollutants impart sublethal stress on saltmarsh halophytes evident at transcriptomic, proteomic, metabolomic and physiological levels. Halophytes form the basal underpinning of saltmarsh ecosystems and as such changes to their physiology may have subsequent detrimental effects on higher levels of ecological organisation. This concept forms the basis of a mechanistic framework for toxicity: an adverse outcome pathway (AOP). AOPs facilitate the use of molecular and biological markers to predict consequent effects on population parameters, and community structure and function. We reviewed the current literature on common anthropogenically enhanced metals (Cu, Zn, Cd and Pb) in saltmarsh ecosystems internationally. The literature amassed to date indicates a lack of data in regard to the impacts of metals at higher levels of biological organisation and insufficient relevant data available to develop reliable AOPs. Collectively, metals impart sublethal stress at the transcriptomic, proteomic, and metabolomic levels and targets of toxic effect are predominantly biomolecules involved in metal transport and detoxification, antioxidant pathways associated with metal-induced oxidative stress, and photosynthetic biochemistry. Thus, these targets would be useful biomarkers in future studies designed to develop AOPs in saltmarsh taxa.