In January 2017, the International Seabed Authority released a discussion paper on the development of Environmental Regulations for deep-sea mining (DSM) within the Area Beyond National Jurisdiction (the "Area"). With the release of this paper, the prospect for commercial mining in the Area within the next decade has become very real. Moreover, within nations' Exclusive Economic Zones, the exploitation of deep-sea mineral ore resources could take place on very much shorter time scales and, indeed, may have already started. However, potentially toxic metal mixtures may be released at sea during different stages of the mining process and in different physical phases (dissolved or particulate). As toxicants, metals can disrupt organism physiology and performance, and therefore may impact whole populations, leading to ecosystem scale effects. A challenge to the prediction of toxicity is that deep-sea ore deposits include complex mixtures of minerals, including potentially toxic metals such as copper, cadmium, zinc, and lead, as well as rare earth elements. Whereas the individual toxicity of some of these dissolved metals has been established in laboratory studies, the complex and variable mineral composition of seabed resources makes the a priori prediction of the toxic risk of DSM extremely challenging. Furthermore, although extensive data quantify the toxicity of metals in solution in shallow-water organisms, these may not be representative of the toxicity in deep-sea organisms, which may differ biochemically and physiologically and which will experience those toxicants under conditions of low temperature, high hydrostatic pressure, and potentially altered pH. In this synthesis, we present a summation of recent advances in our understanding of the potential toxic impacts of metal exposure to deep-sea meio-to megafauna at low temperature and high pressure, and consider the limitation of deriving lethal limits based on the paradigm Hauton et al.Identifying Toxic Impacts of Deep-Sea Mining of exposure to single metals in solution. We consider the potential for long-term and farfield impacts to key benthic invertebrates, including the very real prospect of sub-lethal impacts and behavioral perturbation of exposed species. In conclusion, we advocate the adoption of an existing practical framework for characterizing bulk resource toxicity in advance of exploitation.

Bathymodiolus azoricus is a mussel from vent fields in the southwest of the Azores Triple Junction (Mid-Atlantic Ridge-MAR). Experimental evidence indicates that B. azoricus is a mixotrophic organism, which obtains energy from a dual endosymbiosis and filter-feeding. Yet the relative contribution of symbiosis and filter-feeding to B. azoricus nutrition is still unclear. To address this question, we developed and individual-based model which describes sulphide and methane uptake by endosymbionts, the energy gained through microbial oxidations, the transfer of energy from endosymbionts to B. azoricus, filter-feeding of particulate organic matter (POC) by B. azoricus and the energetic wastes of the mytilid with respiration. The model accounts for size-dependent relationships obtained from empirical data. External concentrations of H 2 S and CH 4 correspond to estimated values for the Menez Gwen vent field, maximal and minimal values measured at MAR. From in situ observed densities of B. azoricus, productivity predictions at the individual level were upscale to the mytilid population at Menez Gwen and compared to estimated values. Predicted biomass of B. azoricus and its endosymbionts show a very high fitting level with estimated values. Results suggest that the relative contribution of filter-feeding and endosymbiosis varies with B. azoricus size, with small mytilids being strongly dependent on filter-feeding, whilst larger mussels obtain a significant portion of its energy from endosymbiosis. This is related with the variation of gill weight with total weight. Results also suggest that, an individual of a certain size can potentially regulate the relative contribution of filter-feeding and endosymbiosis according to external conditions. However, large B. azoricus exhibit a higher level of nutritional flexibility than small mytilids. The relative contribution of endosymbioisis and filter-feeding to the total energy budget of B. azoricus, as well as the mytilid particulate organic matter requirements, are assessed and discussed under several scenarios.

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The variability of the bioaccumulation of metals (Ag, Cd, Cu, Fe, Mn and Zn) was extensively studied in the mussel Bathymodiolus azoricus from five hydrothermal vent sites inside three main vent fields of increasing depth along the Mid-Atlantic Ridge: Menez Gwen, Lucky Strike and Rainbow. Metal bioaccumulation varied greatly between vent fields and even between sites inside a vent field with B. azoricus showing a great capacity to accumulate metals. The bioaccumulation of these metals also varied significantly among tissues. The main target was the gills where metals were mainly associated with soluble compounds whereas in the digestive gland they were mainly associated with insoluble compounds. Storage of metals under insoluble forms in B. azoricus seems to be a major pathway for the detoxification of both essential and non-essential metals. Mussels from the studied fields can be discriminated following their metallic load but the segregation relies partially on the composition of the metal-enriched fluids.

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