Abstract
Copper (Cu) is an important bioactive trace metal in the marine environment, acting as an essential micronutrient for many marine species. However, it can also be harmful at elevated concentrations, triggering sub-lethal and toxic effects. During the last few decades, studies using electrochemical techniques have established that it is the particular chemical form of Cu, its so-called chemical speciation, and not its total concentration that controls the geochemical and biological behaviour and thus the bioavailability and toxicity of Cu in marine systems. The majority of dissolved Cu in the marine environment is strongly complexed by a heterogeneous mixture of Cu-binding organic ligands (L), reducing the free ionic Cu concentration ([Cu2+]) to femto- and picomolar levels. Free ionic Cu is generally considered the most bioavailable and thus the most beneficial or toxic form to marine organisms. Organic complexation reduces [Cu2+] in most marine systems to levels harmless, but above limitation, to many marine microorganisms. Measuring the inorganic, organic, and free Cu forms in the dissolved phase is critical to assess the fate (i.e., distribution, cycling, and reactivity) and the biological effects of Cu in the marine environment. However, the chemical speciation of Cu in natural saline waters is complex and is technically challenging to evaluate, resulting in cost-intensive and time-consuming analyses. Consequently, despite the recognised importance, little data exists for Cu speciation and Cu-binding ligands in the marine environment, and ligand sources and chemical identities are generally understudied. Additionally, environmental processes and factors influencing and controlling Cu speciation in marine systems are still poorly understood. As a result, four different marine systems (i.e., estuarine, coastal and open ocean waters, as well as a shallow low-temperature hydrothermal vent system) were studied around New Zealand in an effort to advance the current understanding of Cu speciation processes and Cu-binding ligands in the marine environment. Total dissolved Cu concentrations were analysed using Inductively Coupled Plasma-Mass Spectrometry (ICP-MS), while Cu speciation was assessed with adsorptive cathodic stripping voltammetry (AdCSV) with salicylaldoxime (SA) as the competing ligand.
It was found that more than 99.7 % of the total dissolved Cu (CuT) in the sampled marine waters was complexed with organic and inorganic (i.e., sulfides) ligands, demonstrating the decisive role of ligands in controlling Cu speciation, bioavailability, and toxicity in the marine environment. AdCSV detected only one L-class with concentrations of up to 47.4 nM for low CuT areas, such as the region of the Hauraki Gulf and White Island/Whakaari, and L-concentrations ([L]) of up to 114.8 nM for areas enriched in CuT like the Astrolabe Reef and the Whau Estuary. Conditional stability constants (logK) of the CuL-complexes were relatively uniform in the working areas and ranged from 11.4 to 13.6, suggesting a similar chemical nature and source of the prevalent Cu-binding ligands in the sampled marine environments.
On the basis of the ligand distribution in the water column, with higher [L] in the photic zone and decreasing [L] with depth, Cu-binding organic ligands were suggested to predominantly originate from passive or active biological in-situ production of planktonic species or life stages. This assumption was supported by the 48 h bioassay in Chapter 6, during which Cu-stressed mussel embryos actively produced ligands, with logK values similar to those observed in the sampled natural saline waters, to mitigate the toxic effects of the Cu2+ in their environment. The current study also pointed out that the heterogeneous ligand pool in the marine environment might also include some organic ligands of terrestrial nature (i.e., humic substances (HS)), organic ligands derived from chemoautotrophic microorganisms, organic ligands formed abiotically under hydrothermal conditions, organic ligands which originate from yet unknown allochthonous and autochthonous sources, and in the case of hydrothermal systems can include substantial amounts of inorganic ligands such as sulfides.
Further, [L] was always in excess of the [CuT] and as a result [Cu2+] were buffered to femto- and picomolar levels in the Hauraki Gulf region, White Island/Whakaari, and the Whau Estuary. These Cu2+ levels were below the toxicity threshold and above the deficiency threshold of Cu for many marine microorganisms. Only at the Astrolabe Reef, with highly elevated [CuT] in the vicinity of the MV Rena wreck, was the Cu complexation capacity of the natural organic ligands saturated, which led to toxic levels of [Cu2+] in the system. Consequently, the benthic invertebrate recruitment was lower, and different types of organisms were recruiting the high Cu2+ areas, leading to an alteration in the community diversity, structure, and functionality of the Astrolabe Reef. This output indicates the importance of organic ligand production by organisms as a strategy to maintain the delicate balance between ecosystem health and degradation.
Temperature, pH, sample depth, chlorophyll a, particulate organic matter, and nutrient concentrations (i.e., nitrogen oxides, phosphate) had a poor correlation with Cu speciation parameters in the water column of the working areas. Salinity was found to increase Cu toxicity to M. galloprovincialis embryos, while higher dissolved organic carbon (DOC) concentrations decreased Cu toxicity to the mussel embryos. These outputs suggest that the biogeochemical cycling of Cu and its impact on marine life is intricate, dependent on the interactions between interrelated physical, chemical, and biological properties of the water column and the affected organism itself (e.g., ligand production, osmoregulation, and ionoregulation). This complexity precludes a generalisation of Cu speciation, bioavailability, and toxicity for the marine environment and thus highlights the urgent need to develop a site-specific saltwater Biotic Ligand Model (BLM) to assure adequate protection of aquatic life in various marine systems. Further, one of the most interesting findings of this thesis was that DOC and ligand quality (i.e., competition of several metals for the same ligand binding sites (non-specific metal binding affinities) or multidentate binding) were significantly more important in determining Cu bioavailability and toxicity in the sampled natural marine environments relative to DOC and ligand quantity. This finding highlights the necessity to elucidate the sources and chemical natures of organic Cu-binding ligands in order to improve the current understanding of the biogeochemical behaviour of Cu in marine systems.
Moreover, it was demonstrated that the excessive Cu-loss during laboratory-based bioassays owing to container adsorption and bioaccumulation processes of the test organisms, together with the detoxification effect of extracellular Cu, and the variability of intracellular Cu, can lead to a significant misunderstanding of Cu toxicity mechanisms and a misrepresentation of Cu toxicity to test organisms. This latter output questions the reliability of current marine water quality criteria (WQC), which were extrapolated from laboratory-based bioassay tests that did not account for the Cu-loss in solution under laboratory conditions.
Overall, the results from this study added to the body of current knowledge about Cu speciation, bioavailability, and toxicity in marine systems. This study thus provides a good basis to supplement the refinement of marine biogeochemical models of Cu, the establishment of a functional saltwater BLM, and the improvement of environmental risk assessments (i.e., WQC) of Cu in marine environments. As a result of this work, two papers have already been published, one is accepted, subject to minor changes, and three are under preparation.