| Summary: | Copper (Cu) is an important micronutrient, predominantly occurring as organic complexes in marine waters. The composition of the ligands forming these metal complexes has implications for the bioavailability of the trace metals to marine microorganisms, both as nutrients and toxicants. A variety of samples from estuarine, coastal and open ocean waters were studied in an effort to further our understanding of the ligands responsible for controlling the bioavailability of copper. Presented here are findings on the nature and likely identity of such ligands, including humic substances and various thiols. A novel method was developed to measure copper-binding humic substances, which were discovered to be synonymous with iron-binding humic substances. Using the new method on samples from the Mersey Estuary, humics were found to account for around 70% of the total ligand available for copper complexation in the estuarine and coastal samples. Samples from a very different estuarine environment, Sapelo Nature Reserve, Georgia, were then analysed in order to study potential copper limitation in blooms of Thaumarchaeota, prolific to the region and with a high copper requirement. Again, it was discovered that around 70% of the total available ligand for copper were humic substances, but that over 90% of the copper was complexed to thiourea-type thiols, also present in excess of the copper concentration. Comparing competitive ligand exchange (CLE) titrations to independent measurements of thiols and humic substances, the L1 and L2 ligand classes obtained via titrations were found to correlate very well with thiols and humics respectively, providing an indication of the nature of the ligands responsible for copper complexation. Furthermore, these findings suggested that copper was predominantly complexed (90%) as Cu(I), contrary to our current understanding of copper speciation. A study across the seasonal cycle at these stations provided further insight into the complexities of copper speciation. Cu2+ was found to be exceptionally low throughout the study, at sub-femtomolar concentrations, and lowest during the Thaumarchaeota bloom itself. Although previously considered to be limited by Cu2+ concentrations lower than 2 x10-13 M (when induced by the presence of artificial ligands), here it was demonstrated that Thaumarchaeota must be able to access the naturally complexed copper in order to bloom at these levels, posing questions for our understanding of copper bioavailability to these organisms. Finally, the copper speciation of two profiles from Line P of the Northeast Pacific, coastal station P4 and open ocean station P26, were assessed in an attempt to characterise the ligands in ocean samples. Thiols and humic substances were detected at both stations but at concentrations lower than the ligand concentrations measured from titrations, suggesting additional ligands play a part in copper-complexation in the open ocean.
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