New Techniques for Understanding Relationships Between Aquatic Organisms and Toxic Metals

Katie Maginnis for TEI

Most people who have taken a basic chemistry course are able to point out the transition metals on the Periodic Table.  Still,  many of the most experienced chemists remain puzzled about the changes that these metals undergo when exposed to natural ligands in aquatic ecosystems.  Fortunately,  Richard Vachet, Associate Professor of Chemistry is shedding some light on the subject. 

Along with his research team, Vachet has been investigating the speciation of transition metals such as copper to understand the different structural forms created when organic compounds called ligands attach to these metals.

Copper is an important metal to focus on because it appears at toxic concentrations in many natural environments but is  also an essential nutrient for all life forms. What Vachet finds especially interesting about this metal involves the narrow range of copper concentrations at which life in aquatic environments can survive.  As he explains, “If you’re below [the optimal concentration], you’re deficient; you don’t have enough copper for life in a particular setting. But if you’re above it, you’re toxic. There’s a very small range of where things are okay.” 

With this in mind, one might wonder how it is possible for life to exist in natural environments with such delicate and specific conditions. “That is what’s interesting from a chemical standpoint,” Vachet says. “What does nature do, what does biology do, what do organisms do, to keep that fine balance?” 

In order to find out, Vachet and his team have been developing special techniques for studying metals and ligands. By combining the methods of immobilized metal affinity chromatography (IMAC) and mass spectrometry (MS), they have been able to gather useful structural data about the metal-ligand complexes. 

With the process of IMAC, Vachet explains, “You have this column in which you’re able to immobilize metals like copper, so that they stick to the column but still have accessible sites where ligands could bind to them.” With this technique, Vachet can collect water samples and pass them through the column, then “capture” ligands that are attracted to copper.

This method also allows the chemists to gather a more concentrated sample of the copper-ligand complexes.  For instance, Vachet can collect a sample in which the complexes are naturally present in nano-molar concentrations (10-9 moles per liter) and force them into a smaller volume until they occur in micro-molar concentrations (10-6 moles per liter) instead. 

These concentrations are still extremely small, but they are now great enough to be detected using mass spectrometry. “MS is a sensitive analytical tool,” Vachet explains.  “It allows us to measure these things at very low concentrations, and get structural information about the compounds.” While both IMAC and MS have previously been used for other purposes, Vachet and his team are the first to combine the two methods for the study of ligands and their structures.

This technique of gathering data has proven useful in field studies near the Chesapeake Bay.  In their recent study of ligands in estuarine environments, Vachet and his team sampled water from three test sites: the Chester River, the Elizabeth River, and the Atlantic Ocean. Out of the three test sites, ligands with thiol groups were found only in the water samples from the Elizabeth River.  This was also the site with the highest concentration of copper, suggesting that there is a positive correlation between the metal and the ligands. 

What could be responsible for the control of ligand production to match the copper content of the water?  Vachet hypothesizes that microorganisms in the water produce these ligands as a defense mechanism, to regulate the toxicity of the copper in their environment. 

One piece of data that led him to this conclusion  was the structure of thiol (sulfur) groups on the ligands. In water, thiol groups typically don’t last long before they are oxidized and turned into another structure.  In his samples, however, Vachet found many ligands with thiol groups that had not yet been oxidized, suggesting that they must have been produced within the previous 24 hours or so. “For that to be the case,” he states, “they were most likely produced biotically.” 

Further evidence supporting this idea includes the interaction between the ligand’s thiol group and the copper to which it binds. When a thiol group binds to copper, the copper is more likely to be reduced to Cu(I) which is less toxic to living organisms than Cu(II). As Vachet suggests, “This could be one way for organisms to deal with the problem of toxicity, by converting Cu(II) to Cu(I).” 

By using the structural analysis made possible by IMAC and MS, Vachet and his team have been able to gain insight into the possible relationship between organisms and toxic metals. “Structure allows you to understand function,” he explains. “Our idea was that if we can get any kind of structural information about these ligands, we could understand how they might impact the cycling of copper in the environment.”  Vachet is further exploring this idea in his current research and its potential e applications to human biology and the environment.