Clay mineral capture of microcystin: water molecules (light blue), microcystin (dark purple), calcium (orange), chloride (light green), clay layer (orange), negative charges in clay structure (yellow).

photo courtesy Ludmilla Aristilde

Clay mineral capture of microcystin: water molecules (light blue), microcystin (dark purple), calcium (orange), chloride (light green), clay layer (orange), negative charges in clay structure (yellow).

October 25, 2016

Researchers Uncover Sediment Interactions That Filter Toxins in Freshwater

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Summer is the perfect season. Who can forget those long hours of sunshine or tiring laps in Taughannock falls? However, that last experience is not guaranteed, especially if the water has plenty of toxin-producing algae in it. Thankfully, Prof. Ludmilla Aristilde’s, biological and environmental engineering, research could help us understand how to neutralize them.

Aristilde, along with Amy L. Pochodylo grad and Thalia G. Aoki ’15, published a paper in the Journal of Colloid and Interface Science in August that sheds light on the behavior of microcystin, a toxin produced by cyanobacteria, or blue-green algae.

Studying the behavior of microcystins is important because of the prevalence of algal blooms in lakes and waterways during the summer. Algal blooms arise when algae — which are naturally found in waterways — receive too many nutrients from runoff water. In the summer, high temperatures and sufficient sunlight support this excess growth.

The proliferation of microcystins is thus, a natural consequence of these blooms. Consequently, exposure to microcystins is a major public concern. If ingested, microcystins are toxic to the liver and their presence can render lakes and ponds unsuitable for swimming.

“I work at the interface of environmental chemistry and environmental biochemistry. I am concerned about the chemical behavior of contaminants because the behavior of these molecules in the environment is connected to their biological exposure,” Aristilde said.

Using molecular simulations, Aristilde and her team looked at two different types of microcystins, microcystin-LR (MC-LR) and microcystin-LA (MC-LA), to understand how the molecule’s chemistry affects its behavior.

A microcystin molecule is composed of a cyclic peptide — a series of amino acids linked together to make a closed circle. All microcystin molecules have two amino acid residues, aspartate and methyl-aspartate that confer two negative charges to the molecule. However, microcystins differ at two positions in the cyclic peptide chain, giving them their unique identities.

Both the studied microcystins have leucine, a type of amino acid, at one of the positions. However, MC-LR has arginine, a positively charged amino acid, while MC-LA has alanine, a neutral amino acid at the other position. Thus, MC-LR carries a positively charged component that MC-LA lacks.

Aristilde found that these amino acids adopted different shapes when placed in two different solutions. One contained excess sodium ions while the other, calcium ions — both nutrients available in fresh water.

Because a calcium ion carries two positive charges, its solution can effectively neutralize  the two negative charges from aspartate and methyl-aspartate, leaving MC-LR’s positively charged arginine residue to react with negatively charged mineral surfaces. However, because a sodium ion carries only one positive charge, MC-LR’s arginine residue is used to help balance the two negative charges, preventing it from interacting with any minerals.

As for MC-LA, which lacks this positively charged residue, Aristilde found that calcium ions acted as a bridge between negatively charged amino acid residues and negatively charged mineral surfaces to allow them to interact.

From their findings, Aristilde’s team developed theoretical models that shed light on what happens to microcystins in specific aquatic environments, demonstrating how these interactions allow clay particles to trap these toxins.

“The ultimate goal is to have better predictions of how microcystins behave in the environment and why they behave the way they do,” Aristilde said.

Going forward, Aristilde said she hopes to investigate the interactions of even more minerals with  microcystins and use their ability to capture these toxins to engineer a wide range of materials that can trap various contaminants.

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