When harmful algal blooms approach shoreline communities along Lake Erie, residents get worried: in addition to being generally unpleasant, the overabundance of blue-green algae could very well be producing microcystin, a toxin that affects the liver, skin, and nervous system. Water treatment plants that draw drinking water directly from the lake have learned to deal with the toxin, but the additional treatment cost can add up quickly at $3,000 per day.
An additional problem is posed by the fact that not all algal blooms are toxic, not all seemingly clear water is free of microcystin, and not all of the microcystin produced in an algal bloom is dissolved in the water column. Some of it may also adhere to sediments, which would make the toxin stick around for longer than it would if it remained in the water.
To determine if this is the case, and which conditions are most likely to lead to microcystin being retained in sediments, Ohio State University researcher Dr. John Lenhart, along with former colleague Dr. Harold Walker (now at Stony Brook University), is assessing how microcystin and Lake Erie’s clay sediments interact under various conditions, such as the water’s pH.
“Certainly there’s the question of how the microcystin is breaking down in the water column,” says Lenhart. “But there’s also the question of whether the sediment can serve as a reservoir of microcystin, and then subsequently release it if the solution conditions change. So the sediment potentially could become a source as well.”
To date, the researchers have been testing how microcystin adheres to three different types of clay minerals common in Lake Erie sediments under various pH conditions. The pH level indicates how acidic or basic the water is – low pH indicates a high concentration of positively charged hydrogen ions in the water, making it acidic, while higher pH values indicate the presence of negatively charged hydroxide ions that create basic conditions. A pH of 7 is considered “neutral,” where acidic and basic ions are balanced.
“Typical pH is around 7.5 to 8 in most lake natural water environments,” says Lenhart. “So we’ve looked at a range of pH values that have some relevance, from pH 5 to 9. Five is maybe a little bit low, but 9 and above is certainly a value that is observed during algal blooms.”
The results showed that microcystin interacts more strongly with the sediments under lower pH conditions, while a higher pH resulted in very weak interactions and adherence. Those findings are consistent with other research on the interactions between various dissolved compounds and sediments, but so far, microcystin had not been studied much in this context.
Interactions between the sediment and microcystin are based on physical and chemical influences. For example, the microcystin molecule and the sediment particles are negatively charged across the pH range included in the study, but the magnitude of this charge varies. At low pH, the negative charges on the sediment and microcystin are relatively small, so that the resulting repulsion can be overcome by other types of interactions that create attraction between the sediment and the microcystin. At high pH, however, the negative charge is larger, and the electrostatic repulsion produced limits attraction.
But even without knowing exactly what drives these interactions, knowing the conditions under which sediment could hold on to microcystin past the time that it can be detected in the water will help water treatment plant managers better plan for and protect citizens from the effects of harmful algal blooms.
“If we have a better understanding of these interactions, then we can predict to a higher degree of accuracy both microcystin concentrations and the longevity of the microcystin after the blooms,” Lenhart explains. “The utilities that draw their source water from these impacted lakes are challenged with predicting when microcystin concentrations are going to be high, and when they’re going to have to alter their treatment plans to account for that. So any additional information to help them plan accordingly is a benefit to them and will save them some money.”
“If we have a better understanding of these interactions, then we can predict […] both microcystin concentrations and the longevity of the microcystin after the blooms.”
Dr. John Lenhart
Lenhart and his team also examined how the presence of certain natural salts, which produce positively charged calcium or sodium ions, would influence the microcystinsediment interactions. These cations attach to the slight negative charge present in the clay sediments, changing how the equally negative charge of the microcystin allows it to attach to the sediment particle. It turns out that calcium ions, such as would be produced by dissolving limestone rock along shorelines, attract more microcystin to the sediment particle than do sodium ions, likely because calcium ions usually carry a higher positive charge than sodium ions.
Ongoing work will examine the influence of organic materials on the interaction between sediments and microcystin, as the current sediment being used in the experiments has been treated to remove any organic molecules. These organic molecules are produced by the metabolism of living organisms in the water, or as dead algae, for example, decompose, and Lenhart describes them as “bulk organic matter” of a variety of molecular shapes and sizes. These organics are attracted to sediments and tend to concentrate there, potentially changing the way in which microcystin can
interact with the clay particles.
“The expectation is that the interactions will be enhanced in the presence of the organics, just because organic molecules like microcystin tend to be attracted to other organics,” Lenhart says. “The complicating factor is that microcystin is actually a negatively charged molecule from pH of around 2 to 12, so that negative charge may confound interactions with organic matter, as organic matter is also typically negatively charged.”
While Lenhart cautions that this project, which ends in February 2015, only looks at one of the most common forms of microcystin produced in a Lake Erie algal bloom – making up anywhere from 45 to 100 percent of total microcystin – knowing more about how the toxin interacts with the lake environment will always be helpful to communities that depend on the lake for water and income, and can provide a solid basis for further studies into keeping lakeshore residents safe from the negative impacts of harmful algal blooms.