Investigating Environmental Contaminants Using Radiotracers

A team of scientists from the Toxicology Centre, University of Saskatchewan led by Drs. Markus Hecker, Karsten Liber, Som Niyogi, Esteban Gillio Meina, Kerstin Bluhm, Ms. Katherine Raes and a group of skilled technicians is using selenium radiotracers, produced at McMaster University’s nuclear reactor, to investigate selenium bioaccumulation in aquatic ecosystems. While selenium is an essential micronutrient for many organisms at low concentrations, it can become toxic to oviparous vertebrates, particularly fish, if its concentration increases in the water.

Selenium is a contaminant of concern in the environment because it can potentially bioaccumulate – increasing in concentration through aquatic food webs, and eventually posing a risk to aquatic lifeforms. Scientists believe that the critical step in accumulation and movement between levels of the food web is the initial uptake of selenium as the inorganic anions selenite (IV), or selenate (VI). However, very little is known about how this process occurs, even in important food sources like periphyton – the microalgae communities that grow to form filaments or sheets that cover submerged surfaces in many freshwater environments. Dr. Hecker’s team is interested in gaining a better understanding of the factors that influence selenium uptake in typical  Canadian periphyton. Their work will aid the development of improved models that can predict uptake and trophic transfer – that is, the movement of selenium between levels of the food chain – in fast-moving freshwater ecosystems.

The research project with the scientists preparing “samplers” – bundles of some 900 frosted microscope slides that can support the growth of naturally occurring periphyton. The team placed the samplers in two different mountain creeks in British Columbia that have low background selenium concentrations, and left them in situ through the summer of 2021. After approximately four months, the research team loaded the samplers into containers filled with site water, and transported them to Saskatoon for analysis.

Back in the lab, the team measured how the periphyton sourced from the different creeks interacted with selenium using selenium-75 (Se-75) radiotracers. The radiotracers were produced by Dr. Derek Morim, a Research Associate with McMaster University’s Isotope Development team. Dr. Morim began by bombarding elemental selenium (isotopically enriched in selenium-74) with neutrons in the core of the McMaster Nuclear Reactor to generate selenium-75; he then chemically processed the radionuclide to transform it into selenate and selenite salts. By mixing precise amounts of non-radioactive selenium salts with the Se-75 radiotracers, the Saskatoon-based scientists could quantify how much selenium the periphyton were taking up simply by measuring the amount of radioactivity accumulating in each sample.

The team’s analysis of periphyton from the two different creek sites showed a remarkable difference when exposed to selenite. “It was amazing to see how much more uptake was observed in the samples collected at Creek I,” remarked Dr. Gillio Meina. “We believe that it is likely due to the different algae composition and we are currently in the process of trying to quantify the amount of algae and qualify the different types present in both samples.”

Dr. Hecker’s team also found very pronounced differences between the uptake of selenium (IV) and selenium (VI) in samples from Creek 1. “This result was not as surprising,” noted Dr. Gillio Meina. “We know from previous research that selenite and selenate use different transporters to get into the algae.  For example, we know that sulphate competes with selenate more than with phosphate. Thus, we tested if the presence of increasing sulphate concentrations in water reduced the accumulation of selenate in natural periphyton.” They evaluated this hypothesis by combining the samples with different sulphate and selenate concentrations. As the scientists expected, uptake of selenate was further reduced in the presence of sulphate due to competition for that transporter.

Excited by their recent findings, the team has requested more shipments of ⁷⁵Se radiotracers which they will use to repeat and verify their initial findings, and to advance their research further. “When it comes to biological systems, you want to ensure that the results are real, as opposed to accidental chances or simple random variance; and some results creates more questions than answers,” says Dr. Gillio Meina. “Thus, we also hope to study other phenomena, such as the influence of phosphate on selenite uptake, as well as probing if there are differences between natural periphyton and lab-grown algae, effects of light, pH and other water chemistry variables that could influence the uptake of selenium by natural periphyton. We are excited that our work can help to shed light on the biokinetics and mechanistic toxicology of selenium, an ubiquitous element that, if not monitored, can affect aquatic systems worldwide.”

“We are excited that our work can help to shed light on the biokinetics and mechanistic toxicology of selenium, an ubiquitous element that, if not monitored, can affect aquatic systems worldwide.”