A microorganism reports on environmental toxins by whether it knows which way is up or down
Pollution, and the resulting toxic habitats, is an increasing problem worldwide. While prevention is a clear priority, finding and minimizing existing damage is an urgent undertaking. Once identified, a key question is: just how toxic is the site? Determining the safe limits of pollutants is a non-trivial task, and while the individual chemicals can be measured, the biological consequences are difficult to understand without biological testing, also known as bioassessment. Just as coal miners used canaries to test the safety of the mines, living test systems are still indispensable. For several decades, scientists have been using a variety of alternative model organisms that can inform us about toxicity and they are progressively fine-tuning the information that they gain gather from these experiments.
Technologies such as mass spectrometry and chromatography specifically measure the quantity of chemicals in environmental samples; however, these assays are best for known chemicals. Researchers will often only find the chemical signatures that they know they should be looking for and they may miss pollutants that they don’t know they should be looking for. Organic molecules frequently undergo transformations upon mixing with water, a pH change, or UV exposure. The unexpected products of these reactions may not be detected by the assay. An additional confounding factor is that environmental pollutants exist in mixtures, and mixtures may have different effects than simply the sum of their components, particularly in a biological context. While powerful, these analytical techniques reveal little about biological effects and it is difficult to accurately predict the toxicity of a chemical mixture without testing in a living model system.
Scientists have used a variety of models, ranging from bacteria to plants to fish, and each model provides a variety of clues about toxicity. Each has varied richness of information that it can report based on growth, movement, appearance, behavior, and survival. These responses are clues about the effects and implications of pollutants. For water pollution alone, there are a host of models. There is duck weed, a fast-growing water plant which is particularly sensitive to heavy metals and pesticides. These pollutants cause observable damage such as leaf or root death, and loss of chlorophyll. Another model is the bioluminescent bacteria Vibrio fischeri, which has been used for over three decades to indicate the toxicity of fresh and salt water. Their natural luminescence becomes gradually extinguished as the bacteria become unhealthy and begin to die. Scientists have also used the movements and survival of microscopic crustaceans to assess water safety. Fish are yet another sensitive indicator of water pollutants and video tracking their swimming behavior can suggest whether their health has been affected.
In an ongoing collaboration between the Kohat University in Pakistan and the University of Erlangen in Germany, scientists have been using another organism, Euglena gracilis, to monitor water pollution. E. gracilis is a single cellular organism that propels itself with a single flagellum. Because it is a relatively inexpensive and informative model system, scientists developed an automatic bioassay using E. gracilis. Since its development in the late 1990s, it has been applied in several locations of industrial contamination in Pakistan. One particularly interesting aspect of E. gracilis is its ability to respond to the effects of gravity and the assay monitors the microorganism’s motility and orientation. Recently, Dr. Azizullah was specifically looking into the gravitactic ability of E. gracilis, or the ability to know which way is up and down. Within E. gracilis, there are organelles that are denser than their surroundings and gravity pulls these downward in the cell. This in turn stretches the membrane of E. gracilis and this stretching activates ion channels in the membrane. Influx of ions through these channels causes a cascade of signals that trigger E. gracilis to re-orient itself. Pollutants such as heavy metals, organic chemicals, or increased salinity can all interfere with this mechanism. Heavy metals are particularly toxic because they bind to these ion channels and prevent signal transmission.
As populations grow and industries develop, the demands for water – particularly fresh water – grow correspondingly. With a limited supply of clean water, it is imperative to try to prevent pollution, but, failing that, to have informative assays to measure the degree of water pollution. For humans, it can take years or decades before the detrimental effects of a toxin manifest themselves. By that point, the link to the toxic source may be difficult to establish, and the damage will already be done. While model organisms do not translate directly into the effects experienced by humans, they do provide early indicators of toxic conditions. As these assays evolve, they can become more sensitive and more informative about the nature of environmental toxins. ~K.E.D.C.