EVERYONE knows plants absorb sunlight to power photosynthesis. What is less well known is that they also radiate light – and this botanical fluorescence is now being harnessed to monitor the safety of drinking water supplies.
Normally, after chlorophyll molecules in plants absorb light, photosynthesis converts light energy into chemical energy that converts carbon dioxide and water into carbohydrate and oxygen. But about 1 or 2 per cent of the light absorbed is re-emitted at a wavelength of 685 nanometres, at the infrared end of the spectrum. Called chlorophyll fluorescence, this process can be measured by sensitive light detectors.
Elias Greenbaum and his colleagues at the Oak Ridge National Laboratory in Tennessee wanted to see if they could use this fluorescence to monitor water quality. Drinking water reservoirs are extremely vulnerable to spills from factories or farms, or even terrorists armed with toxic chemicals. But testing for contaminants takes at least a few hours – long enough for the water to be pumped into homes. Now Greenbaum and his team believe they have hit on a way to give an instant warning of chemical attacks or toxic spills, by using microalgae living in the water as natural biosensors.
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First the team had to find out if toxic chemicals affect an organism’s ability to photosynthesise, and hence its fluorescence. They looked at four chemicals that would be extremely dangerous if found in sufficient concentrations in drinking water: methyl parathion (MPt), potassium cyanide, diuron and paraquat. MPt is an insecticide that is structurally and functionally similar to the nerve gas sarin. Severe cyanide poisoning can cause nervous and respiratory failure. And diuron and paraquat are commodity agrochemicals. Both are toxic to humans.
The team compared alga-laden water from the Clinch River in Tennessee with samples containing algae growing in water contaminated by the different chemicals. Using flashes of light to induce photosynthesis in the algae, they found that the resulting fluorescence was indeed impaired by the chemicals.
Each chemical affects the algae differently, generating a unique fluorescence signature. The researchers are now trying to work out how the chemicals affect the algae. Cyanide, for instance, is known to slow down photosynthesis by binding to enzymes called cytochromes, which normally ferry electrons around.
The team has also shown that the system works fast enough to test flowing water, so it could be used to monitor water quality in real time. DARPA, the research wing of the Pentagon, is evaluating the technology, and a major corporation is said to be planning to license it.
