Illustration by Carl Wiens

Any goldfish owner knows how hard it is to keep algae out of a fish tank once it's become a nutrient-rich broth of fish food and fish poop. But an unsightly aquarium is a trifle compared to the aquatic-life-choking algal blooms that result when manure-rich agricultural runoff, concentrated with phosphorus and nitrogen, drains into our nation's waterways.

Now Walter Adey, a scientist at the Smithsonian Institution, has discovered a way to take advantage of algae's affinity for tainted water to use the plant as a sort of natural cleanser. At a test facility in Florida, Adey's algae-covered "turf scrubbers" have been set in contaminated water, where they are removing nutrients from the Suwannee River watershed.

And Walter Mulbry, a microbiologist with the U.S. Department of Agriculture, says that the phosphorus- and nitrogen-rich algae harvested from the screens make a fine fertilizer. The algae "degrades as the plants grow," Mulbry explains, "so it doesn't leach nutrients into the groundwater at the same rate as conventional fertilizer." Water near algae-fertilized cropland stays cleaner. And fish breathe easier.