change to a purple color. The precise change in dye color is recorded at intervals by an automated monitoring system, allowing hundreds of cultures to be checked by a single operator. Bacteria initially identified in this way as having good potential for degrading pollutants are then passed on for further tests.

On the slimy underwater surfaces of rocks, stems, and leaves in streams and lakes is a thin layer of microscopic life. This ubiquitous submerged film of algae, bacteria, fungi, and protozoans is called periphyton. It can be used like a book to read the health of its aquatic world.

Monitoring environmental quality is a key task of biotechnology, and what better way to do it than to use organisms themselves as sensitive, built-in record-keepers. The advantage of periphyton as an environmental watchdog is that it is found everywhere and stays put. All scientists needed was to find an easy-to-measure characteristic of periphyton that changes in a consistent and predictable way with pollution levels, signaling changes in the microorganisms' surroundings.

They found such a characteristic in the average lipid (fat) composition of organisms in the periphyton. Lipids are made up of various types of fatty acids. The amount, type, and distribution of fatty acids found in periphyton are clues to the health of the microorganisms. Lipids are used by organisms for two different purposes: as part of their outer membrane structure and as a store of energy. Compared with periphyton from clean waters, samples of periphyton from polluted sites were found to have rel-