The sensitive nature of algae makes them respond quickly to disturbances in water quality, making them an excellent warning system for ecological problems. Algae thrive in all aquatic habitats, with each of the more than 20,000 species having a specific affinity for environmental conditions. Those affinities give scientists clues about water conditions. "Most algae live a very short life, usually less than a week," explains Euan Reavie, lead scientist at NRRI's Ely station. "If something happens in their environment-the pH balance changes, or the nutrients increase-a new algal community that is tolerant of the disturbance will quickly replace them." Although their life is short, algae leave clues behind that scientists can read later. Diatom algae have intricate "glass house" cell walls made of biogenic silica, and each species' cell wall exhibits a unique architecture.