Of the many protists in aquatic environments, the euglenoids seem to be the best bio-indicators. These small creatures—they are typically among the largest algal cells, but are often thought to act more like animals than other algae—are considered to be unicellular flagellates and live in water habitats that can vary in pH and light. Some euglenoids are capable of photosynthesis; others do not carry on photosynthesis (they apparently lost their chloroplasts over time) and are predatory. But in general, because their population thrives under high nutrient levels—and human pollution often carries the decomposing organics needed by euglenoids—they are useful bio-indicators of polluted water.