When they reproduce into large populations, they are informally known as tides or harmful blooms. Recent textbooks call most of these single-celled microorganisms protists. During evolution, protists gave rise to macroscopic plants and animals. Pfiesteria, with its 24-stage life cycle, defies neat categorization. At some stages, it swims about powered by two flagella and so is considered a dinoflagellate.
Dinoflagellates are key players in aquatic ecosystems.
Large numbers of them live only in the pores of ice, for example, where they photosynthesize and provide food for other organisms. Pfiesteria can photosynthesize, but only if it has stolen the green organelles called chloroplasts from true algal cells. Although Pfiesteria can subsist in this way, says JoAnn M. Burkholder of North Carolina State University in Raleigh, it changes its eating habits depending on what's available. When fish are around, it can turn into a predator. It abandons its usual swirling swim pattern and makes a beeline for its prey in response to an unknown cue, perhaps in fish oil or excrement.