Mass mortalities of invertebrates, fish, birds, and mammals caused by algal-produced toxins are occurring in modern environments. In addition to direct effects of these toxins, the large mass of organic material produced by algal blooms can lead to oxygen depletion during decay, which indirectly causes death of some biota. Toxin-producing algae occupy a wide range of modern marine, brackish, and freshwater environments. Their growth is favored by warm water temperatures, increased inorganic carbon concentrations (e.g., CO₂), and abundant nutrient supplies in aquatic environments. Cyanobacteria (blue-green algae) are responsible for most of the disease and death caused by algal toxicity today.

Based on characteristics and occurrences of algae in modern aquatic environments and on observations from the fossil record, we propose that toxin-producing algae were present in the geologic past and were an important factor in Phanerozoic mass extinctions. The geologic record demonstrates a pronounced increase in abundance and environmental range of algae, including stromatolitic cyanobacterial mats, coincident with major Phanerozoic mass extinctions. During these past events of algal expansion, population decline of metazoan taxa could have been caused by effects of algal blooms, including algal-produced toxins, at a scale sufficient to generate a fossil record of mass extinction. Environmental changes such as climatic warming, sea level fluctuation, and increased nutrient supply may have promoted algal blooms over vast expanses of marine to freshwater environments. From the increasing frequency of modern, toxin-producing algal blooms, which may be related to global warming, another massive biotic crisis could be forthcoming.