Symptom or Complication: Algae Mats at Mass Extinctions

ROLE OF TOXIN-PRODUCING ALGAE IN PHANEROZOIC MASS EXTINCTIONS: EVIDENCE FROM MODERN ENVIRONMENTS AND THE GEOLOGIC RECORD

CASTLE, James W.,

Dept. of Environmental Engineering and Earth Sciences, Clemson University, 340 Brackett Hall, Clemson, SC 29634-0919, jcastle@clemson.edu

RODGERS, John H. Jr,

Dept. of Forestry and Natural Resources, Clemson University, 261 Lehotsky Hall, Clemson, SC 29634-0317

Abstract:

Observations from modern environments and evidence from the geologic record support the hypothesis that toxin-producing algae were present in the geologic past and played an important role in Phanerozoic mass extinctions. Mass mortalities of invertebrates, fish, birds, and mammals caused by algal-produced toxins are occurring in modern environments. Several types of human illness, some resulting in death, are attributed to toxins produced by algae. In addition to direct effects of these toxins, the large mass of organic material produced by algal blooms can result in dissolved oxygen depletion during decay and indirectly cause death of some biota. Toxin-producing algae occupy a wide range of modern marine, brackish, and freshwater environments. Their growth in aquatic environments is favored by warm water temperatures, increased inorganic carbon concentrations (e.g. CO2), and abundant bioavailable nutrient supplies. Modern, toxin-producing algal blooms are occurring at increasing frequency, which may be related to global warming. Cyanobacteria (blue-green algae) are responsible for most of the disease and death caused by algal toxicity today. The rock 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, declines in populations of metazoan taxa may have been caused by lethal effects of algal blooms, including algal-produced toxins, at a scale sufficient to generate a fossil record of mass extinction. Past environmental changes such as climatic warming, sea-level change, and increased nutrient supply may have promoted algal blooms over vast expanses of marine to freshwater environments. Environmental stressors including UV irradiation, drought, physical injury, and changes in water chemistry can induce algae in modern environments to produce increased quantity and potency of toxins. Catastrophic events such as volcanism and impacts may have been a source of environmental stress that caused or contributed to increased production or potency of algal-produced toxins in the geologic past.

Symptom or complication?

Is this a symptom of the mass death: the lack of grazers allows the return of bacterial mats? Or is it one of the complicating causes of mass death during extinctions? Or both?