Illustration of Alexandrium life cycle Red tide research

Dinoflagellates such as Alexandrium usually reproduce by asexual fission: One cell grows and then divides into two cells, then two into four, four into eight, and so on. When growth is unchecked by environmental conditions—such as a shortage of nutrients or light, or grazing by animals — harmful algae populations can accumulate to visually spectacular but catastrophic levels.

For some species, a decline in available nutrients provokes a switch to sexual reproduction and a new life stage. Cells produce gametes, which fuse to form motile diploid zygote (called a planozygote). Planozygotes will form dormant resting cyst that settle into bottom sediments. These cysts can survive for years, allowing a species to withstand nutrient starvation, extreme winter temperatures, or even ingestion by animals. When favorable conditions resume, the cysts germinate and populate the water column with a new generation of photosynthetically active cells primed for another bloom.

The cyst stage represents an effective strategy for survival and dispersal. With every switch into the cyst stage, a bloom can be carried into new waters by ocean currents, fish, or even humans (via ballast water discharge) and then deposited as a “seed” population that colonizes a new area. (Illustration by Jack Cook, © Woods Hole Oceanographic Institution)