Cyst germination is controlled by a variety of external factors including temperature, light, and oxygen concentrations. Some species have an internal “clock” control. When these conditions are favorable, the cysts break open and a swimming cell emerges. Within a few days of “hatching,” the cell reproduces by simple division.
With abundant nutrients and optimal conditions, cells will reproduce exponentially. A single cell can divide into several hundred cells within weeks. If enough cells bloom, shellfish can become contaminated, poisoning animals and humans who eat them.
When conditions are no longer favorable for growth (e.g., nutrients are depleted), growth stops and gametes are formed.
Two gametes fuse to form a motile, zygotic cell called a planozygote.
This zygote will then thicken its cell wall and condense its cytoplasm to become a cyst. These dormant resting cyst fall to the ocean floor for germination in subsequent years.
Cysts of dinoflagellates lay dormant on the ocean floor, buried in sediment. Left undisturbed, they can stay in this state for years. When oxygen is present and environmental conditions are favorable for growth, germination may begin.
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.