Alexandrium

The genus Alexandrium is a particularly well-known group of bloom-forming marine dinoflagellates due to their impacts on human health.  This genus is globally distributed, and can be found in sub-polar, temperate, and tropical coastal waters. Currently there are more than 30 recognized species and three well supported species complexes. Species within this genus previously were identified by their size and thecal plate structure, but with the advance of molecular phylogenetic analyses the taxonomy of this genus has frequently been revised.

In nature, the roughly spherical Alexandrium cells may be found as individuals or in short chains in rapidly growing populations. Each cell has two flagella, which they use to migrate vertically in the water column depending on light intensity, time of day, and water movement, allowing them to inhabit optimum growing conditions.  They are predominantly autotrophs, relying on photosynthesis, but a few species are mixotrophic under specific life phases and conditions.  The dark to reddish brown cells of these species can vary in size from 20- 80 µm in diameter and therefore are not visible with the naked eye unless in high concentrations, which can discolor coastal waters and produce what are often referred to as "red tides" or harmful algal blooms. Occasionally, blooms can even be bioluminescent, although it is extremely variable depending on the population.

Like many dinoflagellates, Alexandrium species have two main life history phases: a planktonic phase where motile cells are in the water column at various reproductive stages, and their cystic life phase where they lie dormant in the sediments. This alternating life cycle allows the species to survive as cysts during unfavorable growing conditions such as cold winter water temperatures, and in some locations high concentration of cysts accumulate and establish a seed bank that initiates recurring blooms year after year. In some regions, researchers map the location and abundance of these benthic cysts, and use these data to predict and model bloom dynamics. Alexandrium blooms occur seasonally and are influenced by a multitude of abiotic conditions from temperature, salinity, nutrient availability and even weather patterns. They also are impacted by biological factors including predation, infection from viruses, bacteria, parasites or cell encystment. Many scientists around the world research the conditions that promote bloom duration and development to model and predict future blooms as they can be detrimental to human health and fisheries.