Alexandrium

Alexandrium is a genus of marine dinoflagellates that includes a diverse group of species known for their role in harmful algal blooms. Members are photosynthetic microalgae that can also ingest organic material (mixotrophy) and are typically equipped with two flagella and a thecal plate structure. They range in size from about 20 to 60 micrometers and occur in coastal and estuarine waters worldwide, often blooming under favorable environmental conditions.

Several Alexandrium species produce saxitoxins and related compounds that accumulate in shellfish. When humans consume contaminated seafood, they can develop paralytic shellfish poisoning (PSP), a potentially serious or fatal condition. Toxin production varies by species, strain, and environment, so PSP risk is monitored through toxin analyses of shellfish and through cell counts of Alexandrium in water samples.

Ecologically, Alexandrium species are important contributors to coastal HAB events. They can form resting cysts that settle in sediments and germinate to initiate new blooms, allowing persistence across seasons. Sexual reproduction leads to cyst formation, and cyst beds can influence bloom timing and intensity. Environmental factors such as water temperature, stratification, light, and nutrient inputs affect bloom dynamics and toxin production.

Taxonomy within Alexandrium is complex and has been revised with molecular data. Notable members include the A. tamarense complex (often split into A. tamarense, A. fundyense, and A. catenella) along with species such as A. minutum and A. ostenfeldii. The genus remains a focal point for studies of dinoflagellate diversity, bloom ecology, and public health implications.