Alexandrium is a genus of dinoflagellates, which are single-celled organisms found in marine environments. These microorganisms are known for their role in harmful algal blooms, often referred to as "red tides." During these blooms, they can produce toxins that affect marine life and pose risks to human health through contaminated seafood. The most notable species within Alexandrium is Alexandrium fundyense, which is responsible for paralytic shellfish poisoning (PSP). This condition can occur when humans consume shellfish that have fed on these toxic dinoflagellates. Monitoring and research on Alexandrium are essential for managing the health of marine ecosystems and ensuring food safety.