Saxitoxin is produced by several species of marine and freshwater algae and blue-green algae (cyanobacteria).

• A. Marine and freshwater algae and cyanobacteria
• B. Fungi
• C. Bacteria
• D. Plants

Answer: (A)

Saxitoxin is a potent neurotoxin produced by certain photosynthetic microorganisms—specifically some marine and freshwater algae and cyanobacteria. This means it can accumulate in shellfish during algal blooms, posing a risk to humans who consume contaminated seafood. The toxin blocks voltage-gated sodium channels, interrupting nerve signaling and causing paralysis, which is why saxitoxin is associated with paralytic shellfish poisoning. The other groups listed do not describe the organisms known to produce saxitoxin in environmental contexts, so identifying marine and freshwater algae together with cyanobacteria as producers best fits the established source of this toxin.