Some vegetables and fruits in nature are edible and others are dangerous.
Mushrooms are a well known example.
This is also true for microscopic algae. It is important for consumers to make informed choices. The presence of a toxin doesn't necessarily cause an adverse reaction, it is the dosage that matters. Toxic blue-green algae may occur in natural lakes, ponds and reservoirs as a result of nutrients from agricultural run off and human activity. Human toxicity is rare because we normally avoid using water that contains algae.