Explanation:

 Justification: Statement 1: Cyanobacteria, formerly called "blue-green algae" are relatively simple, primitive life forms closely related to bacteria. Typically much larger than bacteria, they photosynthesize like algae. Depending upon the species, cyanobacteria can occur as single cells, filaments of cells, or colonies.
Statement 2: Nutrient-rich bodies of water such as some lakes or ponds may support rapid growth of cyanobacteria. With the right conditions, a "clear" body of water can become very turbid with green, blue-green or reddish-brown colored algae within just a few days. High concentrations of an alga species in a water body form "blooms."
Statement 3: Blue-green algae, like true algae, make up a portion of the phytoplankton in many water bodies. However, blue-green algae are generally not eaten by other aquatic organisms, and thus are not an important part of the food chain. “Keystone species” are crucial parts of the food chain; without them there will be a partial collapse of the food chain. So, 3 is incorrect.
Statement 4: Blue-green algae blooms can alter lake ecosystems by:
• reducing oxygen levels
• preventing the growth of beneficial algae
• producing toxins that are directly harmful to fish and other organisms People may become ill after swimming or water skiing in lakes with toxic blue-green algae.