Like algae, they are photosynthetic. Like other bacteria they lack internal organelles bound by membranes. Also like bacteria, they produce a variety of toxins, whose toxicity is a function of dose, or exposure.
Cyanobacteria exist in many environments including soil, marine and freshwater.
The cause of the blooms can vary. One well established condition that promotes blooms of cyanobacteria is the relative amounts of phosphorus and nitrogen in the water. However, there is an important difference between cyanobacteria and algae such as diatoms. Some blue-greens can fix nitrogen from atmospheric N2 gas. In general algal growth in freshwater lakes is limited by the the availability of phosphorus, with plenty of available dissolved nitrogen in the form of nitrate or ammonia.