4 begins the paragraph by introducing the subject: bacteria. 3 follows 4 since it talks about the principle kinds of bacteria. 32 is a mandatory pair. 3 introduces cyanobacteria and 2 gives general information about it. 1 follows by giving general information. The presence of the word 'however’ in 1 suggests that 1 cannot begin the paragraph, it will only come at a later point. 5 follows 1. ‘that interval' in 5 refers to the ‘gap’ mentioned in 1.