What were the historical limitations of previous classification systems that led to and are solved by the three domain system that is in use today?

What were the historical limitations of previous classification systems that led to and are solved by the three domain system that is in use today? Why is the gene coding for ribosomal RNA (rRNA) used for establishing phylogenetic relationships and how does this compare to previous methods for placing organisms in appropriate taxa? What are the factors that limit our ability to fully classify and understand microbial diversity and what implications do these challenges have for environmental and health management? Why are algae important in nature? How are algae, fungi and lichens related and how is this relationship defined? Explain how the presence of algae can indicate either pollution or productivity of a body of water. There are two types of viruses, DNA viruses and RNA viruses, and each group has subgroups that utilize different nucleic acid biosynthetic pathways. Which of these two types of viruses do you think is responsible for the deadliest human disease(s)? Specify the disease(s), the viruses, and their biosynthetic pathways. Please explain why you think this or these are the deadliest and give examples from each subgroup to support your answer. Has the group of deadliest diseases changed over time? If so, why do you think this is true?