Which of the following is NOT a toxic by-product of oxygen metabolism?

Got a Microbiology Exam coming up...need help finding some of the answers on my study guide.

1.) Which of the following is NOT a toxic by-product of oxygen metabolism?
superoxide
hydroxide radicals
hydrogen peroxide
dihydrogen oxide

2.) Which bacterial cellular constituent is most liable to change under changing environmental conditions?
DNA
peptidoglycan polyamines
proteins

3.) Which of the following could describe the relationship between the cyanobacterium Nostoc and E. coli related to the carbon cycle?
Nostoc produces carbohydrates from lithotrophy, and E. coli breaks down the carbohydrates to generate CO2.
Nostoc produces carbohydrates from phototrophy, and E. coli breaks down the carbohydrates to generate CO2.
Nostoc produces CO2 that E. coli turns into carbohydrates.
E. coli produces carbohydrates that Nostoc uses as energy to produce more carbohydrates.