Which explains how the calvin cycle and the krebs cycle differ

explanation:

b,bacteria convert nitrates

into nitrogen gas.

d,plants and animals take in

and use nitrogen compounds.

e,bacteria break down waste    

products and return nitrogen  

to the soil.

a,bacteria convert atmospheric

nitrogen to ammonia.

c,bacteria convert ammonia

to nitrites and nitrates.

well, that's easy; the answer's c, "it reduces contamination of the water and substrate of the estuary."