Ammonia oxidation

Why is nitrogen important to living systems?
How are these metabolic pathways similar to photosynthesis?
From the Urakawa et al. (2012) paper: Why was nitrite production used to measure the activity of the microbes in the study?
What is a bioassay? Why did the scientists use it in this study?
Which organisms showed sensitivity to oil toxicity? What happened to nitrite production in these organisms? How did nitrite production relate to growth?