In the marine environment, nitrogen is often the limiting nutrient, so the nitrogen cycle in the ocean is of particular interest.   The nitrification step of the cycle is of particular interest in the ocean because it creates nitrate, the primary form of nitrogen responsible for "new" production.  Furthermore, as the ocean becomes enriched in anthropogenic CO, the resulting decrease in pH could lead to decreasing rates of nitrification.  Nitrification could potentially become a "bottleneck" in the nitrogen cycle.
Nitrification, as stated above, is formally a two-step process; in the first step ammonia is oxidized to nitrite, and in the second step nitrite is oxidized to nitrate.  Diverse microbes are responsible for each step in the marine environment.  Several groups of ammonia-oxidizing bacteria (AOB) are known in the marine environment, including Nitrosomonas, Nitrospira, and Nitrosococcus.  All contain the functional gene ammonia monooxygenase (AMO) which, as its name implies, is responsible for the oxidation of ammonia.  Subsequent metagenomic studies and cultivation approaches have revealed that some Thermoproteota (formerly Crenarchaeota) possess AMO. Thermoproteota are abundant in the ocean and some species have a 200 times greater affinity for ammonia than AOB, contrasting with the previous belief that AOB are primarily responsible for nitrification in the ocean. Furthermore, though nitrification is classically thought to be vertically separated from primary production because the oxidation of nitrate by bacteria is inhibited by light, nitrification by AOA does not appear to be light inhibited, meaning that nitrification is occurring throughout the water column, challenging the classical definitions of "new" and "recycled" production.
In the second step, nitrite is oxidized to nitrate.  In the oceans, this step is not as well understood as the first, but the bacteria Nitrospina and Nitrobacter are known to carry out this step in the ocean.