The nitrogen fixing performed by cyanobacteria and some archaea transforms nitrogen in the form of N2, which is very common, into the form NO3 and NH4, which are much less common. This is important because most ocean plants -- particularly the plankton that makes up the majority of oceanic plant life -- need nitrogen in order to transform sunlight into energy but cannot use it in the common N2 form. So the nitrogen fixation carried out by cyanobacteria and archaea transform nitrogen into a form in which it is usable by other ocean organisims.