The availability of nitrogen (N) is one of the factors that controls the productivity of the oceans, and there has been great interest in determining the magnitudes and pathways of N inputs to the world’s oceans through biological N2 fixation. Biological N2 fixation is the reduction of N2 gas to biologically available ammonium, and this is performed by a diverse but limited number of bacterial and archaeal genera. Cyanobacteria are generally assumed to be the major N2-fixing microorganisms in the open ocean. Atmospheric N2 is one of the important external sources of N to the surface waters of the oceans, and thus provides the stoichiometric nutrient flux to support the export of carbon to the deep ocean. This is of importance in ocean–atmosphere fluxes and feedbacks that help to constrain the atmospheric concentrations of the greenhouse gas CO2. There continues to be controversy over whether the oceanic denitrification losses of N are balanced by N2 fixation inputs. There are large uncertainties in the estimates of basin- and global-scale denitrification and N2 fixation rates from biogeochemical calculations, perhaps due to the many assumptions required to scale these processes globally from either biogeochemical or biological data. Conversely, there is also a general lack of data on the distributions and activities of N2-fixing microorganisms over the vast scales of the ocean. At the core of resolving these issues is the identification of the organisms involved and determining how they function, the factors that limit their growth, and their roles in food webs. The difficulties in determining the roles of open ocean microorganisms in N2 fixation are the nature of N2-fixing microorganisms themselves, the dilute nature of microbial populations in the oligotrophic ocean, and the general difficulty in cultivating microorganisms from the ocean. Despite these hurdles, over the past few years much has been learned about the microorganisms primarily responsible for N2 fixation in the surface waters of the open ocean.
Nitrogen fixation by marine cyanobacteria. Trends Microbiol. Jan 10 2011
Discrepancies between estimates of oceanic N2 fixation and nitrogen (N) losses through denitrification have focused research on identifying N2-fixing cyanobacteria and quantifying cyanobacterial N2 fixation. Previously unrecognized cultivated and uncultivated unicellular cyanobacteria have been discovered that are widely distributed, and some have very unusual properties. Uncultivated unicellular N2-fixing cyanobacteria (UCYN-A) lack major metabolic pathways including the tricarboxylic acid cycle and oxygen-evolving photosystem II. Genomes of the oceanic N2-fixing cyanobacteria are highly conserved at the DNA level, and genetic diversity is maintained by genome rearrangements. The major cyanobacterial groups have different physiological and ecological constraints that result in highly variable geographic distributions, with implications for the marine N-cycle budget.