Measurements of Organism Abundances and Activities

N2 fixation is an important process in ecosystems, and performed by a wide variety of different microorganisms, both free-living and symbiotic. In the marine environment, most N2 fixation occurs in the water column and in association with unicellular microorganisms, in contrast to the multicellular plants and associated microorganisms in soil on land. Measuring N2 fixation rates, and determining abundances of diazotrophs is critical for understanding N inputs in marine ecosystems, and yet is challenging because of the low densities of N2-fixing microorganisms, and the large scales of ocean basins. Typical methods for measuring rates include the acetylene reduction and 15N uptake methods, as well as biogeochemical estimates from nutrient concentrations and natural abundances of isotopes. Advances in technology and computational methods have provided new approaches for evaluating N2 fixation in the oceans, including high throughput sequencing and gene amplification, stable isotope probing and remote sensing.