Oceanic nitrogen (N2) fixation has recently been identified as a significant part of the oceanic nitrogen (N) cycle and may directly influence the sequestration of atmospheric CO2 in the oceans by providing a new source of N to the upper water column. The prokaryotic microorganisms that convert N2 gas to reactive N are an unique subcomponent of planktonic ecosystems and exhibit a variety of complex dynamics including the formation of microbial consortia and symbioses and, at times, massive blooms. Accumulating evidence indicates that iron (Fe) availability may be a key controlling factor for these planktonic marine diazotrophs. The primary pathway of Fe delivery to the upper oceans is through dust deposition. N2 fixers may therefore be directly involved in global feedbacks with the climate system and these feedbacks may also exhibit complex dynamics on many different time-scales.

The hypothesized feedback mechanisms will have the following component parts: The rate of N2 fixation in the world's oceans can have an impact on the concentration of the greenhouse gas, carbon dioxide (CO2), in the atmosphere on time-scales of decades (variability in surface biogeochemistry) to millennia (changes in the total NO3 - stock from the balance of N2 fixation and denitrification). CO2 concentrations in the atmosphere influence the climate. The climate system, in turn, can influence the rate of N2 fixation in the oceans by controlling the supply of Fe on dust and by influencing the stratification of the upper ocean. Humans also have a direct role in the current manifestation of this feedback cycle by their influence on dust production, through agriculture at the margins of deserts, and by our own production of CO2 into the atmosphere. The circular nature of these influences can lead to a feedback system, particularly on longer time-scales.

This collaborative and interdisciplinary group of investigators, led by Dr. Anthony Michaels, will study each of the components of this system and then to model the hypothesized feedback processes. Because of the interaction of the various parts of this system, keyed around the unique behavior and biogeochemistry of the prokaryotic microorganisms that can fix N2, this feedback loop should exhibit complex behaviors on a variety of time-scales. In this research, we will conduct a targeted series of experiments and field observations to understand and parameterize each of the pieces of this global process including the direct control of marine N2 fixation by dust deposition. This understanding will then feed a modeling process that examines the complex dynamics of this system on time-scales of years to millennia. The modeling process will be evaluated by comparison with data on the time-dependent behavior of ocean biogeochemistry