Dinitrogen fixation as a source for new production in the Mediterranean Sea: A review

Biological fixation of dinitrogen (N2) by prokaryotic organisms is an important pathway contributing to new production in N limited areas of the oceans and in other aquatic environments. N2 fixation was also hypothesized to contribute significantly to the nutrient impoverished Mediterranean Sea. Here we review the available literature on N2 fixation in the Mediterranean Sea. We examine evidence for past N2 fixation and examine the present Mediterranean for known diazotrophic organisms, measured rates, estimates for nitrogen fixation, and the contribution of N2 fixation to new and total production in both the western and eastern basins. Historical evidence, based mostly on isotopic composition of 15N/14N, indicates that N2 fixation contributed significantly to organic matter in the Mediterranean. Overall results from recent (<10 yr) studies show that the potential for N2 fixation exists and a diversity of diazotrophs are recorded across the Mediterranean. Actual volumetric and areal rates of N2 fixation range from 0 to 17 nmol N L-1 d-1 and 0- 50 μmol N m-2 d-1 respectively. Their contribution to new and total production range from 1-2% in the easternmost Levantine Basin to ~50% of new production during the stratified summer months in the north western Mediterranean.