High rates of microbial dinitrogen fixation and sulfate reduction associated with the Mediterranean seagrass Posidonia oceanica

Seagrass meadows of Posidonia oceanica represent hotspots of productivity in the oligotrophic Mediterranean Sea. The lack of dissolved inorganic nitrogen (DIN) in the seawater suggests that the N-demand of these meadows might be in part supported by microbial dinitrogen (N-2) fixation. However, currently there are no direct N-2 fixation measurements available for this productive marine macrophyte. Here we investigated N-2 fixation activity associated with P. oceanica leaf, rhizome and root pieces. In 15N(2) incubations, the roots exhibited highest rates of N-2 fixation. The rates varied considerably between replicates, presumably due to a patchy microbial colonization of the roots. Additions of organic carbon compounds (acetate, glucose, sucrose or algal lysate) did not enhance the N-2 fixation rates. Sulfate reduction rates measured alongside were also highest in root incubations. Correspondingly, sequences of the nifH gene (a marker gene for the iron protein of the N-2-fixing enzyme nitrogenase) related to known sulfate-reducing bacteria were retrieved from P. oceanica roots. Other nifH sequences clustered with known heterotrophic diazotrophs previously identified in other marine macrophytes. In particular, many sequences obtained from P. oceanica roots were similar (>94%) to a saltmarsh rhizosphere-associated heterotrophic diazotroph, indicating that heterotrophic lifestyle might be common among marine macrophyte-associated diazotrophs. (C) 2016 Elsevier GmbH. All rights reserved.