Significant nitrogen fixation activity associated with the phyllosphere of Mediterranean seagrass Posidonia oceanica: first report

The present study provides first estimates of dinitrogen (N-2) fixation activities associated with the phyllosphere of Posidonia oceanica in the Mediterranean Sea. N2 fixation rates in the phyllosphere reached values up to 2.53 mu g N gDW h(-1) or 15.2 mg N m(-2) d(-1) (integrated to daily rates and by areal basis), which is higher than most rates reported in the phyllosphere of tropical seagrasses. These rates can potentially supply the total N demand of P. oceanica and may contribute significantly to the N budget in the Mediterranean Sea. Our results showed differences in N-2 fixation rates among leaf segments of different age with higher activities in older segments that were heavily epiphytized and higher activities during summer when epiphyte biomass accumulation was higher than in spring. N-2 fixation activities occurred during day and night, suggesting that the community of N-2 fixers in the phyllo sphere of Posidonia oceanica is complex and is not attributable to a single group of diazotrophs. Molecular analysis of nifH genes (coding for the nitrogenase enzyme) of epiphytic samples in the phyllosphere of P. oceanica revealed bacterial nifH sequences, closely related to as-yet uncultured and unidentified bacteria. To support the ecological importance of N-2 fixation in P. oceanica beds, more measurements with wider spatial and temporal scope are needed.