Dinitrogen fixation by exposed communities on the rim of Tikehau atoll (Tuamotu Archipelago, French Polynesia)

Various types of sub-aerially exposed microbial mats, including emergent mats, beach sand, beach rock and Kopara mats, are widespread on the 78 km (25 km(2)) of rim surrounding the Tikehau atoll lagoon. These mats form laminated accretions or diffuse microbial communities growing under high insolation and temperatures, and are therefore subject to desiccation. Both heterocystous and non-heterocystous cyanobacteria occur in these mats. Using acetylene reduction techniques, nitrogenase activity was observed at all sites over a period of 5 years and was 3-17 times higher during daylight than at night in all communities except for beach rock. N-15(2) measurements indicated a molar ratio of acetylene reduction to N-2 fixed of 1.6 for all exposed communities. Estimated N-2 fixation ranged from 1.44 to 8.0 mg N m(-2) day(-1) in these exposed communities (mean of 4.66 mg N m(-2) day(-1)) with beachrock showing the highest rates. For the whole reef rim, daily N-2 fixation amounted to 98.42 kg N day(-1) which represents 28% of the rate of fixation in the entire lagoon (area 400 km(2)).