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

Abstract

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 km2) 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. 15N2 measurements indicated a molar ratio of acetylene reduction to N2 fixed of 1.6 for all exposed communities. Estimated N2 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 N2 fixation amounted to 98.42 kg N day−1 which represents 28% of the rate of fixation in the entire lagoon (area 400 km2).