Abstract
Various types of microbial mats are widespread in the SW New Caledonia lagoon. Both heterocystous (Nodularia harveyana) and non-heterocystous (Hydrocoleum cantharidosmum, H. lyngbyaceum) cyanobacteria dominate these mats. Using the acetylene reduction technique, nitro- genase activity was observed at all sites. Heterocystous cyanobacteria fix N2 during the daytime, whereas non-heterocystous cyanobacteria fix N2 during the night. The intensity of nitrogenase activ- ity depended on the level of light energy received during daylight. An estimation of nitrogen fixation by benthic cyanobacteria (16.4 ± 5.4 mg N2 m -2 d -1 ) at 21 m depth (average depth of the lagoon) represented 19% of the nitrogen requirement for benthic primary production.
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