High abundance of diazotrophic picocyanobacteria (<3 μm) in a Southwest Pacific coral lagoon

In the present study we detected unicellular diazotrophic cyanobacteria along a nutrient gradient in New Caledonia's coral lagoon (24 000 km(2)) using whole-cell hybridization of specific Nitro821 probes (TSA-FISH, Tyramide Signal Amplification of Fluorescent In Situ Hybridization). The specificity of this probe was confirmed on cultures and in the natural environment, Surprisingly, the community of Nitro821-targeted cells was numerically dominated by picocyanobacteria (97%, 1 to 1.5 mu m). These organisms were either free living (63%), recovered with the 0.2 to 3 mu m size fraction or associated (37%) to particles or planktonic dinoflagellates from larger size fractions (3 to 10 mu m and >10 mu m). Diazotrophic picocyanobacterial abundance ranged from 3 to 140 cells ml(-1) along the nutrient gradient and was highest at the oligotrophic lagoonal station. These cells may contribute to N(2) fixation from the < 10 mu m size fraction (4.4 to 8 nmol N(-1) 24 h(-1)), which was 4 times higher than the >10 mu m size fraction (0 to 1.9 nmol N(-1) 24 h(-1)). In addition, in the middle of the lagoon daylight (15)N(2) fixation could explain 78% of (15)N(2) fixation over 24 h. The results presented in the present study reveal for the first time a significant concentration of photosynthetic diazotrophs within the marine picoplankton community.