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

Abstract

In the present study we detected unicellular diazotrophic cyanobacteria along a nutri- ent 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 µm). These organisms were either free living (63%), recovered with the 0.2 to 3 µm size frac- tion or associated (37%) to particles or planktonic dinoflagellates from larger size fractions (3 to 10 µm and >10 µm). 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 con- tribute to N2 fixation from the 10 µm size fraction (0 to 1.9 nmol N -1 24 h -1 ). In addition, in the middle of the lagoon day- light 15 N2 fixation could explain 78% of 15 N2 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.