AME 51:45-53 (2008)  -  DOI: https://doi.org/10.3354/ame01185

ABSTRACT: In the present study we detected unicellular diazotrophic cyanobacteria along a nutrient gradient in New Caledonia’s coral lagoon (24000 km²) 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 fraction 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 contribute to N₂ fixation from the <10 µm size fraction (4.4 to 8 nmol N^–1 24 h^–1), which was 4 times higher than the >10 µm size fraction (0 to 1.9 nmol N^–1 24 h^–1). In addition, in the middle of the lagoon daylight ¹⁵N₂ fixation could explain 78% of ¹⁵N₂ 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.

KEY WORDS: Diazotrophs · Cyanobacteria · Picoplankton · FISH · Pacific · Coral lagoon · Symbiosis