Horizon 1 1 17 ACL : Articles dans des revues avec comité de lecture répertoriées par l'AERES Frontiers in Microbiology art. 554 [14 ] Candidatus Atelocyanobacterium thalassa nitrogen fixation nitrogenase nifH oligotyping UCYN-A New Caledonia Western Tropical South Pacific NOUVELLE CALEDONIE PACIFIQUE ZONE TROPICALE 2018 fdi:010072713 ENG Frontiers in Microbiology 1664-302X ISI:000429285900001 10.3389/fmicb.2018.00554 https://www.documentation.ird.fr/hor/fdi:010072713 https://horizon.documentation.ird.fr/exl-doc/pleins_textes/divers18-04/010072713.pdf 9 Horizon (IRD) Nitrogen (N-2) fixation is a major source of nitrogen that supports primary production in the vast oligotrophic areas of the world's oceans. The Western Tropical South Pacific has recently been identified as a hotspot for N-2 fixation. In the Noumea lagoon (New Caledonia), high abundances of the unicellular N-2-fixing cyanobacteria group A (UCYN-A), coupled with daytime N-2 fixation rates associated with the <10 mu m size fraction, suggest UCYN-A may be an important diazotroph (N-2-fixer) in this region. However, little is known about the seasonal variability and diversity of UCYN-A there. To assess this, surface waters from a 12 km transect from the mouth of the Dumbea River to the Dumbea Pass were sampled monthly between July 2012 and March 2014. UCYN-A abundances for two of the defined sublineages, UCYN-A1 and UCYN-A2, were quantified using qPCR targeting the nifH gene, and the nifH-based diversity of UCYN-A was characterized by identifying oligotypes, alternative taxonomic units defined by nucleotide positions with high variability. UCYN-A abundances were dominated by the UCYN-A1 sublineage, peaked in September and October and could be predicted by a suite of nine environmental parameters. At the sublineage level, UCYN-A1 abundances could be predicted based on lower temperatures (<23 degrees C), nitrate concentrations, precipitation, wind speed, while UCYN-A2 abundances could be predicted based on silica, and chlorophyll a concentrations, wind direction, precipitation, and wind speed. Using UCYN-A nifH oligotyping, similar environmental variables explained the relative abundances of sublineages and their associated oligotypes, with the notable exception of the UCYN-A2 oligotype (oligo43) which had relative abundance patterns distinct from the dominant UCYN-A2 oligotype (oligo3). The results support an emerging pattern that UCYN-A is comprised of a diverse group of strains, with sublineages that may have different ecological niches. By identifying environmental factors that influence the composition and abundance of UCYN-A sublineages, this study helps to explain global UCYN-A abundance patterns, and is important for understanding the significance of N2 fixation at local and global scales. 034 ; 036 UR235