Distributions and abundances of sublineages of the N-2-fixing cyanobacterium Candidatus Atelocyanobacterium thalassa (UCYN-A) in the New Caledonian coral lagoon

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 (