UC Santa Cruz

Nitrogen (N2) fixation is a major source of nitrogen that supports primary production in the vast oligotrophic areas of the world’s oceans. The Southwestern Pacific Ocean has been identified as a potential hotspot for N2 fixation based on empirical and modeled data. In the southwest Lagoon of New Caledonia (SLNC), high abundances of the unicellular N2-fixing cyanobacteria group A (UCYN-A), coupled with daytime N2 fixation rates associated with the < 10 µm size fraction, suggest UCYN-A may be an important diazotroph (N2-fixer) in this region. However, little is known about the seasonal variability and diversity of UCYN-A in this region. To assess this, surface waters from a 12 km transect from the mouth of the Dumbéa River to the Dumbéa 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 using oligotyping. Total 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 (<23C), nitrate concentrations, precipitation, and wind speed, while UCYN-A2 abundances could be predicted based on silicate, chlorophyll a concentrations, wind direction, precipitation and wind speed. Using UCYN-A nifH oligotyping, new UCYN-A sublineages were discovered, and similar environmental variables explained the relative abundances of sublineages and their associated oligotypes, with the notable exception of a UCYN-A2 oligotype (oligo43) which had relative abundance patterns distinct from the dominant UCYN-A2 oligotype (oligo3). The results support the emerging picture that UCYN-A is comprised of a diverse group of strains, with sublineages that likely 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.