Distribution and drivers of symbiotic and free-living diazotrophic cyanobacteria in the western tropical South Pacific

Abstract

The abundance and distribution of cyanobacterial diazotrophs were quantified in two regions (Melanesian archipelago, MA; and subtropical gyre, SG) of the western tropical South Pacific using nifH quantitative polymerase chain reaction (qPCR) assays. UCYN-A1 and A2 host populations were quantified using 18S rRNA qPCR assays including one newly developed assay. All phylotypes were detected in the upper photic zone (0-50m), with higher abundances in the MA region. Trichodesmium and UCYN-B dominated and ranged from 2.18 × 102 to 9.41 × 106 and 1.10 × 102 to 2.78 × 106nifH copies L-1, respectively. Het-1 (symbiont of Rhizosolenia diatoms) was the next most abundant (1.40 × 101-1.74 × 105 nifH copiesL-1) and co-occurred with het-2 and het-3. UCYN-A1 and A2 were the least abundant diazotrophs and were below detection (bd) in 63 and 79, respectively, of 120 samples. In addition, in up to 39% of samples in which UCYN-A1 and A2 were detected, their respective hosts were bd. Pairwise comparisons of the nifH abundances and various environmental parameters supported two groups: A deep-dwelling group (45m) comprised of UCYN-A1 and A2 and a surface group (0-15m) comprised of Trichodesmium, het-1 and het-2. Temperature and photosynthetically active radiation were positively correlated with the surface group, while UCYN-A1 and A2 were positively correlated with depth, salinity, and oxygen. Similarly, in a meta-analysis of 11 external datasets, all diazotrophs, except UCYN-A were correlated with temperature. Combined, our results indicate that conditions favoring the UCYN-A symbiosis differ from those of diatom diazotroph associations and free-living cyanobacterial diazotrophs.