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Abstract. The abundance and distribution of cyanobacterial diazotrophs were quantifiedin two regions (Melanesian archipelago, MA; and subtropical gyre, SG) of thewestern tropical South Pacific using nifH quantitative polymerasechain reaction (qPCR) assays. UCYN-A1 and A2 host populations were quantifiedusing 18S rRNA qPCR assays including one newly developed assay. Allphylotypes were detected in the upper photic zone (0–50 m), with higherabundances in the MA region. Trichodesmium and UCYN-B dominated andranged from 2.18  ×  10 2 to 9.41  ×  10 6 and1.10  ×  10 2 to 2.78  ×  10 6  nifH copiesL −1 , respectively. Het-1 (symbiont of Rhizosolenia diatoms) wasthe next most abundant (1.40  ×  10 1 –1.74  ×  10 5 nifH copies L −1) and co-occurred with het-2 and het-3. UCYN-A1and A2 were the least abundant diazotrophs and were below detection (bd) in63 and 79, respectively, of 120 samples. In addition, in up to 39 % ofsamples in which UCYN-A1 and A2 were detected, their respective hosts werebd.Pairwise comparisons of the nifH abundances and variousenvironmental parameters supported two groups: a deep-dwelling group (45 m)comprised of UCYN-A1 and A2 and a surface group (0–15 m) comprised ofTrichodesmium, het-1 and het-2. Temperature and photosyntheticallyactive radiation were positively correlated with the surface group, whileUCYN-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 resultsindicate that conditions favoring the UCYN-A symbiosis differ from those ofdiatom diazotroph associations and free-living cyanobacterial diazotrophs.

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