LAUSR

Cyanobacterial N2‐fixing microorganisms (diazotrophs) play a critical role in nitrogen and carbon cycling in the oceans; hence, accurate measurements of diazotroph abundance are imperative for understanding ocean biogeochemistry. Marine diazotroph abundances are often assessed using qPCR of the nifH gene, a sensitive, taxa‐specific, and time/cost‐efficient method. However, the validity of nifH abundance as a proxy for cell concentration has recently been questioned. Here, we compare nifH gene abundances to cell counts for four diazotroph taxa (Trichodesmium, Crocosphaera, Richelia, and Calothrix) on two cruises to the North Pacific Subtropical Gyre, one of the largest habitats for marine diazotrophs. nifH:cell relationships were strong and significant for Crocosphaera, Richelia, and Calothrix (nifH:cell 1.51–2.58; R2 = 0.89–0.96) but were not significant for Trichodesmium, despite previous studies reporting significant nifH:cell relationships for this organism. Limited available data suggest that empirical nifH:cell can vary among studies but that relationships are usually significantly linear and >1:1. Our study indicates that nifH gene abundance, while not a direct measure of cells, is a useful quantitative proxy for diazotroph abundance.