Abstract
Cyanobacteria are thought to be the main N
2
-fixing organisms
(diazotrophs) in marine pelagic waters, but recent molecular analyses indicate
that non-cyanobacterial diazotrophs are also present and active. Existing data
are, however, restricted geographically and by limited sequencing depths. Our
analysis of 79,090 nitrogenase (
nifH
) PCR amplicons encoding
7,468 unique proteins from surface samples (ten DNA samples and two RNA samples)
collected at ten marine locations world-wide provides the first in-depth survey
of a functional bacterial gene and yield insights into the composition and
diversity of the
nifH
gene pool in marine waters. Great
divergence in
nifH
composition was observed between sites.
Cyanobacteria-like genes were most frequent among amplicons from the warmest
waters, but overall the data set was dominated by
nifH
sequences most closely related to non-cyanobacteria. Clusters related to Alpha-,
Beta-, Gamma-, and Delta-Proteobacteria were most common and showed distinct
geographic distributions. Sequences related to anaerobic bacteria
(
nifH
Cluster III) were generally rare, but preponderant in
cold waters, especially in the Arctic. Although the two transcript samples were
dominated by unicellular cyanobacteria, 42% of the identified
non-cyanobacterial
nifH
clusters from the corresponding DNA
samples were also detected in cDNA. The study indicates that non-cyanobacteria
account for a substantial part of the
nifH
gene pool in marine
surface waters and that these genes are at least occasionally expressed. The
contribution of non-cyanobacterial diazotrophs to the global N
2
fixation budget cannot be inferred from sequence data alone, but the prevalence
of non-cyanobacterial
nifH
genes and transcripts suggest that
these bacteria are ecologically significant.