Gammaproteobacterial diazotrophs and nifH gene expression in surface waters of the South Pacific Ocean

Abstract

In addition to the cyanobacterial N2-fixers (diazotrophs), there is a high nifH gene diversity of non-cyanobacterial groups present in marine environments, yet quantitative information about these groups is scarce. N2 fixation potential (nifH gene expression), diversity and distributions of the uncultivated diazotroph phylotype γ-24774A11, a putative gammaproteobacterium, were investigated in the western South Pacific Ocean. γ-24774A11 gene copies correlated positively with diazotrophic cyanobacteria, temperature, dissolved organic carbon and ambient O2 saturation, and negatively with depth, chlorophyll a and nutrients, suggesting that carbon supply, access to light or inhibitory effects of DIN may control γ-24774A11 abundances. Maximum nifH gene-copy abundance was 2 × 104 l−1, two orders of magnitude less than that for diazotrophic cyanobacteria, while the median γ-24774A11 abundance, 8 × 102 l−1, was greater than that for the UCYN-A cyanobacteria, suggesting a more homogeneous distribution in surface waters. The abundance of nifH transcripts by γ-24774A11 was greater during the night than during the day, and the transcripts generally ranged from 0–7%, but were up to 26% of all nifH transcripts at each station. The ubiquitous presence and low variability of γ-24774A11 abundances across tropical and subtropical oceans, combined with the consistent nifH expression reported in this study, suggest that γ-24774A11 could be one of the most important heterotrophic (or photoheterotrophic) diazotrophs and may need to be considered in future N budget estimates and models.

High levels of heterogeneity in diazotroph diversity and activity within a putative hotspot for marine nitrogen fixation

“…It must be noted that amplicon sequencing approaches can only reconcile relative abundances and therefore do not allow for the absolute quantification of colonial versus singlecelled diazotrophs and that it is difficult to directly equate diazotroph communities to N 2 fixation activity. However, we identified a range of photoautotrophic, photoheterotrophic and heterotrophic bacteria that share high similarities in nifH sequences to those recovered from similar oceanic environments (for example, Langlois et al, 2005;Moisander et al, 2010;Bombar et al, 2013;Moisander et al, 2014;Thompson et al, 2014).…”

“…Three nifH OTUs were dominant across the data set, comprising 67% of total nifH DNA sequences retrieved. These included OTU6956, which was 100% identical to Trichodesmium erythraeum (IMS 101; hereafter Trichodesmium) in nifH amino acid (aa) composition, OTU6352, which was 100% identical in aa composition to UCYN-A and clustered with the UCYN-A1 ecotype (Supplementary Figure S1), and OTU4713, which shared 91% similarity in aa composition to the γ-proteobacteria Pseudomonas stutzeri, and clustered within the γ-24774A11 clade (Moisander et al, 2008(Moisander et al, , 2014. Despite the ubiquity of these dominant OTUs, significant partitioning of diazotroph population structure was observed between the ATS and Coral Sea (Figure 3; ANOSIM, Global R: 0.471, Po0.001) and between the Austral spring and winter sampling (ANOSIM, Global R: 0.399, Po0.001).…”

“…Indeed, Trichodesmium has long been recognised as an important member of the phytoplankton in this region (Hallegraeff and Jeffrey, 1984;Burford et al, 1995Burford et al, , 2009, and unicellular diazotrophs are assumed to be highly active here as well (Montoya et al, 2004). However, compared with the South Pacific Ocean, where unicellular diazotrophs, including Candidatus Atelocyanobacterium thalassa (UCYN-A), Crocosphaera watsonii and the γ-proteobacterial clade γ-24774A11, are known to be abundant (Moisander et al, 2010(Moisander et al, , 2014, we currently lack any detailed understanding of patterns in the diversity, activity and ecology of diazotrophs within tropical Australian waters.…”

“…N 2 fixation is mediated by a diverse range of microorganisms (Zehr et al, 2003), including the photoautotrophic cyanobacterium Trichodesmium, as well as unicellular cyanobacteria, diatomassociated cyanobacteria and heterotrophic bacteria, all of which are distributed across tropical and subtropical latitudes (Capone et al, 1997(Capone et al, , 2005Montoya et al, 2004;Moisander et al, 2008Moisander et al, , 2010Moisander et al, , 2014Foster et al, 2009).…”

Climate relevant trace gases (N2O and CH4) in the Eurasian Basin (Arctic Ocean)

“…Also, the diazotrophic community may be dominated by heterotrophic bacteria (Farnelid et al, 2011(Farnelid et al, , 2013Moisander et al, 2014;Bentzon-Tilia et al, 2015). In this study, significant N 2 and N 2 O fixation rates were determined in surface waters and in ice brine during the same expedition cruise.…”