Gammaproteobacterial diazotrophs and nifH gene expression in surface waters of the South Pacific Ocean.
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 × 10(4) l(-1), two orders of magnitude less than that for diazotrophic cyanobacteria, while the median γ-24774A11 abundance, 8 × 10(2) 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.
Department of Ocean Sciences, University of California Santa Cruz, Santa Cruz, CA, USA.
Pia H
PH
Moisander
Department of Ocean Sciences, University of California Santa Cruz, Santa Cruz, CA, USA.
Tracy
T
Serros
Department of Ocean Sciences, University of California Santa Cruz, Santa Cruz, CA, USA.
Ryan W
RW
Paerl
Department of Ocean Sciences, University of California Santa Cruz, Santa Cruz, CA, USA.
Roxanne A
RA
Beinart
Department of Ocean Sciences, University of California Santa Cruz, Santa Cruz, CA, USA.
Jonathan P
JP
Zehr
D000458:Cyanobacteria / Q000235:genetics; D020563:Gammaproteobacteria / Q000235:genetics* / Q000378:metabolism; D018628:Gene Dosage; D015870:Gene Expression; D009586:Nitrogen Fixation / Q000235:genetics*; D010088:Oxidoreductases / Q000235:genetics* / Q000378:metabolism; D010140:Pacific Ocean; D010802:Phylogeny; D012623:Seawater / Q000382:microbiology*; D013696:Temperature; D014871:Water Microbiology
D010088:Oxidoreductases; C029130:nitrogenase reductase