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. 2016 Dec;5(6):1016-1026.
doi: 10.1002/mbo3.385. Epub 2016 Jun 28.

Possible association of diazotrophs with marine zooplankton in the Pacific Ocean

Kazi Md Azimuddin  1 , Junya Hirai  2 , Shotaro Suzuki  1 , Md Nurul Haider  1 , Aiko Tachibana  1 , Keigo Watanabe  1 , Minoru Kitamura  3 , Fuminori Hashihama  4 , Kazutaka Takahashi  5 , Koji Hamasaki  1
Affiliations

Possible association of diazotrophs with marine zooplankton in the Pacific Ocean

Kazi Md Azimuddin et al. Microbiologyopen. 2016 Dec.

Abstract

Dinitrogen fixation, the biological reduction in N2 gas to ammonia contributes to the supply of new nitrogen in the surface ocean. To understand the diversity and abundance of potentially diazotrophic (N2 fixing) microorganisms associated with marine zooplankton, especially copepods, the nifH gene was studied using zooplankton samples collected in the Pacific Ocean. In total, 257 nifH sequences were recovered from 23 nifH-positive DNA extracts out of 90 copepod samples. The nifH genes derived from cyanobacteria related to Trichodesmium, α- and γ-subdivisions of proteobacteria, and anaerobic euryarchaeota related to Methanosaeta concilii were detected. Our results indicated that Pleuromamma, Pontella, and Euchaeta were the major copepod genera hosting dinitrogen fixers, though we found no species-specific association between copepods and dinitrogen fixers. Also, the digital PCR provided novel data on the number of copies of the nifH gene in individual copepods, which we report the range from 30 to 1666 copies per copepod. This study is the first systematic study of zooplankton-associated diazotrophs, covering a large area of the open ocean, which provide a clue to further study of a possible new hotspot of N2 fixation.

Keywords: nifH; Marine; Pacific Ocean; microorganisms; zooplankton.

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Figures

Figure 1
Figure 1
Zooplankton sampling locations 2011–2014: values (in parentheses) indicate the number of nifH‐positive zooplankton samples over the total number of samples analyzed at each site.
Figure 2
Figure 2
Phylogenetic tree constructed based on analysis of 257 nifH‐translated amino acid sequences using neighbor‐joining method. Bold type denotes sequences in this study (sequences with 100% similarity are grouped); boldface numbers (in parentheses) represent the number of retrieved clones from each copepod. Bootstrap values (>50%) are indicated at branch points; scale bars are estimated sequence divergences (10%); and P, phylotype.
Figure 3
Figure 3
(A) Comparison of the nifH abundance of individual copepods (no nifH gene was detected from sample KT‐8). (B) Comparison of the average nifH copies of three copepod genera. SC, Scolecithricella; LB, Labidocera. Error bars indicate standard deviation.
Figure 4
Figure 4
Average nifH copy number from individual copepods. Numerical values except 1666 and 1278 represent average nifH copy numbers determined by ddPCR. 1666 and 1278 are from single measurement each.
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References

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