Basin scale variability of active diazotrophs and nitrogen fixation in the North Pacific, from the tropics to the subarctic Bering Sea

TY - JOUR

T1 - Basin scale variability of active diazotrophs and nitrogen fixation in the North Pacific, from the tropics to the subarctic Bering Sea

AU - Shiozaki, Takuhei

AU - Bombar, Deniz

AU - Riemann, Lasse

AU - Hashihama, Fuminori

AU - Takeda, Shigenobu

AU - Yamaguchi, Tamaha

AU - Ehama, Makoto

AU - Hamasaki, Koji

AU - Furuya, Ken

PY - 2017/6/1

Y1 - 2017/6/1

N2 - Nitrogen-fixing microorganisms (diazotrophs) provide biologically available nitrogen to plankton communities and thereby greatly influence the productivity in many marine regions. Various cyanobacterial groups have traditionally been considered the major oceanic diazotrophs, but later noncyanobacterial and presumably heterotrophic diazotrophs were also found to be widespread and potentially important in nitrogen fixation. However, the distribution and activity of different diazotroph groups is still poorly constrained for most oceanic ecosystems. Here we examined diazotroph community structure and activity along a 7500 km south-north transect between the central equatorial Pacific and the Bering Sea. Nitrogen fixation contributed up to 84% of new production in the upper waters of the subtropical gyre, where the diazotroph community included the gammaproteobacterium γ-24774A11 and highly active cyanobacterial phylotypes (>50% of total nifH transcript abundance). Nitrogen fixation was sometimes detectable down to 150 m depth and extended horizontally to the edge of the gyre at around 35°N. Nitrogen fixation was even detected far north on the Bering Sea shelf. In the Alaskan Coastal Waters on the Bering Sea shelf, low nitrate together with high dissolved iron concentrations seemed to foster diazotroph growth, including a prominent role of UCYN-A2, which was abundant near the surface (1.2×105 nifH gene copies L−1). Our study provides evidence for nitrogen fixation in the Bering Sea and suggests a clear contrast in the composition of diazotrophs between the tropical/subtropical gyre and the separate waters in the cold northern regions of the North Pacific.

AB - Nitrogen-fixing microorganisms (diazotrophs) provide biologically available nitrogen to plankton communities and thereby greatly influence the productivity in many marine regions. Various cyanobacterial groups have traditionally been considered the major oceanic diazotrophs, but later noncyanobacterial and presumably heterotrophic diazotrophs were also found to be widespread and potentially important in nitrogen fixation. However, the distribution and activity of different diazotroph groups is still poorly constrained for most oceanic ecosystems. Here we examined diazotroph community structure and activity along a 7500 km south-north transect between the central equatorial Pacific and the Bering Sea. Nitrogen fixation contributed up to 84% of new production in the upper waters of the subtropical gyre, where the diazotroph community included the gammaproteobacterium γ-24774A11 and highly active cyanobacterial phylotypes (>50% of total nifH transcript abundance). Nitrogen fixation was sometimes detectable down to 150 m depth and extended horizontally to the edge of the gyre at around 35°N. Nitrogen fixation was even detected far north on the Bering Sea shelf. In the Alaskan Coastal Waters on the Bering Sea shelf, low nitrate together with high dissolved iron concentrations seemed to foster diazotroph growth, including a prominent role of UCYN-A2, which was abundant near the surface (1.2×105 nifH gene copies L−1). Our study provides evidence for nitrogen fixation in the Bering Sea and suggests a clear contrast in the composition of diazotrophs between the tropical/subtropical gyre and the separate waters in the cold northern regions of the North Pacific.

U2 - 10.1002/2017gb005681

DO - 10.1002/2017gb005681

M3 - Journal article

SN - 0886-6236

VL - 31

SP - 996

EP - 1009

JO - Global Biogeochemical Cycles

JF - Global Biogeochemical Cycles

IS - 6

ER -