The relative contributions of unicellular and filamentous diazotrophs to N2 fixation in the South China Sea and the upstream Kuroshio

• Published in: Deep-sea research. Part I, Oceanographic research papers Oceanographic research papers, 2014-03, Vol.85, p.56-71
• Main Authors: Lee Chen, Yuh-ling, Chen, Houng-Yung, Lin, Yen-Huei, Yong, Tze-Ching, Taniuchi, Yukiko, Tuo, Sing-how
• Format: Article
• Language: English
• Subjects: Animal and plant ecology; Animal, plant and microbial ecology; Bacteria; Biological and medical sciences; Correlation analysis; Filamentous diazotroph; Fundamental and applied biological sciences. Psychology; Kuroshio; N2-fixation rate; Nitrogen; Oceanography; Plankton; Sea water ecosystems; Size fraction; South China Sea; Synecology; Unicellular diazotroph
• ISSN: 0967-0637; 1879-0119
• DOI: 10.1016/j.dsr.2013.11.006

We studied the seasonal, diel, and vertical distribution of phytoplankton N2 fixation to understand the relative contributions of unicellular and filamentous nitrogen fixers (diazotrophs) to N2 fixation and nitrogen recycling in the northern South China Sea (SCS) and the neighboring upstream Kuroshio. N2-fixation rates were measured by the 15N2 tracer technique (addition by bubble) on unicellular (10 or 20µm, mostly Trichodesmium and Richelia) fractionated by 10- or 20-µm mesh sizes. The mean depth-integrated total (unicellular+filamentous) N2-fixation rates in the SCS (51.7±6.2µmolNm−2d−1) averaged 1/3 of that in the Kuroshio (142.7±29.6µmolNm−2d−1), with higher rates in the winter than in other seasons in the SCS and the opposite seasonal pattern in the Kuroshio. Unicellular diazotrophs contributed 65% of the total N2 fixation in the SCS, which were negatively correlated with surface temperature and, as for total N2 fixation, were higher in the winter when Trichodesmium spp. were scarce. In comparison, the unicellular diazotrophs contributed 50% of total N2 fixation in the Kuroshio, and their contributions were not significantly correlated with surface temperature. In both the SCS and the Kuroshio, the unicellular N2 fixation was more important during the night than during the day, and in the deep euphotic layer than in the surface layer, even in the daytime. Our results show that the unicellular diazotrophs were important N2 fixers and contributed significantly to N2 fixation in the tropical marginal seas, more so in the SCS than the Kuroshio. •Unicellular diazotrophs is the dominated N2 fixers in the South China Sea.•Unicellular diazotrophs' N2 fixation was important in the deep euphotic layer.•Nighttime N2 fixation by unicellular diazotrophs was more important than the daytime.