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N2 fixation rate and diazotroph community structure in the western tropical North Pacific Ocean
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Run Zhang1, *, Dongsheng Zhang2, Min Chen1, Zhibing Jiang2, Chunsheng Wang2, *, Minfang Zheng1, Yusheng Qiu1, Jie Huang3
Acta Oceanologica Sinica | 2019, 38(12) : 26 - 34
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Acta Oceanologica Sinica | 2019, 38(12): 26-34
• Marine Chemistry •
N2 fixation rate and diazotroph community structure in the western tropical North Pacific Ocean
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Run Zhang1, *, Dongsheng Zhang2, Min Chen1, Zhibing Jiang2, Chunsheng Wang2, *, Minfang Zheng1, Yusheng Qiu1, Jie Huang3
Affiliations
  • 1 College of Ocean and Earth Sciences, Xiamen University, Xiamen 361102, China
  • 2 Key Laboratory of Marine Ecosystem and Biogeochemistry, State Oceanic Administration/Second Institute of Oceanography, Ministry of Natural Resources, Hangzhou 310012, China
  • 3 Ministry of Education Key Laboratory for Earth System Modeling/Department of Earth System Science, Tsinghua University, Beijing 100084, China
Published: 2019-12-25 doi: 10.1007/s13131-019-1513-4
Outline
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In the present study, we report N2 fixation rate (15N isotope tracer assay) and the diazotroph community structure (using the molecular method) in the western tropical North Pacific Ocean (WTNP) (13°–20°N, 120°–160°E). Our independent evidence on the basis of both in situ N2 fixation activity and diazotroph community structure showed the dominance of unicellular N2 fixation over majority of the WTNP surface waters during the sampling periods. Moreover, a shift in the diazotrophic composition from unicellular cyanobacteria group B-dominated to Trichodesmium spp.-dominated toward the western boundary current (Kuroshio) was also observed in 2013. We hypothesize that nutrient availability may have played a major role in regulating the biogeography of N2 fixation. In surface waters, volumetric N2 fixation rate (calculated by nitrogen) ranged between 0.6 and 2.6 nmol/(L·d) and averaged (1.2±0.5) nmol/(L·d), with <10 μm size fraction contributed predominantly (88%±6%) to the total rate between 135°E and 160°E. Depth-integrated N2 fixation rate over the upper 200 m ranged between 150 μmol/(m2·d) and 480 μmol/(m2·d) (average (225±105) μmol/(m2·d). N2 fixation can account for 6.2%±3.7% of the depth-integrated primary production, suggesting that N2 fixation is a significant N source sustaining new and export production in the WTNP. The role of N2 fixation in biogeochemical cycling in this climate change-vulnerable region calls for further investigations.

western tropical North Pacific Ocean (WTNP)  /  N2 fixation  /  15N isotope tracer assay  /  unicellular diazotroph
Run Zhang, Dongsheng Zhang, Min Chen, Zhibing Jiang, Chunsheng Wang, Minfang Zheng, Yusheng Qiu, Jie Huang. N2 fixation rate and diazotroph community structure in the western tropical North Pacific Ocean[J]. Acta Oceanologica Sinica, 2019 , 38 (12) : 26 -34 . DOI: 10.1007/s13131-019-1513-4
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1 Introduction
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N2 fixation plays a major role in adding biologically essential N nutrients and drive biogeochemical cycles for the oligotrophic tropical/subtropical oceans (Gruber and Sarmiento, 1997; Karl et al., 2002; Deutsch et al., 2007). The western tropical North Pacific Ocean (WTNP) is a region of significant importance to global climate system and fisheries wherein material and heat exchanges take place actively (Lehodey et al., 1997; Carpenter, 1998; Clement and DiNezio, 2014). The WTNP also seems as a favorable environment for N2 fixation as it is characterized by low nutrient low chlorophyll (LNLC) conditions in the permanently stratified upper water column (Cabrera et al., 2015). However, our knowledge of N2 fixation is not abundant in the WTNP in general.
Available N2 fixation data are few and directly measured rates are far from abundant in the WTNP (Kitajima et al., 2009; Shiozaki et al., 2013; Chen et al., 2014; Kim et al., 2017). At a fixed station (13°N, 135°E) in the Western Pacific Warm Pool, 15N2 tracer assay based N2 fixation rate fell in a range of 1–15 μmol/(m3·d) for the upper water column, likely suggesting that the Western Pacific Warm Pool, and probably the adjacent waters also, is suitable for N2 fixation (Shiozaki et al., 2013). In general, large spatial scale sampling spanning the WTNP is lacking to date. Besides the N2 fixation rate, the diazotroph community structure is a key variable factor when looking into the dynamics of N2 fixation and evaluating its biogeochemical importance in the context of the ongoing rapid climatic changes (Church et al., 2009; Fu et al., 2014). The filamentous cyanobacterium Trichodesmium (Capone et al., 1997) and heterocystous endosymbiont Richelia (Carpenter et al., 1999) had traditionally been considered as the dominant oceanic N2 fixers. Recently, using molecular approaches targeting the nifH gene, marine diazotrophs including unicellular cyanobacteria (groups UCYN-A, -B, -C) and non-cyanobacterial diazotrophs (bacteria and archaea) are widely distributed and actively fix N2 in global oceans (Zehr et al., 2001; Church et al., 2008; Shiozaki et al., 2014). Studies have also indicated that the UCYN-A (Atelocyanobacterium) and UCYN-B (Crocosphaera) may equally contribute to or even exceed the amount of N2 fixed by Trichodesmium in some regions in the oligotrophic Pacific Ocean (Zehr et al., 2001; Montoya et al., 2004). Diazotroph community structure and activity along a long south-north transect between the central equatorial Pacific and the Bering Sea had been examined, revealing large latitudinal variability in the western Pacific (Shiozaki et al., 2017). However, the composition and zonal pattern of diazotrophic groups and their relative contribution to N2 fixation contribution in the WTNP remain largely unknown.
In the present study, large spatial range sampling in the WTNP were conducted to (1) measure the N2 fixation rate; (2) examine the bulk N2 fixation rate and of two size fractions (10 μm as a category); and (3) diazotroph community structure, which is vital to understanding N2 fixation dynamics and its biogeochemical impact in the WTNP. Our results will add knowledge concerning N2 fixation in the undersampled WTNP and provide insights to understand C and N dynamics in the WTNP, which is now experiencing significant environmental changes (Cravatte et al., 2009).
2 Materials and methods
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2.1 Oceanographic setting
The westward-flowing North Equatorial Current (NEC) is a prevailing surface current in the sampling area (13°–20°N) of the WTNP, and plays a vital role in the North Pacific circulation as it partitions the flow to the subtropical gyre to the north and the tropical gyre to the south (Qiu et al., 2015). The NEC bifurcates at ~13°N when meeting the western boundary and flows northward as the Kuroshio and southward as the Mindanao Current (Qiu et al., 2015) (Fig. 1). Outside the sampling area, surface currents also include the eastward North Equatorial Countercurrent (NECC) in the south and the eastward North Pacific Subtropical Countercurrent (STCC) in the north. The WTNP is featured by strong light radiation. Due to surface stratification over almost the entire year, the WTNP is oligotrophic with low levels of nutrients and chlorophyll a (Cabrera et al., 2015).
2.2 Seawater sampling
A total of 38 stations were sampled on cruises DY27-1 (8–26 July 2012), DY27-2 (20–22 August 2012), DY27-3 (8–19 September 2012), and west–east DY29-1 (30 May–4 June 2013) onboard the R/V Haiyang Liuhao in the WTNP, covering a spatial range ~14°–20°N, 120°–160°E (Fig. 1). Seawater samples were collected from 0, 30, 50, 75, 100, 150, 200 m using a Niskin bottle rosette sampler attached with conductivity-temperature-depth (CTD) sensors (SBE 917Plus, Sea-Bird) or an acid-cleaned bucket. Ocean Data View was used to draw sampling maps.
2.3 Environmental variables
The chlorophyll a concentration was determined using a Turner Trilogy fluorometer while on the sea. Macro-nutrients were measured after conventional protocols (Hansen and Koroleff, 1999). Nitrate and nitrite were measured on a SKALAR SAN++ flow injection autoanalyzer after reduction using the cadmium–copper reduction method. The detection limit for nitrate analysis was 0.2 μmol/L in the range of 0.2–10 μmol/L. Phosphate was determined by the standard molybdenum blue method with a detection limit of 0.02 μmol/L.
2.4 Nitrogen isotopic ratio of suspended particulate organic matter (δ15NPN)
Bulk suspended particulate organic nitrogen (PN) for measuring natural 15N abundance was collected from 0, 30, 50, 75, 100, 150, 200 m by filtering ~10 L of seawater onto precombusted (450°C, 4 h) GF/F glass fiber filters. The filters containing POM were stored frozen (–20°C) while on the sea. PN content and 15N abundance were measured on a Finnigan MAT DELTAplus XP isotope ratio mass spectrometer that was coupled to a Carlo Erba NC2500 elemental analyzer (EA-IRMS). Nitrogen isotopic composition was presented in the conventional δ notation as follows:
${\delta ^{15}}{\rm{N}} = \left(\frac{{{{{R}}_{{\rm{sample}}}}}}{{{{{R}}_{ {\rm{standard}}}}}} - 1\right) \times 1\;000,$
where Rsample and Rstandard are the 15N/14N ratios of the sample and the standard air N2, respectively. International isotope reference materials (IAEA-N-2 and USGS35) were used for calibration. The reproducibility of δ15N measurements was better than 0.2‰. To measure size fractionated δ15NPN, ~25 L of surface seawater was prefiltered through a Millipore polycarbonate (pore size 10 μm) membrane and then by a GF/F membrane at seven stations. The particles collected on the GF/F filter are of the <10 μm size class. Particles collected on the prefilter were washed onto a GF/F filter and are thus of the >10 μm size class. The filter samples were frozen while on the sea. The validation of the size fractionation experiment was conducted at seven stations (see Supplementary Information).
2.5 N2 fixation rate (NFR)
Vertical samples for incubation of N2 fixation and primary production were collected at nine stations down to 200 m during the 2012 cruise. N2 fixation rates were measured using 15N2 tracer assay (Montoya et al., 1996). The technique is the same as Zhang et al. (2015). Briefly, the duplicate seawater samples in clear transparent glass bottles were amended with 15N2 (2 mL 15N2 per L seawater) and fitted with screens to simulate light intensities (0 m: 100%; 25 m: 50%; 50 m: 10%; 100 m: 1%; 150 m: 1%; 200 m: 0.1%), and then placed in a deck incubator with flowing surface seawater for 24 h. The depth-integrated rate was obtained after trapezoidal integration. The rate obtained using this method may represent underestimates due to the insufficient dissolution of 15N2 gas (Mohr et al., 2010). However, we suggest that this factor did not undermine our discussion, mainly the degree of underestimation of absolute rates will be significantly lowered after 24 h incubation (Klawonn et al., 2015; Wannicke et al., 2018). Second, the size characteristics of NFR will be slightly affected compared to that of the absolute NFR.
Post-incubation NFR size fractionation (Bonnet et al., 2009) was conducted at seven stations. Each incubated sample was prefiltered through a Millipore polycarbonate filter (pore size 10 μm) and then by a precombusted glass fiber filter. The particles collected on the GF/F filter are of the <10 μm size class. The particles collected on the polycarbonate prefilters were transferred to another glass fiber filter and thus are of the >10 μm size class. The size fractionated samples were frozen while on the sea. N2 fixation samples were also measured on EA-IRMS for PN content and 15N abundance. The contribution of each size fraction to the total NFR was estimated subsequently.
2.6 Nitrogenase gene (nifH) analysis-derived diazotroph community structure
During the 2013 cruise, diazotroph community structure at the genus level was examined based on nitrogenase (nifH) relative abundance at 11 stations (P1–P11) along a meridional section (125°–155°E). Surface (~0 m) seawater samples were collected at 10 stations (P1–P10), while depth profile down to 100 m (25, 50, 75 and 100 m) was sampled at the easternmost Sta. P11. For each sample, ~4 L of seawater was filtered onto a polycarbonate filter (0.2 μm poresize, Millipore) after being prefiltered through a 200 μm mesh size silk and then frozen immediately in liquid N2 until extraction in the land laboratory. The filters were cut into small pieces with pre-sterilized scissors and then transferred into tubes. Then, the total DNA was extracted using a Fast DNA Spin kit (MP Biomedicals, USA) based on the manufacturer’s protocols.
The nitrogenase gene (nifH) was amplified using a nested Polymerase Chain Reaction (PCR) protocol (Zehr and Turner, 2001). The reaction mixture (50 µL final volume) contained 5 µL PCR 10× buffer (TaKaRa, China), 8 µL of a 25 mmol/L MgCl2 solution, 8 µL of 200 mmol/L dNTPs (TaKaRa, China), 0.5 µL of each primer (100 µmol/L, Sangon Biotech, China), 0.5 µL of Taq DNA polymerase (TaKaRa, China), 2 µL template, and 25.5 µL of deionized water. For each DNA extraction sample, triplicate PCR reactions were conducted with one additional negative control. Primers used in the first-round PCR reaction were nifH3 (5′-TTYTAYGGNAARGGNGG-3′) and nifH4 (5′-ATRTTRTTNGCNGCRTA-3′). In the second-round PCR reaction, nifH1 and nifH2 were used with adaptors and sample specific barcodes added (see Supplementary Information). PCR products of each sample were run on a 1% agarose gel and amplified nifH fragments were excised and purified with a QIAquick Gel extraction kit (Qiagen, USA). None of the 11 negative controls resulted in a visible band when run on a gel. Illumina sequencing was performed using a Miseq PE300 kit (Shanghai Majorbio Bio-Pharm Biotechnology Co. Ltd., Shanghai, China). The purified PCR products with specific barcodes were quantified using a QuantiFluorTM-ST (Promega).
Pair-end (PE) reads data (raw data) were obtained from Miseq sequencing. The quality control was conducted using Trimmomatic. Bases in the reads end were filtered if the quality value below 20. After this quality control step, the reads with length below 50 bp were removed. The filtered PE reads merged into one sequence, the minimum overlap length was 10 bp. The different samples were separated according to the adaptors and tags, and in the meanwhile the forward-reverse orientation of sequences was adjusted. The adaptors and tags needed to be totally identity and the maximum mismatched base numbers of primers were 2 bp. In total, 554 564 sequences were obtained from 14 samples (Table S1). The trimmed sequences have been deposited in the National Center for Biotechnology Information (NCBI) Sequence Read Archive under accession number SRP 153943.
The unique tags were extracted from the trimmed sequences. The singletons were removed from the unique tags. Then these unique tags were clustered as representative sequences of OTUs by Usearch 7.1 with cutoff value 0.97. The chimera was removed during the clustering process. All unique tags were mapped to the representative sequences of OUTs. The representative sequences of OTUs were assigned to the taxonomy classification by RDP classifier (Wang et al., 2007) within Qiime platform (Caporaso et al., 2010) with FGR database (Fish et al., 2013). The threshold was set as 0.7. According to the taxa classification information, all sequences were regrouped into several nifH clusters as follows, those belonging to Cyanobacteria were split into Trichodesmium, UCYN-B (Crocosphaera) and other cyanobacteria, the other sequences were stayed in class level including Alpha-proteobacterial, Beta-proteobacteria, Delta-proteobacteria, Gamma-proteobacterial, Proteobacteria unclassified, Clostridia and others. The regrouped sequences were used to analyze the community composition of each sample.
2.7 Primary production
In the 2012 cruises, primary production (PP) was measured via a 14C tracer assay (Wolfe and Schelske, 1967). Surface (~0 m) samples were collected at 19 stations and depth profiles were collected at three stations (135°, 150°, 155°E). Briefly, 100 mL of seawater was filled into three acid-cleaned transparent polycarbonate bottles (one dark+two light) immediately after collection, and 0.8 μCi NaH14CO3 was then added to each bottle. The bottles were placed in a deck incubator with flowing surface seawater for 24 h. At the end of incubation, each sample was gently (<100 mm Hg) filtered using 0.2 μm-pore size mixed cellulose ester membranes and stored frozen (20°C) while on board. Before analysis, the sample filters were acid fumigated to remove inorganic carbon. The incorporated 14C radioactivity was then measured on a liquid scintillation counter (Perkin-Elmer TriCarb 2900TR). The PP rates (μmol/(L·d), calculate by carbon) were calculated as follows:
${{PP}} = \frac{{({{{R}}_{\rm{S}}} - {{{R}}_{\rm{B}}}) \times c_{{\rm{TCO}}_2}}}{{{{R}} \times \Delta {{t}}}},$
where RS and RB are the radioactivities of 14C (μCi) in light and dark bottles after correction for quenching, respectively. R and $c_{{\rm{TCO}}_2} $ are the added radioactivity of NaH14CO3 and the total carbon dioxide (μmol/L, calculated by carbon) in seawater, respectively. To estimate the depth-integrated PP (mmol/(m2·d)), trapezoidal integration was used.
3 Results
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3.1 Environmental variables
LNLC conditions were observed during sampling (Figs 1 and 2a). Sea surface temperature (SST) generally exceeded 28°C. Not surprisingly, dissolved inorganic nitrogen (nitrate+nitrite) was generally low in the upper water column (100 m) (Fig. 3f). Moreover, low concentrations of soluble reactive phosphorus were observed (Fig. 3g). The shipboard-measured chlorophyll a concentration was generally <0.05 mg/m3 in surface waters. Along the meridional section (PO section, ~125°–150°E) sampled in September 2012, relatively higher surface chlorophyll a was observed for the western stations located in the Philippine Sea. The depth wherein subsurface chlorophyll maximum (SCM) was present was found to be 75–125 m along the PO section, with an increasing trend westward (Fig. 3e).
3.2 Natural δ15NPN
Nitrogen isotopic ratios (δ15N) in natural suspended particulate nitrogen (PN) varied from −1.6‰ to +4.2‰ ((1.9±1.3)‰, n=27) in surface waters (Fig. 2b). Size fractionation experiment of surface suspended particulate organic matter (POM) showed that δ15N in <10 μm PN was significantly lower than that of >10 μm PN ((1.7±1.3)‰ vs (5.1±3.2)‰; n=7, p<0.05, t-test).
3.3 N2 fixation rate (NFR)
N2 fixation activity was evident by the 15N2 tracer assay, which caused bulk δ15NPN when terminating the incubation at no lesser than 35‰ for surface water samples. The surface N2 fixation rate ranged between 0.6 and 2.6 nmol/(L·d) and averaged at (1.2±0.5) nmol/(L·d) (n=27, Fig. 2c). For the size fractionated N2 fixation rate, <10 μm size fraction contributed predominantly (88%±6%, n=7; Fig. 4a) to the total rate between 135°E and 160°E. NFR seems to be higher at the subsurface (Fig. 3d). Depth-integrated N2 fixation rate fell within a range between 150 and 480 μmol/(m2·d) ((225±105) μmol/(m2·d), n=9; Fig. 2d).
3.4 Diazotroph community structure
In 2013, UCYN-B comprised ~100% of the diazotroph community at the genus level based on the nifH gene abundance at most stations (9 of 11 stations visited). However, for the two stations at the western end of the sampling area, the relative abundance of Trichodesmium increased from ~8% at Sta. P2 to 96.8% at the westernmost Sta. P1 (Fig. 4b). A decreasing relative abundance of UCYN-B with increasing depth was observed at Sta. P11 (Fig. 4c).
3.5 PP
Surface water 14C uptake derived PP averaged at (0.20±0.14) μmol/(L·d) over the study area, with the highest value (0.57 μmol/(L·d) being encountered at the westernmost station in the Luzon Strait (Fig. 2d). Depth-integrated PP averaged at (33.0±16.9) mmol/(m2·d) (n=3).
4 Discussion
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4.1 Zonal variability of the diazotroph community structure in the WTNP
This is the first time that a large-scale zonal distribution of the diazotroph community structure has been reported in the WTNP to the best of our knowledge. We found a clear longitudinal shift in 2013 in the diazotroph community structure from UCYN-B, which dominate over most of the WTNP, to the other well-known keystone diazotrophic cyanobacteria, Trichodesmium spp., that dominate in the vicinity of the western boundary current (Kuroshio) observed in this study (Fig. 4b). Consistently, a previous study in the adjacent western Pacific warm pool (13.5°N, 136°E) found high abundance of unicellular diazotrophic cyanobacteria Crocosphaera watsonii (Kim et al., 2017). UCYN-B are restricted to warm tropical waters and high abundance occurred in upper water column (as shown at station P11) which positively correlated to the temperature in the open ocean (Church et al., 2008; Shiozaki et al., 2014). Our finding is also consistent with previous observations of relatively abundant Trichodesmium in the Kuroshio Current, especially after its contact with the island mass along its main path (Zhang et al., 2012; Zhang et al., 2014; Shiozaki et al., 2014; Jiang et al., 2018). Our results along with available studies from other oceanic regions suggest that a spatial niche partitioning between diazotroph groups may be a common feature in the western tropical Pacific, with nano diazotrophs predominating the remote oceanic waters and micro Trichodesmium spp. usually predominating the waters close to large land mass, as in the western tropical South Pacific (WTSP) (Bonnet et al., 2009; Shiozaki et al., 2014). The diazotroph community structure is a key variable when looking into the dynamics of N2 fixation and evaluating its biogeochemical importance in the context of the ongoing rapid climatic changes, thus needs to be further investigated in future studies in the WTNP (Church et al., 2009; Fu et al., 2014).
We propose that the zonal pattern of the diazotroph community structure and size fractionation of NFR may be tightly related to shift in environmental conditions in the WTNP. For instance, as a key limiting element for N2 fixation, iron (Fe) variability is largely regulated both by the surface current (NEC) and atmospheric deposition (Jickells and Moore, 2015). In the study area, the prevailing westward-flowing NEC transports the most oligotrophic surface waters to the west, while it becomes more productive after receiving more nutrients (from aeolian input) westward. This is evidenced by a gradual eastward decrease in surface chlorophyll a (Figs 1 and 2a) and shoaling of SCM depth (Fig. 3e) along the NEC flow pathway. Although no concurrent dissolved iron (dFe) measurements in this study are available and published dFe data in surface waters of the WTNP are also lacking as well, study in the subtropical western North Pacific showed dFe generally <0.2 nmol/L in the surface waters (Brown et al., 2005), indicating a severe Fe deficiency in the remote WTNP waters as well. There is evidence suggesting a better adaptation for UCYN-B to poor Fe environments (especially for the more remote waters) than filamentous Trichodesmium (Saito et al., 2011; Jacq et al., 2014). Consistently, relatively active N2 fixation can take place even over the sampled WTNP (Figs 3d and 4b). In contrast, the phosphorus scarcity, which is usually observed in the warm open oceanic waters, may be alleviated by diazotrophs via dissolved organic phosphorus (DOP) utilization (Dyhrman and Haley, 2006; Dyhrman et al., 2006). Consistently, macro-scale exhaustion of surface phosphate along with relatively active N2 fixation has been observed in the western Pacific Ocean (Hashihama et al., 2009), probably indicating the complementary source of P (such as utilization of DOP) by N2 fixers. In any case, the exact causes for the shift in diazotroph community structure and controls on N2 fixation characteristics need to be investigated intensively in the WTNP.
4.2 N2 fixation as a new N source to the WTNP ecosystem
N2 fixation rates in the present study generally fall in the range reported in the adjacent Western Pacific Warm Pool (Shiozaki et al., 2017) and Sta. ALOHA in the NPSG (Böttjer et al., 2017). Depth-integrated N2 fixation rate in 2012 ((271.7±180.4) μmol/(m2·d), n=3) are generally higher than the reported total atmospheric N deposition flux of 26 μmol/(m2·d) in the western Pacific Ocean (Martino et al., 2014). Our concurrent measurements in 2012 show that depth-integrated N2 fixation accounts for 6.2%±3.7% of the depth-integrated PP ((33.0±16.9) mmol/(m2·d), n=3) after Redfield stoichiometry (C:N=106:16), which is similar to the surface ratio (5.9%±3.9%). If we adopt the typical f-ratio (=nitrate assimilation/PP) value range (0.05–0.20) reported in the adjacent Western Pacific Warm Pool with a similar LNLC condition (Shiozaki et al., 2013), we estimate that N2 fixation may represent a new N flux comparable (30% or higher) to that of nitrate assimilation. The oligotrophic oceanic system, such as the Western Pacific Warm Pool, is generally featured by rapid recycling (low f-ratios) along with active N2 fixation (Shiozaki et al., 2013), thereby suggesting that the contribution of N2 fixation in local C and N cycles probably exceeds such NFR/PP ratios in the WTNP.
The influence of N2 fixation to local biogeochemistry is reflected also by the relatively low δ15NPN values (1.9‰±1.3‰ for surface PN), especially for the western part of the study area (Fig. 2b). Although both N2 fixation and atmospheric deposition can input isotopically light N to the surface ocean (Zhang et al., 2011; Ren et al., 2017), the latter is less likely a dominant factor controlling δ15NPN distribution in this study. As our results show that the stations closer to the major land mass, which are expected to receive more aeolian N, are actually featured by much higher (rather than lower) δ15NPN values (Fig. 2b). Hence, we suggest that N2 fixation is the main factor for the observed spatial variation of δ15NPN values. Consistently, low δ15N values were also observed in the suspended particulate organic matter during 2014 (Yang et al., 2017) and sediment trap samples in the WTNP (Kim et al., 2017), and N2 fixation has been speculated to be the main reason. N2 fixation derived fresh organic matter is featured by relatively 15N-depleted isotope ratios, ~1‰, because the substrate for this pathway (atmospheric N2 δ15N=0‰) is depleted in 15N and an insignificant amount of isotope fractionation takes place during this process (Carpenter et al., 1997).
The WTNP is a region vulnerable to climate variability and the main forcing mechanisms include warming, freshening, and changed atmospheric dust deposition (Cravatte et al., 2009; Kodama et al., 2011; Jickells and Moore, 2015). Therefore, how N2 fixation and relevant biological pump processes in the WTNP may respond to climatic changes needs to be addressed in future studies (Kim et al., 2017).
5 Conclusions
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Our results showed that unicellular fraction is the dominant contributor to N2 fixation during sampling periods in summers of 2012 and 2013 in most of the sampling area, whereas micro-diazotroph Trichodesmium dominate in the west end (Kuroshio). Such zonal variations of diazotroph community structure may be a complex result of shift in environmental conditions in the sub-regions of the WTNP, i.e., NEC transporting most oligotrophic waters westward while atmospheric dust deposition increase on the other hand. N2 fixation acts as a significant exogenous new N source and plays an important in driving biogeochemical cycles in the WTNP, wherein enhanced environmental changes occur in the context of climatic changes.
Acknowledgements
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We are grateful to China Ocean Mineral Resources R&D Association (COMRA) for arranging the cruise and providing background data. We thank the captain and crew of the R/V Haiyang Liuhao for their invaluable assistance while sampling on the sea. We thank Xiguang Deng, Binbin Guo, Youcheng Bai, Xinyu Xu for technical assistance. Thanks are also due to COMRA scientists, Jianfang Chen, Zhi Yang and Wei Zhuang, for helpful discussions. We also thank the invaluable comment by anonymous reviewers.
Funding
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  • The National Basic Research Program of China under contract No. 2015CB452903; the Foundation of China Ocean Mineral Resources R&D Association under contract No. DY135-E2-2-03; the Science and Technology Basic Resources Investigation Program of China under contract No. 2017FY201403; the National Natural Science Foundation of China under contract Nos 41676174, 41206104 and 41876198.
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Appendix
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Year 2019 volume 38 Issue 12
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Article Info
doi: 10.1007/s13131-019-1513-4
  • Receive Date：2018-12-28
  • Online Date：2026-04-01
  • Published：2019-12-25
Article Data
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History
  • Received：2018-12-28
  • Accepted：2019-02-19
Funding
The National Basic Research Program of China under contract No. 2015CB452903; the Foundation of China Ocean Mineral Resources R&D Association under contract No. DY135-E2-2-03; the Science and Technology Basic Resources Investigation Program of China under contract No. 2017FY201403; the National Natural Science Foundation of China under contract Nos 41676174, 41206104 and 41876198.
Affiliations
    1 College of Ocean and Earth Sciences, Xiamen University, Xiamen 361102, China
    2 Key Laboratory of Marine Ecosystem and Biogeochemistry, State Oceanic Administration/Second Institute of Oceanography, Ministry of Natural Resources, Hangzhou 310012, China
    3 Ministry of Education Key Laboratory for Earth System Modeling/Department of Earth System Science, Tsinghua University, Beijing 100084, China

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表12种不同金属材料的力学参数
科
Family		 属数
Number of
genus		 种数
Number of
species		 占总种数比例
Percentage of
total species (%)		 属
Genus		 种数
Number of
species		 占总种数比例
Percentage of total
species (%)
鹅膏菌科Amanitaceae 2 11 5.26 鹅膏菌属 Amanita 10 4.78
小菇科 Mycenaceae 2 12 5.74 丝盖伞属 Inocybe 5 2.39
多孔菌科 Polyporaceae 8 14 6.70 蜡蘑属 Laccaria 5 2.39
红菇科 Russulaceae 3 23 11.00 小皮伞属 Marasmius 6 2.87
小菇属 Mycena 11 5.26
光柄菇属 Pluteus 5 2.39
红菇属 Russula 17 8.13
栓菌属 Trametes 5 2.39
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