Concentration and δ15N of amino acids in size-fractionated particles from the eastern tropical North Pacific ODZ

This dataset includes the concentration and δ15N of bulk N and amino acids in size-fractionated particles. Particle samples were collected aboard the R/V Sally Ride (cruise SR2011) from December 23 – 30, 2020 with a McLane large volume pumping system (WTS-LV). These data assess the N sources utilized by different phytoplankton communities inhabiting the two chlorophyll maxima and the transformation of particulate organic matter within the oxygen-deficient zone.

Methods
Two particle fractions were collected at five stations in the eastern tropical North Pacific region. At each station, seawater was filtered sequentially through a 53 µm Nitex mesh and then a pre-combusted 142-mm-diameter glass fiber filter (GF-75: 0.3 µm retention size; or GF/F: 0.7 µm nominal retention size) with the McLane pump in situ. These filters were promptly frozen upon retrieval.
Each 53 µm Nitex mesh was immersed in approximately 100 ml of MilliQ water and subjected to a 5-minute sonication. The solution was filtered onto pre-combusted 47-mm glass fiber filters (GF/F, 0.7 µm pore size), which retain the > 53 µm particle fraction.

The GF-75 and GF/F filters (0.3 or 0.7 – 53 µm fractions) were subsampled, packed in tin capsules, and analyzed for concentration and δ15N of bulk materials by an elemental analyzer isotope-ratio mass spectrometer at the UC Davis Stable Isotope Facility. Bulk δ15N analysis was not performed for the > 53 µm fraction due to the limited amount of collected material.

The sample pre-treatment procedures for δ15N-amino acids were based on the method detailed in Zhang et al. (2021). The glass fiber filters underwent hydrolysis with 6N HCl for 22 hours at 110°C. Hydrophobic impurities were eliminated from the hydrolysates through liquid-liquid extraction using n-hexane/dichloromethane (6:5, v/v), followed by evaporation to dryness in a vacuum evaporator (RapidVap, Labconco). The samples were subsequently redissolved in 0.05N HCl and further purified via cation-exchange resin, following procedures adapted from Takano et al. (2010) to remove metal ions and salts. The purified amino acids in the samples were completely dried under vacuum.

Amino acids in the samples were separated and collected as individual fractions using an ICS-5000+ Ion-exchange chromatography system with the instrumental method adapted from Zhang et al. (2021). For each sample, Phe, Glu, and their corresponding IC procedural blanks were collected from 1 – 3 replicate injections. The IC-collected fractions were sequentially converted to NO2- and N2O through a two-step process involving hypochlorite oxidation and azide reduction, as described in McIlvin and Altabet (2005); Zhang et al. (2007); Zhang and Altabet (2008); and Zhang et al. (2021). δ15N-N2O was determined with a GV IsoPrime IRMS.

本数据集包含分级颗粒物（size-fractionated particles）中的总氮（bulk N）、氨基酸的浓度及氮15同位素比值（δ15N）。样本于2020年12月23日至30日搭载"莎莉·莱德号"（R/V Sally Ride）研究船SR2011航次采集，采用麦克莱恩大体积抽水系统（McLane large volume pumping system, WTS-LV）完成采样。本数据集旨在解析栖息于两处叶绿素最大值层的不同浮游植物群落所利用的氮源，以及缺氧区内颗粒有机质的转化过程。

实验方法
本研究在东热带北太平洋海域的5个站位设置了两个颗粒物分级组分。每个站位的海水依次通过53 μm尼隆筛网（Nitex mesh）过滤，再采用原位麦克莱恩泵将海水过滤至预灼烧的142 mm直径玻璃纤维滤膜（GF-75：截留粒径0.3 μm；或GF/F：标称截留粒径0.7 μm）。采集后的滤膜立即冷冻保存。

将每个53 μm尼隆筛网浸入约100 mL密理博（MilliQ）超纯水，超声处理5分钟，将所得溶液过滤至预灼烧的47 mm玻璃纤维滤膜（GF/F，孔径0.7 μm），该滤膜可截留>53 μm的颗粒物组分。

对GF-75和GF/F滤膜（对应0.3/0.7 μm~53 μm组分）进行分样，封装于锡囊（tin capsules）中，由加州大学戴维斯分校稳定同位素实验室（UC Davis Stable Isotope Facility）使用元素分析仪-同位素比值质谱联用系统（elemental analyzer isotope-ratio mass spectrometer），测定其总物质的浓度及δ15N值。由于>53 μm组分的采集样本量有限，未对其开展总氮δ15N分析。

氨基酸δ15N的样本前处理流程参照Zhang等（2021）的方法执行。将玻璃纤维滤膜置于6 mol/L HCl中，于110℃下水解22小时。采用正己烷/二氯甲烷（体积比6:5）进行液液萃取，去除水解液中的疏水性杂质，随后置于真空浓缩仪（RapidVap, Labconco）中蒸干。将样本重新溶解于0.05 mol/L HCl中，参照Takano等（2010）的方法采用阳离子交换树脂进一步纯化，以去除金属离子与盐分。纯化后的样本氨基酸经真空完全干燥。

采用ICS-5000+离子交换色谱系统（ICS-5000+ Ion-exchange chromatography system），参照Zhang等（2021）的仪器方法，将样本中的氨基酸分离并收集为单个组分。对每个样本，通过1~3次平行进样收集苯丙氨酸（Phe）、谷氨酸（Glu）及其对应的离子色谱（IC）程序空白。参照McIlvin与Altabet（2005）、Zhang等（2007）、Zhang与Altabet（2008）及Zhang等（2021）的方法，通过次氯酸盐氧化和叠氮化物还原的两步反应，将离子色谱收集的组分依次转化为NO2-和N2O。采用GV IsoPrime同位素比值质谱仪（GV IsoPrime IRMS）测定δ15N-N2O。