Data for contribution of marine phytoplankton and bacteria to ocean alkalinity

The contributions of phytoplankton and bacteria cells to alkalinity (AT) were measured in seawater samples obtained from 205 locations including the East Sea, the North Pacific Ocean, the Bering Sea, the Chukchi Sea, and the Arctic Ocean. We attributed the differences in AT values measured for unfiltered versus filtered samples to AT components contributed by phytoplankton (retained on a 0.7 µm filter) and by phytoplankton and bacteria combined (AT-BIO; retained on a 0.45 µm filter). The AT-BIO values reached 10-19 μmol kg-1 in the East Sea and the North Pacific Ocean, and progressively decreased to a level of 1 μmol kg-1 with distance towards the Arctic Ocean. The study shows that the AT-BIO values are non-negligible in coastal and open ocean environments and need to be considered when assessing the accuracy of carbon parameters calculated using the thermodynamic models that use measured AT as an input parameter.

Methods
Sampling locations

Sampling was carried out on board the R/V ARAON during the study from 17 July 2020 to 15 September 2020. The sampling locations were approximately evenly spaced along a transect from the East Sea to the North Pacific Ocean, the Bering Sea, the Chukchi Sea, and the Arctic Ocean. A total of 205 surface seawater samples were collected in 4-L polycarbonate bottles from the underway clean seawater line. To obtain depth profiles, a total 24 discrete samples were collected in 1-L high density polyethylene bottles using a rosette sampler (SeaBird Electronics, SBE 911 plus) at two sites (76.003ºN, 170.588ºW; 74.499ºN, 162.262ºW) in the Arctic Ocean.

Determination of AT, AT-BIO, AT-PLANKTON, and AT-BACTERIA

For each seawater sample the AT values of an unfiltered and a filtered (0.7 μm and 0.45 μm) sample were measured within 12 h of collection. The total contribution of all biogenic particles to the AT of the unfiltered seawater (AT-BIO; Equation 2) was determined as the difference between the measured AT values of the unfiltered sample and the 0.45 μm-filtered sample, followed by subtraction of the CaCO3 contribution (AT-PIC), which was calculated from the particulate inorganic carbon (PIC) concentration determined as described in Text S1. In this calculation we assumed that the particulate matter contribution to AT mostly comprised phytoplankton and bacteria cells, and CaCO3 shells. The contribution of phytoplankton cells to AT (AT-PLANKTON; Equation 3) was determined as the difference in measured AT between the unfiltered sample and the 0.7 μm-filtered sample, followed by subtraction of AT-PIC. The contribution of bacteria (AT-BACTERIA; Equation 4) was determined by calculating the difference in measured AT values between the 0.7 μm-filtered sample and the 0.45 μm-filtered sample.

          AT-BIO = AT-PLANKTON + AT-BACTERIA = AT-unfiltered – AT-0.45 μm-filtered – AT-PIC      (2)

          AT-PLANKTON = AT-unfiltered – AT-0.7 μm-filtered – AT-PIC                                      (3)

          AT-BACTERIA = AT- 0.7 μm-filtered – AT-0.45 μm-filtered                                           (4)

The precisions of measurements for three biological AT components were estimated from the root sum square of the standard deviations of all parameters involved ( ; see those relevant parameters in Equations 2, 3, 4, and Text S2) and were found to be 1.1 μmol kg-1 for AT-BIO, AT-PLANKTON, and AT-BACTERIA. Note that the AT-PIC component was not considered in the estimation of the precisions for AT-BIO and AT-PLANKTON because measurable PIC values were found only for 6 of the 205 samples.

For more details, see the methods section in the manuscript "Contribution of marine phytoplankton and bacteria to ocean alkalinity: an uncharacterized component".

本研究针对取自东海、北太平洋、白令海、楚科奇海以及北冰洋共205个站位的海水样品，测定了浮游植物与细菌细胞对总碱度（alkalinity, AT）的贡献。我们将未过滤与过滤样品的AT测定值差异，归因于浮游植物（截留于0.7 μm滤膜）以及浮游植物与细菌共同（AT-BIO；截留于0.45 μm滤膜）所贡献的AT组分。其中AT-BIO值在东海与北太平洋可达10~19 μmol·kg⁻¹，随着向北冰洋的距离增加，其值逐步降低至1 μmol·kg⁻¹水平。本研究表明，AT-BIO值在近岸与开阔大洋环境中均不可忽视，在以实测AT作为输入参数的热力学模型计算碳参数时，需考虑该组分的影响。

## 方法
### 采样站位
本研究于2020年7月17日至9月15日期间，在"ARAON"号海洋科考船上完成采样。采样站位沿东海至北太平洋、白令海、楚科奇海及北冰洋的断面近似均匀分布。研究团队通过走航清洁海水采集管路，使用4 L聚碳酸酯瓶共采集205份表层海水样品。为获取深度剖面，在北冰洋的两个站位（76.003°N, 170.588°W；74.499°N, 162.262°W），使用采水器（SeaBird Electronics, SBE 911 plus）搭配1 L高密度聚乙烯瓶，共采集24份离散分层样品。

### 总碱度（AT）、AT-BIO、AT-PLANKTON与AT-BACTERIA的测定
对于每份海水样品，我们在采样后12小时内分别测定未过滤样品以及0.7 μm、0.45 μm过滤样品的AT值。未过滤海水的全部生源颗粒对AT的总贡献（AT-BIO；公式2）通过以下方式计算：先求取未过滤样品与0.45 μm过滤样品的AT测定值之差，再减去碳酸钙组分贡献（AT-PIC），其中AT-PIC由补充材料S1中所述方法测定的颗粒无机碳（particulate inorganic carbon, PIC）浓度计算得到。本计算假设颗粒物质对AT的贡献主要来自浮游植物、细菌细胞以及碳酸钙壳体。浮游植物细胞对AT的贡献（AT-PLANKTON；公式3）通过以下方式计算：先求取未过滤样品与0.7 μm过滤样品的AT测定值之差，再减去AT-PIC。细菌对AT的贡献（AT-BACTERIA；公式4）通过计算0.7 μm过滤样品与0.45 μm过滤样品的AT测定值之差得到。

$$ ext{AT-BIO} = ext{AT-PLANKTON} + ext{AT-BACTERIA} = ext{AT}_ ext{未过滤} - ext{AT}_ ext{0.45 μm过滤} - ext{AT-PIC} ag{2}$$
$$ ext{AT-PLANKTON} = ext{AT}_ ext{未过滤} - ext{AT}_ ext{0.7 μm过滤} - ext{AT-PIC} ag{3}$$
$$ ext{AT-BACTERIA} = ext{AT}_ ext{0.7 μm过滤} - ext{AT}_ ext{0.45 μm过滤} ag{4}$$

研究团队通过所有参与参数的标准差的方和根估算了三种生源AT组分的测量精度（详见公式2、3、4及补充材料S2中的相关参数），结果显示AT-BIO、AT-PLANKTON与AT-BACTERIA的测量精度均为1.1 μmol·kg⁻¹。需要注意的是，由于205份样品中仅6份样品检出可测量的PIC值，因此在估算AT-BIO与AT-PLANKTON的精度时未考虑AT-PIC组分的影响。

更多细节可参阅论文"Contribution of marine phytoplankton and bacteria to ocean alkalinity: an uncharacterized component"的方法章节。