Carbon uptake rate calculated for sea-ice core samples during POLARSTERN cruise ARK-XXVII/3 (IceArc) in 2012

Net Primary Production was measured using the 14**C uptake method with minor modifications. Melted sea ice samples were spiked with 0.1µCi ml**-1 of 14**C labelled sodium bicarbonate (Moravek Biochemicals, Brea, USA) and distributed in 10 clear bottles (20 ml each). Subsequently they were incubated for 12 h at -1.3°C under different scalar irradiances (0-420 µmol photons m**-2 s**-1) measured with a spherical sensor (Spherical Micro Quantum Sensor US-SQS/L, Heinz Walz, Effeltrich, Germany). At the end of the incubation, samples were filtered onto 0.2 µm nitrocellulose filters and the particulate radioactive carbon uptake was determined by liquid scintillation counting using Filter count scintillation cocktail (Perkin Elmer, Waltham, USA). The carbon uptake values in the dark were subtracted from the carbon uptake values measured in the light incubations. Dissolved inorganic carbon (DIC) was measured for each sample using the flow injection system (Hall and Aller, 1992). The DIC concentration was taken into account to calculate the amount of labeled bicarbonate incorporated into the cell. Carbon fixation rates were normalized volumetrically and by chlorophyll a. Photosynthesis-irradiance curves (PI curves) were fitted using MATLAB® according to the equation proposed by Platt et al. (1980) including a photoinhibition parameter (beta) and providing the main photosynthetic parameters: maximum Chla normalized carbon fixation rate if there were no photoinhibition (Pb) and the initial slope of the saturation curve (alpha). The derived parameters: light intensity at which photosynthesis is maximal (Im), the carbon fixation rate at that maximal irradiance (Pbm) and the adaptation parameter or photoacclimation index (Ik) were calculated according to Platt et al. (1982).

本数据集采用14C摄取法（14C uptake method）并经小幅改良测定净初级生产力（Net Primary Production）。具体实验流程如下：将海冰融解样品添加0.1微居里·毫升⁻¹的14C标记碳酸氢钠（14C labelled sodium bicarbonate）（购自美国布雷市Moravek Biochemicals公司），随后分装至10个透明玻璃瓶（单瓶容积20毫升）。将样品置于-1.3℃环境中，在0~420微摩尔光子·米⁻²·秒⁻¹的不同标量辐照度下培养12小时，辐照度通过球形微量子传感器US-SQS/L（德国埃夫尔特特里茨Heinz Walz公司）测定。培养结束后，将样品过滤至0.2微米硝酸纤维素滤膜，采用Filter Count闪烁液（美国沃尔瑟姆Perkin Elmer公司）通过液体闪烁计数法测定颗粒态放射性碳摄取量。以暗培养组的碳摄取值作为本底，从光培养组的测定值中扣除该本底。采用流动注射系统（Hall与Aller，1992）测定每个样品的溶解无机碳（dissolved inorganic carbon, DIC）浓度，并结合该浓度计算掺入细胞的标记碳酸氢钠总量。将碳固定速率按体积及叶绿素a（chlorophyll a）进行标准化处理。根据Platt等（1980）提出的拟合方程，借助MATLAB®拟合光合-辐照度曲线（PI曲线），该方程包含光抑制参数（beta），可输出两项核心光合参数：无光抑制条件下的最大叶绿素a标准化碳固定速率（Pb），以及饱和曲线的初始斜率（alpha）。随后根据Platt等（1982）的方法，计算得到三项衍生光合参数：达到最大光合速率时的光强（Im）、该最大辐照度下的碳固定速率（Pbm），以及光适应指数（photoacclimation index, Ik）。