CO2 and vitamin B12 interactions determine bioactive trace metal requirements of a subarctic Pacific diatom

Phytoplankton growth can be limited by numerous inorganic nutrients and organic growth factors. Using the subarctic diatom Attheya sp. in culture studies, we examined how the availability of vitamin B(12) and carbon dioxide partial pressure (pCO(2)) influences growth rate, primary productivity, cellular iron (Fe), cobalt (Co), zinc (Zn) and cadmium (Cd) quotas, and the net use efficiencies (NUEs) of these bioactive trace metals (mol C fixed per mol cellular trace metal per day). Under B(12)-replete conditions, cells grown at high pCO(2) had lower Fe, Zn and Cd quotas, and used those trace metals more efficiently in comparison with cells grown at low pCO(2). At high pCO(2), B(12)-limited cells had ~50% lower specific growth and carbon fixation rates, and used Fe ~15-fold less efficiently, and Zn and Cd ~3-fold less efficiently, in comparison with B(12)-replete cells. The observed higher Fe, Zn and Cd NUE under high pCO(2)/B(12)-replete conditions are consistent with predicted downregulation of carbon-concentrating mechanisms. Co quotas of B(12)-replete cells were 5- to 14-fold higher in comparison with B(12)-limited cells, suggesting that >80% of cellular Co of B(12)-limited cells was likely from B(12). Our results demonstrate that CO(2) and vitamin B(12) interactively influence growth, carbon fixation, trace metal requirements and trace metal NUE of this diatom. This suggests the need to consider complex feedback interactions between multiple environmental factors for this biogeochemically critical group of phytoplankton in the last glacial maximum as well as the current and future changing ocean.

浮游植物（Phytoplankton）的生长可受多种无机营养盐与有机生长因子的共同限制。本研究以亚北极硅藻（subarctic diatom）角刺藻属（Attheya sp.）为实验材料开展培养实验，探究维生素B₁₂（vitamin B₁₂）与二氧化碳分压（carbon dioxide partial pressure, pCO₂）的可获得性如何影响其生长速率、初级生产力、细胞内铁（Fe）、钴（Co）、锌（Zn）与镉（Cd）的细胞配额，以及上述生物活性微量金属的净利用效率（NUEs，即每摩尔细胞内微量金属每日固定的碳摩尔数）。在维生素B₁₂充足条件下，高pCO₂培养的细胞其Fe、Zn与Cd细胞配额更低，且相较于低pCO₂培养的细胞，对这些微量金属的利用效率更高。在高pCO₂环境中，维生素B₁₂限制的细胞其比生长速率与固碳速率较维生素B₁₂充足的细胞降低约50%，对Fe的利用效率降低约15倍，对Zn与Cd的利用效率降低约3倍。本研究观测到的高pCO₂/维生素B₁₂充足条件下Fe、Zn与Cd更高的净利用效率，与碳浓缩机制（carbon-concentrating mechanisms）的预测下调结果一致。维生素B₁₂充足细胞的Co细胞配额较维生素B₁₂限制细胞高5~14倍，这表明维生素B₁₂限制细胞中超过80%的细胞内Co可能来源于维生素B₁₂。本研究结果证实，二氧化碳与维生素B₁₂可交互影响该硅藻的生长、固碳过程、微量金属需求及微量金属净利用效率。这意味着在末次冰盛期（last glacial maximum）以及当前与未来的海洋环境变化背景下，针对这一生物地球化学关键类群浮游植物，需要考虑多种环境因子间复杂的反馈交互作用。