Photosynthesis by marine diatoms contributes significantly to the global carbon cycle. Due to the low concentration of CO₂ in seawater, many diatoms use extracellular carbonic anhydrase (eCA) to enhance the supply of CO₂ to the cell surface. While much research has investigated how the requirement for eCA is influenced by changes in CO₂ availability, little is known about how eCA contributes to CO₂ supply following changes in the demand for carbon. We therefore examined how changes in photosynthetic rate influence the requirement for eCA in three centric diatoms. Modeling of cell surface carbonate chemistry indicated that diffusive CO₂ supply to the cell surface was greatly reduced in large diatoms at higher photosynthetic rates. Laboratory experiments demonstrated a trend of an increasing requirement for eCA with increasing photosynthetic rate that was most pronounced in the larger species, supporting the findings of the cellular modeling. Microelectrode measurements of cell surface pH and O₂ demonstrated that individual cells exhibited an increased contribution of eCA to photosynthesis at higher irradiances. Our data demonstrate that changes in carbon demand strongly influence the requirement for eCA in diatoms. Cell size and photosynthetic rate will therefore be key determinants of the mode of dissolved inorganic carbon uptake.

中文翻译：

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硅藻对外部碳酸酐酶的需求受到溶解无机碳供需的影响

海洋硅藻的光合作用对全球碳循环做出了重大贡献。由于海水中CO _{2浓度较低，许多硅藻利用胞外碳酸酐酶(eCA)来增强CO 2}向细胞表面的供应。_{虽然许多研究都调查了 CO 2}可用性变化对 eCA 的需求有何影响，但对于碳需求变化后eCA 如何促进 CO ₂供应却知之甚少。因此，我们研究了光合速率的变化如何影响三个中心硅藻对 eCA 的需求。细胞表面碳酸盐化学模拟表明，在较高的光合速率下，大型硅藻向细胞表面扩散的CO ₂供应大大减少。实验室实验表明，随着光合作用速率的增加，对 eCA 的需求量也随之增加，这在较大的物种中最为明显，这支持了细胞模型的发现。_{细胞表面 pH 和 O 2}的微电极测量表明，单个细胞在较高辐照度下表现出 eCA 对光合作用的贡献增加。我们的数据表明，碳需求的变化强烈影响硅藻对 eCA 的需求。因此，细胞大小和光合速率将是溶解无机碳吸收模式的关键决定因素。