Microalgae induce a CO₂-concentrating mechanism (CCM) to overcome CO₂-limiting stress in aquatic environments by coordinating inorganic carbon (Ci) transporters and carbonic anhydrases (CAs). Two mechanisms have been suggested to facilitate Ci uptake from aqueous media: Na⁺-dependent HCO₃⁻ uptake by solute carrier (SLC) family transporters and accelerated dehydration of HCO₃⁻ to CO₂ by external CA in model diatoms. However, studies on ecologically and industrially important diatoms including Chaetoceros gracilis, a common food source in aquacultures, are still limited. Here, we characterized the CCM of C. gracilis using inhibitors and growth dependency on Na⁺ and CO₂. Addition of a membrane-impermeable SLC inhibitor, 4,4′-diisothiocyano-2,2′-stilbenedisulfonic acid (DIDS), or the transient removal of Na⁺ from the culture medium did not impair photosynthetic affinity for Ci in CO₂-limiting stress conditions, but addition of a membrane-impermeable CA inhibitor, acetazolamide, decreased Ci affinity to one-third of control cultures. In culture medium containing 0.23 mM Na⁺ C. gracilis grew photoautotrophically by aeration with air containing 5% CO₂, but not with the air containing 0.04% CO₂. These results suggested that C. gracilis utilizes external CAs in its CCM to elevate photosynthetic affinity for Ci rather than plasma-membrane SLC family transporters. In addition, it is possible that low level of Na⁺ may support the CCM in processes other than Ci-uptake at the plasma membrane specifically in CO₂-limiting conditions. Our findings provide insights into the diversity of CCMs among diatoms as well as basic information to optimize culture conditions for industrial applications.

微藻通过协调无机碳 (Ci) 转运蛋白和碳酸酐酶 (CA)，诱导 CO ₂浓缩机制 (CCM) 以克服水生环境中限制CO _{2 的}压力。已提出两种机制来促进水介质中的 Ci 吸收：溶质载体 (SLC) 家族转运蛋白对Na ⁺依赖性 HCO ₃ ^{- 的}吸收和模型硅藻中外部 CA将 HCO ₃ ^-加速脱水为 CO ₂。然而，包括生态和重要的工业硅藻研究纤细角毛藻，水产养殖中常见的食物来源，仍然是有限的。在这里，我们表征了C的 CCM 。gracilis使用抑制剂和生长依赖于 Na ⁺和 CO ₂。添加不透膜的 SLC 抑制剂、4,4'-二异硫氰基-2,2'-二苯乙烯二磺酸 (DIDS) 或从培养基中瞬时去除 Na ⁺不会削弱光合亲和力对 CO _{2 中的}Ci限制性压力条件下，但添加膜不可渗透的 CA 抑制剂乙酰唑胺，降低了对三分之一的对照培养物的 Ci 亲和力。在含有 0.23 mM Na ^{+ 的} 培养基中，通过用含有 5% CO _{2 的}空气曝气而不是在含有 0.04% CO ₂的空气中曝气，纤细纤毛自养生长。这些结果表明C. gracilis在其 CCM 中利用外部 CA 来提高对 Ci 而不是质膜 SLC 家族转运蛋白的光合亲和力。此外，低水平的 Na ^{+ 有}可能支持 CCM 在除质膜上的 Ci 吸收以外的过程中，特别是在 CO ₂限制条件下。我们的研究结果提供了对硅藻中 CCM 多样性的见解，以及优化工业应用培养条件的基本信息。