Department of Marine Sciences, University of Georgia, Athens, Georgia 30602, USA.
Plant Physiol. 2013 Jun;162(2):1142-52. doi: 10.1104/pp.113.217737. Epub 2013 May 8.
Many microalgae induce an extracellular carbonic anhydrase (eCA), associated with the cell surface, at low carbon dioxide (CO2) concentrations. This enzyme is thought to aid inorganic carbon uptake by generating CO2 at the cell surface, but alternative roles have been proposed. We developed a new approach to quantify eCA activity in which a reaction-diffusion model is fit to data on (18)O removal from inorganic carbon. In contrast to previous methods, eCA activity is treated as a surface process, allowing the effects of eCA on cell boundary-layer chemistry to be assessed. Using this approach, we measured eCA activity in two marine diatoms (Thalassiosira pseudonana and Thalassiosira weissflogii), characterized the kinetics of this enzyme, and studied its regulation as a function of culture pH and CO2 concentration. In support of a role for eCA in CO2 supply, eCA activity specifically responded to low CO2 rather than to changes in pH or HCO3(-), and the rates of eCA activity are nearly optimal for maintaining cell surface CO2 concentrations near those in the bulk solution. Although the CO2 gradients abolished by eCA are small (less than 0.5 μm concentration difference between bulk and cell surface), CO2 uptake in these diatoms is a passive process driven by small concentration gradients. Analysis of the effects of short-term and long-term eCA inhibition on photosynthesis and growth indicates that eCA provides a small energetic benefit by reducing the surface-to-bulk CO2 gradient. Alternative roles for eCA in CO2 recovery as HCO3(-) and surface pH regulation were investigated, but eCA was found to have minimal effects on these processes.
许多微藻在低二氧化碳（CO2）浓度下诱导细胞表面的细胞外碳酸酐酶（eCA）。这种酶被认为通过在细胞表面生成 CO2 来帮助无机碳的摄取，但也提出了替代作用。我们开发了一种新的方法来定量 eCA 活性，该方法通过拟合反应-扩散模型来获取无机碳中（18）O 去除的数据。与以前的方法不同，eCA 活性被视为表面过程，从而可以评估 eCA 对细胞边界层化学的影响。使用这种方法，我们测量了两种海洋硅藻（拟菱形藻和威氏海链藻）的 eCA 活性，表征了这种酶的动力学，并研究了其作为培养 pH 和 CO2 浓度函数的调节。支持 eCA 在 CO2 供应中的作用，eCA 活性特异性响应低 CO2，而不是 pH 或 HCO3（-）的变化，并且 eCA 活性的速率几乎是最佳的，可将细胞表面 CO2 浓度维持在接近溶液主体的浓度附近。尽管 eCA 消除的 CO2 梯度很小（主体和细胞表面之间的浓度差小于 0.5μm），但这些硅藻中的 CO2 摄取是由小浓度梯度驱动的被动过程。对短期和长期 eCA 抑制对光合作用和生长的影响的分析表明，eCA 通过减少表面到主体的 CO2 梯度提供了微小的能量益处。还研究了 eCA 在 HCO3（-）和表面 pH 调节中的 CO2 回收中的替代作用，但发现 eCA 对这些过程的影响最小。
