Herfort Lydie, Thake Brenda, Roberts James
School of Biological Sciences, Queen Mary, University of London, Mile End Road, London E1 4NS, UK.
New Phytol. 2002 Dec;156(3):427-436. doi: 10.1046/j.1469-8137.2002.00523.x.
• Bicarbonate acquisition mechanisms and the kinetics of dissolved inorganic carbon (DIC) use in photosynthesis and calcification were investigated in Emiliania huxleyi. • Photosynthesis was measured using O evolution and C incorporation and calcification was measured with C. Noncalcifying (coccolith-free) cells were produced from calcifying (coccolith-bearing) cells of the same strain of E. huxleyi, so that photosynthesis could be monitored independently from calcification. • Neither photosynthesis nor calcification was saturated at the ambient DIC concentration of seawater. In coccolith-bearing cells, both processes showed biphasic kinetics with DIC concentration, with a hiatus located at 1 mM. The same biphasic pattern and similar rates of photosynthesis were found in the coccolith-free cells. Inhibitor experiments showed that E. huxleyi acquires bicarbonate mainly by an anion exchange protein, but external carbonic anhydrase can be activated at low concentrations of DIC. • We conclude that the biphasic kinetics of photosynthesis and calcification are caused by the presence of two bicarbonate acquisition mechanisms and also, since calcification does not enhance photosynthesis in this coccolithophore, we question the current view that the two processes are tightly coupled.
• 研究了赫氏颗石藻中碳酸氢盐获取机制以及光合作用和钙化过程中溶解无机碳（DIC）利用的动力学。
• 通过测量氧气释放和碳固定来测定光合作用，用碳来测定钙化作用。从同一株赫氏颗石藻的钙化（含颗石）细胞中产生非钙化（无颗石）细胞，以便能够独立于钙化作用监测光合作用。
• 在海水的环境DIC浓度下，光合作用和钙化作用均未达到饱和。在含颗石细胞中，这两个过程均呈现出随DIC浓度变化的双相动力学，在1 mM处存在一个间断点。在无颗石细胞中也发现了相同的双相模式和相似的光合作用速率。抑制剂实验表明，赫氏颗石藻主要通过阴离子交换蛋白获取碳酸氢盐，但在低浓度DIC下外部碳酸酐酶可被激活。
• 我们得出结论，光合作用和钙化作用的双相动力学是由两种碳酸氢盐获取机制的存在导致的，而且，由于在这种颗石藻中钙化作用不会增强光合作用，我们对目前认为这两个过程紧密耦合的观点提出质疑。
