Tchernov D, Hassidim M, Luz B, Sukenik A, Reinhold L, Kaplan A
Department of Plant Sciences, Hebrew University of Jerusalem, Israel.
Curr Biol. 1997 Oct 1;7(10):723-8. doi: 10.1016/s0960-9822(06)00330-7.
Many aquatic photosynthetic microorganisms possess an inorganic-carbon-concentrating mechanism that raises the CO2 concentration at the intracellular carboxylation sites, thus compensating for the relatively low affinity of the carboxylating enzyme for its substrate. In cyanobacteria, the concentrating mechanism involves the energy-dependent influx of inorganic carbon, the accumulation of this carbon--largely in the form of HCO3(-)-in the cytoplasm, and the generation of CO2 at carbonic anhydrase sites in close proximity to the carboxylation sites.
During measurements of inorganic carbon fluxes associated with the inorganic-carbon-concentrating mechanism, we observed the surprising fact that several marine photosynthetic microorganisms, including significant contributors to oceanic primary productivity, can serve as a source of CO2 rather than a sink during CO2 fixation. The phycoerythrin-possessing cyanobacterium Synechococcus sp. WH7803 evolved CO2 at a rate that increased with light intensity and attained a value approximately five-fold that for photosynthesis. The external CO2 concentration reached was significantly higher than that predicted for chemical equilibrium between HCO3- and CO2, as confirmed by the rapid decline in the CO2 concentration upon the addition of carbonic anhydrase. Measurements of oxygen exchange between water and CO2, by means of stable isotopes, demonstrated that the evolved CO2 originated from HCO3- taken up and converted intracellularly to CO2 in a light-dependent process.
We report net, sustained CO2 evolution during photosynthesis. The results have implications for energy balance and pH regulation of the cells, for carbon cycling between the cells and the marine environment, and for the observed fractionation of stable carbon isotopes.
许多水生光合微生物具有无机碳浓缩机制，该机制可提高细胞内羧化位点处的二氧化碳浓度，从而弥补羧化酶对其底物相对较低的亲和力。在蓝细菌中，浓缩机制涉及无机碳的能量依赖性流入、这种碳（主要以HCO₃⁻的形式）在细胞质中的积累，以及在靠近羧化位点的碳酸酐酶位点处产生二氧化碳。
在测量与无机碳浓缩机制相关的无机碳通量过程中，我们观察到一个惊人的事实，即几种海洋光合微生物，包括对海洋初级生产力有重要贡献的微生物，在二氧化碳固定过程中可作为二氧化碳的来源而非吸收汇。含有藻红蛋白的蓝细菌聚球藻属Synechococcus sp. WH7803以随光强增加的速率释放二氧化碳，其释放值约为光合作用的五倍。达到的外部二氧化碳浓度显著高于HCO₃⁻与CO₂化学平衡所预测的值，添加碳酸酐酶后二氧化碳浓度迅速下降证实了这一点。通过稳定同位素测量水与二氧化碳之间的氧交换表明，释放的二氧化碳源自摄取的HCO₃⁻，并在光依赖过程中在细胞内转化为二氧化碳。
我们报道了光合作用过程中持续的净二氧化碳释放。这些结果对细胞的能量平衡和pH调节、细胞与海洋环境之间的碳循环以及观察到的稳定碳同位素分馏具有重要意义。
