Leggat William, Marendy Elessa M, Baillie Brett, Whitney Spencer M, Ludwig Martha, Badger Murray R, Yellowlees David
Biochemistry and Molecular Biology, James Cook University, Townsville, Qld 4811, Australia. Current address: Centre for Marine Studies, University of Queensland, St Lucia, Qld 4072, Australia.
Biochemistry and Molecular Biology, James Cook University, Townsville, Qld 4811, Australia.
Funct Plant Biol. 2002 Apr;29(3):309-322. doi: 10.1071/PP01202.
Dinoflagellates exist in symbiosis with a number of marine invertebrates including giant clams, which are the largest of these symbiotic organisms. The dinoflagellates (Symbiodinium sp.) live intercellularly within tubules in the mantle of the host clam. The transport of inorganic carbon (Ci) from seawater to Symbiodinium (=zooxanthellae) is an essential function of hosts that derive the majority of their respiratory energy from the photosynthate exported by the zooxanthellae. Immunolocalisation studies show that the host has adapted its physiology to acquire, rather than remove CO2, from the haemolymph and clam tissues. Two carbonic anhydrase (CA) isoforms (32 and 70 kDa) play an essential part in this process. These have been localised to the mantle and gill tissues where they catalyse the interconversion of HCO3 to CO2, which then diffuses into the host tissues. The zooxanthellae exhibit a number of strategies to maximise Ci acquisition and utilisation. This is necessary as they express a form II Rubisco that has poor discrimination between CO2 and O2. Evidence is presented for a carbon concentrating mechanism (CCM) to overcome this disadvantage. The CCM incorporates the presence of a light-activated CA activity, a capacity to take up both HCO3and CO2, an ability to accumulate an elevated concentration of Ci within the algal cell, and localisation of Rubisco to the pyrenoid. These algae also express both external and intracellular CAs, with the intracellular isoforms being localised to the thylakoid lumen and pyrenoid. These results have been incorporated into a model that explains the transport of Ci from seawater through the clam to the zooxanthellae.
甲藻与许多海洋无脊椎动物共生，包括巨蛤，巨蛤是这些共生生物中体型最大的。甲藻（共生藻属）生活在宿主蛤类外套膜小管的细胞内。无机碳（Ci）从海水向共生藻（=虫黄藻）的转运是宿主的一项基本功能，宿主的大部分呼吸能量来自虫黄藻输出的光合产物。免疫定位研究表明，宿主已经调整了其生理机能，以便从血淋巴和蛤类组织中获取而非去除二氧化碳。两种碳酸酐酶（CA）同工型（32 kDa和70 kDa）在这一过程中起着至关重要的作用。它们已被定位到外套膜和鳃组织，在那里催化HCO3向CO2的相互转化，然后CO2扩散到宿主组织中。虫黄藻表现出多种策略来最大限度地获取和利用Ci。这是必要的，因为它们表达的II型核酮糖-1,5-二磷酸羧化酶/加氧酶（Rubisco）对CO2和O2的区分能力较差。有证据表明存在一种碳浓缩机制（CCM）来克服这一缺点。CCM包括存在光激活的CA活性、吸收HCO3和CO2的能力、在藻类细胞内积累高浓度Ci的能力以及Rubisco定位于蛋白核。这些藻类还表达细胞外和细胞内的CA，细胞内同工型定位于类囊体腔和蛋白核。这些结果已被纳入一个模型，该模型解释了Ci从海水通过蛤类到虫黄藻的转运过程。
