大型海藻多数生长在潮间带，随着潮汐的变化，其光合作用低潮时暴露于空气（干出）中、高潮时沉没在水中进行。它们在如此“陆生”与“水生”环境条件下的生理过程已经得到了广泛的研究。然而，有关这些生理过程如何响应大气CO_2浓度变化方面，尚缺乏认识。本文在综述了有关大型海藻类光合作用研究进展的基础上，以常见的几种潮间带海藻为材料，在以下几个方面展开了研究：1）在“陆生”与水生两种环境条件下光合行为的比较；2）干出失水过程中光合作用特性及其与大气CO_2浓度变化的关系；3）高浓度CO_2培养条件下生理特性的变化。主要结果如下：石莼（Ulva lactuca L)、海萝[Gloiopeltis furcata (P. et R) J.Ag]、幅叶藻[Petalonia fascia (Muller) Kutz]在暴露于空气中时，光合速率随着失水量的增加而增加，达到峰值后，随进一步失水而下降；而条浒苔[Enteromorpha clathrata (Roth) Grev.]、坛紫菜（Porphyra haitanensis T.J. Chang et B. F. Zhang）、小杉藻 （Gigartina intermedia Suring）、半叶马尾藻[Sargassum hemiphyllum (Turn.) Ag.]的光合作用在干出初期没有显示增加的现象。严重的失水使得这些海藻的光合效率、CO_2羧化效率和光饱和点显著下降了，而光补偿点和CO_2补偿点则显著升高了。这些海藻在干出状态下光合作用受目前大气CO_2浓度的限制，增加空气中的CO_2浓度明显地促进了其光合作用。空气中CO_2浓度加倍使这些种类的光合速率在轻度失水（＜20%）时提高23 ～ 78%；在严重失水（＞50%）时提高68 ～ 196%。这显示了干出失水过程中光合作用限制因素的转变：从电子转递或Pi再生限制为主转变为以羧化能力限制为主。另外，在露天日光条件下，增加空气中的CO_2浓度，同样促进了干出时的光合作用，但对其促进的程度因一天中干出的时刻（早、午、晚）不同而不同。当沉没于海水中时，海水通常的无机碳浓度（2.2 ～ 2.2 mM）能饱和或基本饱和石莼、坛紫菜的光合作用，其光合作用的温度系数Q_(10)及CO_2的表观羧化效率显著地高于干出状态下的值。用含高浓度CO_2的空气充气培养石莼和条浒苔的结果表明，高浓度CO_2条件下，两种绿藻的相对生长速率有增加的趋势，但未达到显著水平。高浓度CO_2的培养，使石莼在低CO_2条件下的光合能力显著下降了，但对条浒苔的影响不显著；使这两种绿藻对营养盐的吸收速率增加了；同时使条浒苔的叶绿素含量、可溶性蛋白含量略下降了，可溶性碳水化合物含量则显著增高了；并且使条浒苔的硝酸还原酶活性下降了。总之，潮间带海藻在暴露于空气中时，光合作用受目前大气CO_2浓度水平的限制，藻体失水进一步加剧了这种限制作用。大气CO_2浓度增加，将促进潮间带海藻在低潮干出时的光合作用。然而，高浓度CO_2条件下的海藻如何进行有关的生理调整需进一步的研究。
Marine macroalgae are mostly distributed in the intertidal zone, and photosynthesize in air at low tide and in water when tide is high. Physiological processes of the intertidal macroalgae have been well comparatively studied between such "terrestrial" and aquatic environments. However, little is known about the impacts of increasing atmospheric CO_2 on their photosynthesis during emersion. On the basis of reviewing the progresses in the photosynthetic studies of intertidal macroalgae, studies were carried out on the 1) comparative photosynthesis of intertidal macroalgae during submersion and emersion; 2) photosynthesis of intertidal macroalgae during emersion and its relationship with atmospheric CO_2 rise; and 3) the effects of elevated CO_2 concentration in culture. There was an initial increase of emersed photosynthesis in Ulva lactuca, Gloiopeltis furcata and Petalonia fascia while exposed following moderate water loss, and thereafter photosynthesis declined with further water loss; such initial increase was not found in Enteromorpha clathrata, Porphora haitanensis, Gigartina intermedia and Sargassum hemiphyllum. The photosynthetic efficiencies, carboxylating efficiencies and photon saturation points significantly decreased, while the photon and CO_2 compensation points remarkably increased when these macroalgae were desiccated heavily. The emersed photosynthesis appeared to be limited by the present atmospheric CO_2 concentration, and could be significantly enhanced with the atmospheric CO_2 rise. The emersed photosynthesis was raised by 23 ～ 78% with moderate desiccation (＞50%), indicating the limitation switched from electron transfer or Pi regeneration capacities to Rubisco-catalyzed carboxylation. On the other hand, the photosynthetic enhancement at the elevated CO_2 level when emersed was confirmed under sun light, but varied solar radiations at different time (morning, noon, evening) in a day brought about differed extents of the effects. The inorganic carbon composition in seawater could saturate or nearly saturate the photosynthesis of U. lactuca and P. haitanensis while submersed, and the temperature coefficient Q_(10) of photosynthesis and the apparent carboxylating efficiency for CO_2 were significantly higher under submersed than emersed conditions. The culture of U. lactuca and E. clathrata aerated with elevated CO_2 concentration showed that these two green macroalgae grew faster but to a insignificant extent compared to that of the air-cultured. Elevated CO_2 significantly reduced the photosynthesis of U. lactuca, while insignificantly affected that of E. clathrata when measured at low CO_2 level. Nutrients uptake rates in the two algae were remarkably enhanced in CO_2-enriched cultures. The contents of chlorophyll and soluble protein were lowered, while the contents of soluble carbohydrate were higher in high CO_2-grown E. clathrata. Meanwhile, the elevated CO_2 remarkably reduced the activities of nitrate reductase in E. clathrata. In conclusion, the emersed photosynthesis of intertidal macroalgae was limited by the present atmospheric CO_2 concentration, and desiccation increased such CO_2-limitation. Photosynthesis of intertidal macroalgae while exposed at low tide would be enhanced by the atmospheric CO_2 rise. However, further studies are required to explain the responses of the intertidal macroaglae to elevated CO_2 concentrations.