珠江口最大浑浊带区浮游植物对光环境变化的响应及其适应机制

Estuarine environments often drasticly change in the spatical and temporal scales, no exception for underwater light conditions. The land-derived inputs and water-exchanges between innner and outer esturies lead to a dramatic verital mixing within water column; simutaniously, the abundant suspensions and dissolved matters in the water column also lead to a steep light decay. Furthermore, the light with different wave-bands such as photosynthetically active radiation (PAR, 400-700 nm) and ultraviolet radiation(UVR, 280-400 nm) have differential decay rates in water, which results in the great changes in light quality with increasing water depth. Therefore, the light quantity (intensity) and quality (wave-band) vary greatly within different depths of the esturies. Phytoplankton living in estuaries would be periodically transferred from surface to bottom of the water column and vice versa due to the vertical mixing, and suffer from the drastic changes of underwater light environments i.e. being exposed to different light intensities and qulities (i.e., PAR and UV) at different depths. At present, it is usually believed that light environment change is a key reason for estuarine phytoplankton biomass and productivity, while it is unclear so far that how they response and acclimate to such light changes although it is vital for them to survive, to be dominant species and even blooms, and this process mainly lie on how they maintain the activities of Photosystem II. Pearl River has the largest annual land runoffs among Chinese rivers, with a vertial mixing volecity reached as high as 10 cm s-1 in its maximum turbidity zone. Therefore, this project is designed to carry out studies in this area as follows: (1) Using onboard experiments in a seasonal scale, to study the relationships of the mixing-caused changes in field sunlight quantity and quality and photosynthetic carbon fixation by natural phytoplankton assemblages from the maxiamum turbidity zone of the Pearl River estury, in order to reveal the effects of field light environment changes upon estuarine carbon fixation; (2) Laboratorial experiments will be performed using the mono-species of phytoplankton that are the dominating species (mainly diatoms and dinoflagellates) and isolated from the maximum turbidity zone of Pearl River estuary, to study the relationships of light environment changes and photosynthetic rate [including 14C carban fixation rate and maximal quatam yield (Fv/Fm)], and also to further study the effects of such light changes upon the PSII damage and repair processes and the repair-dependent FtsH protein enzyme contents and activities. This study can clearify how phytoplankton response and acclimate to the estuarine light environment changes and potentially elucidate the physilogical reasons for the alteration of field light environment changes on estuarine phytoplankton biomass and production in the photophysiological view of point.

混合剧烈、透明度浅和光质(紫外光与可见光比例)随水深变化大是河口海域最显著的物理特征；生活在其中的浮游植物随混合上下移动，势必经受着剧烈的光环境变化(光强与光质)。当前普遍认为光环境变化是河口浮游植物生物量和生产力变化的最关键因素之一，对于其中间过程—浮游植物如何响应和适应光环境变化而存活下来、成为优势种甚至爆发性生长的内在机理并不清楚，而该中间过程的关键在于它们如何维持光系统II的活性。珠江是我国年入海水量最大的河，其河口最大浑浊带区混合速率极高；因此项目拟在该区域研究原位光环境变化与浮游植物群体光合固碳变化关系的同时，分离、培养优势浮游植物单种，在室内和室外可控条件下研究光环境变化与其光合能力变化的关系，并深入研究光环境变化与光系统II的损伤、修复动力学过程及修复酶变化等的关系，阐明其如何响应和适应该光环境变化，从光生理学角度揭示光环境变化引起河口浮游植物生物量和生产力变化的内在原因。

受陆源输入及内外海水交换动力影响，河口特别是最大浑浊带区水体混合剧烈；生活在其中的浮游植物会随混合而上下移动，经受着剧烈的光环境包括光暗周期、光强、光质等的变化。如何响应和适应这种光环境变化，是河口浮游植物存活下来、成为优势种，甚至爆发性生长所必须面对的首要问题。我们现场调查发现，珠江口最大浑浊带区真光层深度为7-15m，混合层深度在夏季为5-8m，且在5m以下存在低氧现象，面积约1500km2；该区域浮游植物群体光系统活性受控于光环境、且存在明显昼夜节律，河口浮游植物可快速适应河口盐度的梯度性变化，这从光生理学角度解释了河口浮游植物丰度较高，且经常爆发性生长、形成赤潮的内在原因。本项目在实验室内研究了细胞大小不同的代表性浮游植物硅藻和甲藻对光环境变化的响应与适应，首次发现并证实了硅藻与蓝藻和高等植物不同可以在夜间利用呼吸产生的能量修复白天失活的光系统，这为硅藻在全球海洋范围内广泛分布，且成为优势种提供了一个新证据。还发现，除了光强变化会影响硅藻、甲藻光系统活性外，光暗期的变化也会改变其活性，影响细胞生长；同时证实了细胞大小不同的硅甲藻对光暗期的生理响应不同。如光期较短有利于细胞较小的甲藻生长，但光期较长时有利于硅藻细胞的生长；在光强较低时，光期较长更有利于细胞较大的甲藻生长。还证实，与光暗期变化相比，温度的变化对细胞较小的硅藻光合生理及生长的影响更大，而光暗期变化对较大的硅藻影响更大，而且光期越短越有利于缓解温度变化对硅藻生理、生长的影响。进一步证实了光强和光暗期会协同改变细胞的光能吸收和利用能力、暗呼吸及胞内抗氧化系统活性，最终改变其生长。最后，细胞较小浮游植物的最适生长温度高于较大的，因此在一定范围的温度升高对较小细胞的生长更有利。我们阐明了河口浮游植物对光环境变化的响应与适应机理，从细胞大小角度揭示了河口光环境变化会改变浮游植物种群结构，影响河口生态系统。