Biogeography of Photosynthetic Light-Harvesting Genes in Marine Phytoplankton

BackgroundPhotosynthetic light-harvesting proteins are the mechanism by which energy enters the marine ecosystem. The dominant prokaryotic photoautotrophs are the cyanobacterial genera Prochlorococcus and Synechococcus that are defined by two distinct light-harvesting systems, chlorophyll-bound protein complexes or phycobilin-bound protein complexes, respectively. Here, we use the Global Ocean Sampling (GOS) Project as a unique and powerful tool to analyze the environmental diversity of photosynthetic light-harvesting genes in relation to available metadata including geographical location and physical and chemical environmental parameters.
MethodsAll light-harvesting gene fragments and their metadata were obtained from the GOS database, aligned using ClustalX and classified phylogenetically. Each sequence has a name indicative of its geographic location; subsequent biogeographical analysis was performed by correlating light-harvesting gene budgets for each GOS station with surface chlorophyll concentration.
Conclusion/SignificanceUsing the GOS data, we have mapped the biogeography of light-harvesting genes in marine cyanobacteria on ocean-basin scales and show that an environmental gradient exists in which chlorophyll concentration is correlated to diversity of light-harvesting systems. Three functionally distinct types of light-harvesting genes are defined: (1) the phycobilisome (PBS) genes of Synechococcus; (2) the pcb genes of Prochlorococcus; and (3) the iron-stress-induced (isiA) genes present in some marine Synechococcus. At low chlorophyll concentrations, where nutrients are limited, the Pcb-type light-harvesting system shows greater genetic diversity; whereas at high chlorophyll concentrations, where nutrients are abundant, the PBS-type light-harvesting system shows higher genetic diversity. We interpret this as an environmental selection of specific photosynthetic strategy. Importantly, the unique light-harvesting system isiA is found in the iron-limited, high-nutrient low-chlorophyll region of the equatorial Pacific. This observation demonstrates the ecological importance of isiA genes in enabling marine Synechococcus to acclimate to iron limitation and suggests that the presence of this gene can be a natural biomarker for iron limitation in oceanic environments.

【背景】光合捕光蛋白是能量进入海洋生态系统的核心途径。占主导地位的原核光合自养生物为蓝细菌属的原绿球藻（Prochlorococcus）和聚球藻（Synechococcus），二者分别以两种截然不同的捕光系统为特征：结合叶绿素的蛋白复合物，以及结合藻胆素的蛋白复合物。本研究以全球海洋采样（Global Ocean Sampling, GOS）计划为独特且高效的研究工具，针对光合捕光基因的环境多样性展开分析，并结合已公开的元数据——包括地理位置、理化环境参数——开展关联研究。
【方法】本研究从GOS数据库中获取所有捕光基因片段及其元数据，使用ClustalX进行序列比对并开展系统发育分类。每条序列的名称均标注了其采样地理位置；后续通过关联各GOS采样站位的捕光基因组成与表层叶绿素浓度，完成生物地理分析。
【结论与意义】本研究依托GOS数据，在大洋盆地尺度上绘制了海洋蓝细菌捕光基因的生物地理分布图谱，并证实存在环境梯度：叶绿素浓度与捕光系统的多样性显著相关。本研究定义了三类功能迥异的捕光基因：（1）聚球藻的藻胆体（phycobilisome, PBS）基因；（2）原绿球藻的pcb基因；（3）部分海洋聚球藻中存在的铁胁迫诱导（iron-stress-induced, isiA）基因。在营养盐受限的低叶绿素浓度区域，Pcb型捕光系统的遗传多样性更高；而在营养盐充足的高叶绿素浓度区域，PBS型捕光系统则展现出更丰富的遗传多样性。我们将这一现象归因于特定光合策略的环境筛选作用。尤为重要的是，独特的isiA捕光系统存在于赤道太平洋的铁受限、高营养盐低叶绿素区域。该发现证实了isiA基因在帮助海洋聚球藻适应铁限制胁迫中的生态重要性，并表明该基因的存在可作为海洋环境中铁限制的天然生物标志物。