PSI is an essential component of the photosynthetic apparatus of oxygenic photosynthesis. While most of its subunits are conserved, recent data have shown that the arrangement of the light-harvesting complexes I (LHCIs) differs substantially in different organisms. Here we studied the PSI-LHCI supercomplex of Botryococccus braunii, a colonial green alga with potential for lipid and sugar production, using functional analysis and single-particle electron microscopy of the isolated PSI-LHCI supercomplexes complemented by time-resolved fluorescence spectroscopy in vivo. We established that the largest purified PSI-LHCI supercomplex contains 10 LHCIs (~240 chlorophylls). However, electron microscopy showed heterogeneity in the particles and a total of 13 unique binding sites for the LHCIs around the PSI core. Time-resolved fluorescence spectroscopy indicated that the PSI antenna size in vivo is even larger than that of the purified complex. Based on the comparison of the known PSI structures, we propose that PSI in B. braunii can bind LHCIs at all known positions surrounding the core. This organization maximizes the antenna size while maintaining fast excitation energy transfer, and thus high trapping efficiency, within the complex.

光系统I（PSI）是氧合光合作用的光合装置的重要组成部分。尽管大多数亚基是保守的，但最近的数据表明，在不同的生物体中，光采复合物（LHCI）的排列方式大不相同。在这里，我们使用功能分析和单粒子电子显微镜对离体的PSI-LHCI超级复合物进行补充，并在体内进行时间分辨荧光光谱分析，研究了布氏葡萄球菌（Botryococccus braunii）的PSI-LHCI超级复合物，该脂质具有糖和糖生产的潜力。我们确定最大的纯化PSI-LHCI超复合物包含10个LHCI（〜240个叶绿素（Chls））。然而，电子显微镜证明了颗粒中的异质性，并显示了围绕PSI核心的LHCI共有13个独特的结合位点。时间分辨荧光光谱表明，体内的PSI天线尺寸甚至比纯化的复合物还要大。基于对已知PSI结构的比较，我们提出布鲁氏芽孢杆菌中的PSI可以在核心周围所有已知位置结合LHCI。这种组织结构使天线尺寸最大化，同时又保持了复杂区域内快速的激发能量传输，从而提高了捕获效率。