Oxygenic photosynthesis evolved more than 3 billion years ago in cyanobacteria. The increased complexity of photosystem I (PSI) became apparent from the high-resolution structures that were obtained for the complexes that were isolated from various organisms, ranging from cyanobacteria to plants. These complexes are all evolutionarily linked. In this paper, the researchers have uncovered the increased complexity of PSI in a single organism demonstrated by the coexistance of two distinct PSI compositions. The Large Dunaliella PSI contains eight additional subunits, six in PSI core and two light harvesting complexes. Two additional chlorophyll a molecules pertinent for efficient excitation energy transfer in state II transition were identified in PsaL and PsaO. Short distances between these newly identified chlorophylls correspond with fast excitation transfer rates previously reported during state II transition. The apparent PSI conformations could be a coping mechanism for the high salinity.

有氧光合作用在蓝细菌中发展了超过30亿年前。从高分辨率结构获得的光系统I（PSI）复杂性的增加显而易见，该结构是从从蓝细菌到植物的各种生物体中分离得到的复合物获得的。这些复合物都是进化联系在一起的。在本文中，研究人员发现两种不同PSI成分的共存证明了单一生物体中PSI的复杂性增加。大型杜氏藻PSI包含八个额外的亚基，六个在PSI核心中，两个两个光收集复合物。另外两个叶绿素a在PsaL和PsaO中鉴定了与状态II转换中有效激发能量转移相关的分​​子。这些新发现的叶绿素之间的短距离对应于先前在状态II过渡期间报道的快速激发转移速率。明显的PSI构象可能是高盐度的应对机制。