Photosynthesis in deserts is challenging since it requires fast adaptation to rapid night-to-day changes, that is, from dawn’s low light (LL) to extreme high light (HL) intensities during the daytime. To understand these adaptation mechanisms, we purified photosystem I (PSI) from Chlorella ohadii, a green alga that was isolated from a desert soil crust, and identified the essential functional and structural changes that enable the photosystem to perform photosynthesis under extreme high light conditions. The cryo-electron microscopy structures of PSI from cells grown under low light (PSI_LL) and high light (PSI_HL), obtained at 2.70 and 2.71 Å, respectively, show that part of light-harvesting antenna complex I (LHCI) and the core complex subunit (PsaO) are eliminated from PSI_HL to minimize the photodamage. An additional change is in the pigment composition and their number in LHCI_HL; about 50% of chlorophyll b is replaced by chlorophyll a. This leads to higher electron transfer rates in PSI_HL and might enable C. ohadii PSI to act as a natural photosynthesiser in photobiocatalytic systems. PSI_HL or PSI_LL were attached to an electrode and their induced photocurrent was determined. To obtain photocurrents comparable with PSI_HL, 25 times the amount of PSI_LL was required, demonstrating the high efficiency of PSI_HL. Hence, we suggest that C. ohadii PSI_HL is an ideal candidate for the design of desert artificial photobiocatalytic systems.

沙漠中的光合作用具有挑战性，因为它需要快速适应夜间的快速变化，即从黎明的低光 (LL) 到白天的极端高光 (HL) 强度。为了了解这些适应机制，我们从小球藻（一种从沙漠土壤地壳中分离出来的绿藻）中纯化了光系统 I (PSI)，并确定了使光系统能够在极端强光条件下进行光合作用的基本功能和结构变化。_{在低光 (PSI LL} ) 和高光 (PSI _HL )下生长的细胞的 PSI 的低温电子显微镜结构)，分别在 2.70 和 2.71 Å 处获得，表明部分光捕获天线复合物 I (LHCI) 和核心复合物亚基 (PsaO) 从 PSI _HL中消除，以最大限度地减少光损伤。_{另一个变化是 LHCI HL}中的颜料成分及其数量；约 50% 的叶绿素b被叶绿素a取代。这导致 PSI _HL中的电子转移率更高，并可能使C. ohadii PSI 在光生物催化系统中充当天然光合作用剂。将PSI _HL或 PSI _LL连接到电极上并测定它们的感应光电流。获得与 PSI _{HL相当的光电流}，需要 25 倍的 PSI _LL量，证明了 PSI _HL的高效率。因此，我们建议C. ohadii PSI _HL是设计沙漠人工光生物催化系统的理想候选者。