Raanan Hagai, Oren Nadav, Treves Haim, Keren Nir, Ohad Itzhak, Berkowicz Simon M, Hagemann Martin, Koch Moriz, Shotland Yoram, Kaplan Aaron
Dept. of Plant and Environmental Sciences, The Hebrew University of Jerusalem, Edmond J. Safra Campus, Givat Ram, Jerusalem 9190401, Israel.
Dept. of Plant and Environmental Sciences, The Hebrew University of Jerusalem, Edmond J. Safra Campus, Givat Ram, Jerusalem 9190401, Israel; Arid Ecosystems Research Center, The Hebrew University of Jerusalem, Edmond J. Safra Campus, Givat Ram, Jerusalem 9190401, Israel.
Biochim Biophys Acta. 2016 Jun;1857(6):715-22. doi: 10.1016/j.bbabio.2016.02.007. Epub 2016 Feb 16.
Organisms inhabiting biological soil crusts (BSCs) are able to cope with extreme environmental conditions including daily hydration/dehydration cycles, high irradiance and extreme temperatures. The photosynthetic machinery, potentially the main source of damaging reactive oxygen species during cessation of CO(2) fixation in desiccating cells, must be protected to avoid sustained photodamage. We compared certain photosynthetic parameters and the response to excess light of BCS-inhabiting, desiccation-tolerant cyanobacteria Leptolyngbya ohadii and Nostoc reinholdii with those observed in the "model" organisms Nostoc sp. PCC 7120, able to resurrect after mild desiccation, and Synechococcus elongatus PCC 7942 and Synechocystis sp. PCC 6803 that are unable to recover from dehydration. Desiccation-tolerant strains exhibited a transient decline in the photosynthetic rate at light intensities corresponding to the inflection point in the PI curve relating the O(2) evolution rate to light intensity. They also exhibited a faster and larger loss of variable fluorescence and profoundly faster Q(A)(-) re-oxidation rates after exposure to high illumination. Finally, a smaller difference was found in the temperature of maximal thermoluminescence signal in the absence or presence of 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU) than observed in "model" cyanobacteria. These parameters indicate specific functional differences of photosystem II (PSII) between desiccation tolerant and sensitive cyanobacteria. We propose that exposure to excess irradiation activates a non-radiative electron recombination route inside PSII that minimizes formation of damaging singlet oxygen in the desiccation-tolerant cyanobacteria and thereby reduces photodamage.
栖息于生物土壤结皮（BSCs）中的生物能够应对极端环境条件，包括每日的水合/脱水循环、高辐照度和极端温度。光合机构可能是干燥细胞中停止二氧化碳固定期间活性氧损伤的主要来源，必须加以保护以避免持续的光损伤。我们将栖息于生物土壤结皮、耐干燥的蓝细菌奥氏细鞘丝藻（Leptolyngbya ohadii）和雷氏念珠藻（Nostoc reinholdii）的某些光合参数以及对过量光照的响应，与在“模式”生物念珠藻属（Nostoc sp.）PCC 7120（能够在轻度干燥后复活）、聚球藻（Synechococcus elongatus）PCC 7942和聚球藻属（Synechocystis sp.）PCC 6803（无法从脱水状态恢复）中观察到的情况进行了比较。耐干燥菌株在与将氧气释放速率与光强相关的PI曲线拐点对应的光强下，光合速率出现短暂下降。它们在暴露于高光照后还表现出可变荧光更快、更大的损失以及Q(A)(-)再氧化速率显著加快。最后，与“模式”蓝细菌相比，在存在或不存在3-(3,4-二氯苯基)-1,1-二甲基脲（DCMU）的情况下，最大热释光信号温度的差异较小。这些参数表明耐干燥和敏感蓝细菌之间光系统II（PSII）存在特定的功能差异。我们提出，暴露于过量辐照会激活PSII内部的非辐射电子复合途径，从而使耐干燥蓝细菌中损伤性单线态氧的形成最小化，进而减少光损伤。
