| 摘要: | 本研究探討藍綠菌Cyanobacterium aponinum在極端高光逆境下的耐受表型演化與光保護策略。結果顯示，野生型菌株在1200 μmol photons m⁻² s⁻¹ 的高光條件下大部分生長停滯並出現色素降解，僅少數細胞能存活並恢復增殖，突顯其在光系統與調控網路中存在快速適應的潛力。通過篩選與培養耐高光突變株，發現其光系統運作、能量耗散與抗氧化能力均重新達到平衡，且在持續光條件下生長速率與野生型相當，顯示高光適應並未伴隨顯著生長成本。
全基因組分析顯示，耐高光突變株僅累積少量突變事件，其中C. aponinum AL20115HLr-2的rpaA基因發生9 bp插入，導致三個胺基酸插入，可能是驅動高光耐受的核心事件。RpaA作為晝夜節律輸出型轉錄因子，可調控光合作用、碳代謝與修復系統，其插入突變可能透過微調二聚化穩定性及DNA結合親和力，改變下游數百個基因的轉錄輸出，實現對光逆境的快速適應。
色素分析顯示，突變株類胡蘿蔔素顯著上升，葉綠素a含量下降，呈現典型耐高光光合生物的色素重編程模式，透過「降低能量輸入 + 提高光保護機制」的策略，降低光氧化傷害。儘管突變株在光暗循環下生長速率下降，顯示晝夜節律輸出可能受 rpaA 插入突變影響，但其在高光環境下的存活優勢仍明顯。
綜合而言，C. aponinum AL20115HLr-2 的耐高光表型可能是由rpaA 基因插入突變，影響「晝夜節律調控× 光保護色素系統強化 × 能量分配權衡」三者共同驅動的整體適應轉型。研究結果深化了對藍綠菌高光適應機制的理解，並為利用晝夜節律因子提升光合生物耐逆境能力提供理論依據。
;This study investigates the evolution of high-light (HL) tolerance and photoprotective strategies in Cyanobacterium aponinum under extreme HL stress. Wild-type cells exhibited growth arrest and pigment degradation under 1200 μmol photons m⁻² s⁻¹, with only a small fraction surviving and resuming proliferation, highlighting rapid adaptive potential in their photosynthetic and regulatory networks. Selected HL-tolerant mutants maintained stable photosystem function, enhanced energy dissipation, and antioxidative capacity, while sustaining growth rates comparable to the wild type under continuous light, indicating that HL adaptation occurred without significant growth costs.
Whole-genome analysis revealed few accumulated mutations in HL-tolerant strains. In particular, the AL20115HL-2 mutant harbored a 9-bp insertion in the rpaA gene, resulting in three amino acid insertions. RpaA, a key output transcription factor of the circadian clock, regulates photosynthesis, carbon metabolism, and repair pathways. Structural modeling suggests that this insertion may subtly alter RpaA dimerization and DNA-binding affinity, thereby modulating transcriptional outputs of hundreds of downstream genes and enabling rapid adaptation to high light stress.
Pigment analysis showed a significant increase in carotenoids and a decrease in chlorophyll a, reflecting typical photoprotective reprogramming. This "reduced light harvesting + enhanced energy dissipation" strategy effectively mitigates photodamage. Although growth under light-dark cycles was slower, likely due to altered circadian output from the rpaA insertion, the mutant retained a clear survival advantage under HL conditions.
In summary, the high-light-tolerant phenotype of C. aponinum AL20115HLr-2 is likely driven by an insertional mutation in the rpaA gene, which induces an integrated adaptive transition jointly mediated by circadian regulation, enhancement of photoprotective pigment systems, and trade-offs in energy allocation. These findings deepen our understanding of high-light adaptation mechanisms in cyanobacteria and provide a theoretical framework for exploiting circadian regulatory factors to enhance stress tolerance in photosynthetic organisms. |
