<i>Nannochloropsis oceanica</i> (Eustigmatophyceae) mutants resistant to rose bengal demonstrate high tolerance to oxygen

The genus <i>Nannochloropsis</i> (Eustigmatophyceae), including <i>N. oceanica</i>, has attracted considerable interest, due to its ability to accumulate high levels of lipids. Under large-scale cultivation outdoors, cells are exposed to a combination of high oxygen and light irradiance conditions that may promote photoinhibition, so selecting strains able to tolerate oxidative stress is desirable. Rose bengal (RB) reacts with oxygen to form singlet oxygen in the presence of light. Therefore, RB can be a very useful tool for testing the sensitivity of algae to oxidative stress and thus identify strains capable of photosynthesis under oxidative stress. In this study, two mutants of <i>N. oceanica</i>, RB2 and RB113, which are resistant to high concentrations of rose bengal, were phenotypically characterized for their sensitivity to high oxygen concentrations. Both strains exhibited levels of catalase and ascorbate peroxidase 1.5- and 3-fold higher, respectively, than the wild type. In addition, the synthesis of carotenoids was lower than in the wild type, indicating lower oxidative stress. These results were confirmed by chlorophyll fluorescence measurements, which indicated a lower sensitivity of the photosynthetic apparatus of the mutants, especially of RB2, even in the presence of H₂O₂. The results support the hypothesis that resistance to rose bengal may also induce resistance to high dissolved oxygen stress. The ability shown by the mutants to perform photosynthesis more efficiently under high oxygen stress than the wild type makes them promising candidates for outdoor cultures.

微拟球藻属（Nannochloropsis，隶属于真眼点藻纲（Eustigmatophyceae）），包含海洋微拟球藻（N. oceanica），因其可积累高水平脂质而受到学界广泛关注。在大规模户外培养过程中，藻细胞会同时暴露于高氧与高光辐照的复合环境，此种环境可能诱发光抑制，因此筛选耐受氧化胁迫的藻株具有重要应用价值。虎红（rose bengal，RB）在光照条件下可与氧气反应生成单线态氧，因此可作为检测藻类对氧化胁迫敏感性的有效工具，进而筛选出可在氧化胁迫下维持光合作用的藻株。本研究针对两株高浓度虎红耐受型海洋微拟球藻突变株RB2与RB113，对其高氧浓度耐受性开展了表型表征。结果显示，相较于野生型菌株，两株突变株的过氧化氢酶与抗坏血酸过氧化物酶活性分别提升1.5倍与3倍；此外，突变株的类胡萝卜素合成水平低于野生型，提示其自身承受的氧化胁迫程度更低。叶绿素荧光测定结果进一步验证了上述结论：即便在过氧化氢（H₂O₂）处理条件下，突变株（尤其是RB2）的光合机构对氧化胁迫的敏感性仍显著低于野生型。本研究结果支持“虎红耐受性可同时诱导藻株对高溶解氧胁迫产生抗性”这一假说。相较于野生型，突变株在高氧胁迫下可更高效地进行光合作用，这一特性使其成为户外大规模培养的潜在优良候选藻株。