The dinoflagellate Heterocapsa circularisquama is highly toxic to bivalves. However, significant toxicity to finfish species has not been reported. We previously found that H. circularisquama has light-dependent haemolytic agents. Purification and chemical structural analyses revealed that the haemolytic agent H2-a is a porphyrin derivative, which exhibits light-dependent cytotoxicity toward tumour cells. To clarify the biological activity of H2-a further, its antibacterial activities against Gram-positive and Gram-negative bacteria were investigated in this study. A fraction (F5) equivalent to H2-a purified from the methanol extract of H. circularisquama showed potent light-dependent bactericidal activity toward Staphylococcus aureus, and the activity was concentration- and light illumination time-dependent; however, Escherichia coli was highly resistant to F5. Electron microscopic observation suggested that F5 induces morphological changes in S. aureus in a light-dependent manner. Further analysis using other bacterial species showed that the Gram-positive bacterium Bacillus subtilis was more sensitive than the Gram-negative bacteria Pseudomonas aeruginosa and Vibrio alginolyticus. These results indicate that F5 is a photo-induced antibacterial agent with relatively higher specificity to Gram-positive bacteria. Iodometric assay suggested that singlet oxygen was generated from light-illuminated F5. Histidine, a specific singlet oxygen scavenger, markedly inhibited the photosensitising antibacterial activity of F5 against S. aureus, suggesting the involvement of singlet oxygen in antibacterial activity. The antibacterial spectrum of F5 was evidently different from that of 5,10,15,20-tetra (N,N,N-trimethylanilinium) porphyrin tetratosylate, a commercially available porphyrin compound with antibacterial activity. Our results demonstrate that H. circularisquama has a novel antibacterial photosensitiser, a porphyrin derivative, with relatively higher specificity to Gram-positive bacteria. To the best of our knowledge, this is the first study to discover a porphyrin derivative with antibacterial activity in marine microalga.

鞭毛鞭毛异鳞藻对双壳类有高度毒性。然而，尚未报道对有鳍鱼类有明显的毒性。先前我们发现圆形斑块菌具有光依赖性溶血剂。纯化和化学结构分析表明，溶血剂H2-a是卟啉衍生物，对肿瘤细胞表现出光依赖性细胞毒性。为了进一步阐明H2-a的生物学活性，在本研究中研究了其对革兰氏阳性和革兰氏阴性细菌的抗菌活性。从圆形等渗藻的甲醇提取物中纯化的相当于H2-a的馏分（F5）对金黄色葡萄球菌表现出强效的光依赖性杀菌活性，其活性与浓度和光照时间有关；但是，大肠杆菌对F5具有高度抗性。电子显微镜观察表明，F5以光依赖的方式诱导金黄色葡萄球菌的形态变化。使用其他细菌种类进行的进一步分析表明，革兰氏阳性细菌枯草芽孢杆菌比革兰氏阴性细菌铜绿假单胞菌和溶藻弧菌更敏感。这些结果表明F5是对革兰氏阳性细菌具有相对较高特异性的光诱导抗菌剂。碘量法测定表明，单线态氧是由光照下的F5产生的。组氨酸，一种特定的单线态氧清除剂，显着抑制F5对金黄色葡萄球菌的光敏抗菌活性，表明单线态氧与抗菌活性有关。F5的抗菌谱明显不同于5,10,15,20-四（N，N，N-三甲基苯胺）卟啉四甲苯磺酸盐，这是一种可商购的具有抗菌活性的卟啉化合物。我们的结果表明，圆斑鳞茎具有新型抗菌光敏剂，一种卟啉衍生物，对革兰氏阳性细菌具有相对较高的特异性。据我们所知，这是第一个在海洋微藻中发现具有抗菌活性的卟啉衍生物的研究。