Algicidal activities of secondary metabolites of marine macroalgal-derived endophytic fungi

Abstract

Red tide occurrs frequently and causes significant damage to the environment and human health. As a result, development of new efficient and environment friendly red-tide microalgae inhibitors has gained increasing attention in recent times. Algicolous endophytic fungi with unique habitats are promising sources for active agents owing to their abundant secondary metabolites and distinguished activities. In this study, the algicidal activities of 49 marine macroalgal-derived endophytic fungi against phytoplankton Alexandrium tamarense, Prorocentrum donghaiense, Heterosigma akashiwo, and Chattonella marina were examined using 96-well microplate. Four fungal strains, including Aspergillus wentii (pt-1), A. ustus (cf-42), and A. versicolor (dl-29, pt-20), exhibited potent algicidal activities. A total of 32 pure compounds isolated from these fungi were noted to possess different degrees of algicidal activities. Of those, 11 compounds comprising five anthraquinones, two terpenoids, and four steroids showed high 24-h inhibition rates for the four red tide algae, with EC 50(24 h) values ranging from 0.01 to 14.29 μg/mL. Among them, compound 1 (1,5-dihydroxy-3-methoxy-7-methylanthraquinone) presented the strongest activity against H. akashiwo, and could decrease its chlorophyll a (Chl a ) and superoxide dismutase contents and increase the soluble protein, malondialdehyde, and peroxidase contents. These results suggested that the identified anti-algal compound might inhibit the growth of red tide algae by weakening photosynthesis (reducing Chl a content), destroying cell membrane, and damaging the antioxidant system.