Antioxidative and antimicrobial activities of intertidal seaweeds and possible effects of abiotic factors on these bioactivities

To overcome acute physiological stress from frequent exposure to high irradiance levels and emersion stress, seaweeds colonizing intertidal zones generate valuable secondary metabolites. Their chemical composition is influenced by spatial changes in environmental parameters, and bioactivities closely linked to specific compounds change accordingly. We measured antioxidative and antimicrobial activities of 26 species in ten intertidal zones of eastern Guangdong, China, and examined the possible effects of abiotic factors. Most brown algae exhibited higher antioxidative activity and total phenol content than red algae and green algae, while most brown algae and green algae revealed more efficient antimicrobial activity than red algae. Their activities were also affected by the environment of their intertidal habitats. Similar antioxidative ability or total phenol content were found in seaweeds settled in high-, middle- and low-tide zones, while more positively antimicrobial ability was discovered in seaweeds colonizing the low-tide zone than in those in middle- and high-tide zones. These differences were mainly caused by the different stress levels of sun exposure, nutrition and desiccation, as well as temperature and salinity fluctuations in various tidal regions. Seaweeds colonizing coastal waters and experiencing stresses such as low salinity, limited dissolved oxygen (DO) or rich nutrition exhibited superior antioxidative ability and total phenol content. This result was further strengthened by the finding that the antioxidative ability and total phenol content of Ulva fasciata were positively affected by nutrition and negatively influenced by DO or salinity. The antioxidative activity and total phenol content of Sargassum vachellianum were also positively affected by ammonium-nitrogen. No similar trend was found in the antimicrobial activity of seaweeds. Our results suggested an effect of abiotic factors on the antioxidative and antimicrobial activities of intertidal seaweeds in the wild, and may allow for selective gathering high-activity category in terms of algal species and local environmental conditions.