Conspecific cues that induce spore settlement in the biofouling and green tide-forming alga Ulva tepida provide a potential aggregation mechanism

Abstract

Ulva, an important biofouling and green tide-forming alga, forms aggregations that have adverse economic and ecological impacts. However, little is known regarding the aggregation mechanism of Ulva. Knowledge of Ulva aggregation provides clues for controlling this alga in marine biofouling and green tides. We hypothesized Ulva contains conspecific settlement cue(s). Hexane and dichloromethane extracts of U. tepida fronds were found to induce the settlement of conspecific spores. From these extracts, two bioactive compounds that induce spore settlement in U. tepida were isolated: C16:4 at 5 μg mL−1 and di-n-butyl phthalate at 0.1–10 μg mL−1. Fatty acids with different carbon chains (C16–C18) and degrees of saturation, C16:1 and C18:2, previously reported to be present in U. tepida fronds, showed significant inducing activity for spore settlement. C16:1, C16:4, C18:2, and di-n-butyl phthalate all triggered dense settlements of U. tepida spores. The results support our hypothesis and provide insight into possible aggregation mechanisms for Ulva.