Microplastics in bloom-forming macroalgae: Distribution, characteristics and impacts

[Display omitted] •Drifting macroalgae showed a higher microplastics (MPs) abundance than attached form.•MPs abundance in drifting macroalgae increased along the drifting path.•MPs abundance in driting macroalgae was 2204 times higher than that in seawater.•Ulva prolifera has diverse mechanisms to trap microplastics.•U. prolifera has a high tolerance to microplastics. Macroalgal blooms and marine microplastics (MPs), as global challenges for oceans, are both showing a rising trend. However, none is known regarding the interaction of these two important issues. The Yellow Sea suffers the world’s largest green tides and severe MPs pollution as well. Therefore, we tracked the trapping of MPs by drifting Ulva prolifera in the Yellow Sea during the green-tide period. The abundance of MPs in drifting U. prolifera was 595–3917 times higher than that in seawater and increased along the drifting path from south to north in the Yellow Sea. In addition, four mechanisms of trapping plastics (twining, attachment, embedment, and wrapping) on or in U. prolifera were unmasked, which explains why the plant has such strong capacity to trap MPs. Laboratory incubation experiments showed that MPs (0.025–25 mg L−1) did not affect relative growth rate, effective photochemical efficiency of photosystem II (PSII), or saturating irradiance of U. prolifera until reaching an extremely high concentration (100 mg L−1), indicating a high tolerance to MPs. Due to tremendous biomass and coverage of the green tide and increased frequency as well, the plastics trap in drifting macroalgae can alter the spatio-temporal distribution of MPs in the oceans.