Seagrass under siege? Investigating the effects of microplastics on Eelgrass (Zostera muelleri) in a...

Seagrass meadows are critical coastal ecosystems that are increasingly recognised for their potential to form microplastic sinks. This study investigated the impacts of microplastic contamination on Eelgrass (Zostera muelleri), a dominant seagrass in the Indo-Pacific. Through a 90-day mesocosm experiment, Z. muelleri was exposed to three escalating microplastic concentrations (885, 3,540, and 8850 particles.kg^-1 sediment) to evaluate morphological, photosynthetic (pulse amplitude modulation fluorometry), bacterial microbiome (16S rRNA gene amplicon sequencing), and leachate (LC-QToF-MS/MS) responses. Relative to controls (0 particles.kg^-1), no significant morphological effects were detected at current environmental concentrations (885 particles.kg^-1 sediment), but significant declines in biomass (72-86%), rhizome length (110-190%), and leaf count (8-10 fewer leaves) under elevated microplastic loads. Photosynthetic (Y-Yield) efficiency was not affected, and sediment microbiomes exhibited resilience, with no significant shifts in diversity. Chemical analyses identified 82 leachates in sediments, including ten unique to treatments spiked with microplastics, suggesting leachate toxicity may be related to observed biological impacts. These findings suggest that Z. muelleri may be resilient to microplastic pollution at current observed concentrations, but are likely to be negatively affected if microplastic pollution continues to rise. Research on leachate-specific effects is required for targeted mitigation strategies to help conserve these vital ecosystems.