High microplastic-microalga ratios facilitate the short-term growth and dissolved organic matter transformation of marine Porphyridium

Current assessments of microplastic (MP) toxicity on microalgae often overlook the varying microplastic-to-microalgae (MM) ratios found in marine environments, potentially misinterpreting ecological risks. In the present study, we investigate whether adjusting MM ratios could alleviate photoinhibition and stimulate the short-term growth of marine microalgae Porphyridium cruentum, a species noted for its bounded exopolysaccharides (b-EPs). Contrary to the typical toxicity paradigm, we observed that high MM ratios (70 and 140) significantly improved short-term microalgal growth by up to 45.2% and alleviated photosynthetic pigment degradation relative to the control microalgae. Mechanistically, this growth promotion might be driven by a b-EP-mediated "self-protection" process: The C-O/C-O-C bond facilitated MP adsorption to b-EPs, enhancing b-EPs concentration in the high MM groups and promoting the aggregation and sedimentation of MPs. This process resulted in a 37% decrease in the original dissolved organic matter molecular formulas in the high MM groups, creating a low-oxidative habitat that favored initial survival. Crucially, the microalgae demonstrated remarkable plasticity under these extreme loads; in the MM140 group, cells compensated for MP-induced shading by significantly upregulating B-phycoerythrin (+57%), carotenoids (+42%), and chlorophyll a (+19%) to sustain photosynthesis. However, the short-term growth promotion and MP aggregates induced cell shading and phosphate limitation, suppressing photosynthesis-antenna proteins and downregulating DNA replication and nucleotide excision repair pathways—signaling potential long-term risks. This study provides a novel mechanistic understanding of MP-microalga interactions, proposing that microalgae can utilize b-EPs to mitigate acute MP stress at the expense of long-term integrity, a finding that reshapes our understanding of microalgal resilience in polluted oceans.