Dual impacts of elevated pCO2 on the ecological effects induced by microplastics and nanoplastics: A study with Chlamydomonas reinhardtii

Aquatic organisms face increased complexity and severity when exposed to the combined stressors of climate change and micro- and nanoplastics (MNPs), as opposed to facing these stressors individually. This study examined the effects and underlying mechanisms of elevated pCO₂, which leads to freshwater acidification, as well as amino-modified polystyrene MNPs (PS-NH₂ MNPs) of varying sizes (5 μm, 300 nm, 80 nm), on Chlamydomonas reinhardtii under both individual and combined conditions. The results showed a size-dependent toxicity of PS MNPs, with the smaller nanoparticles (80 nm) causing greater toxic inhibition than the larger microparticles (5 μm and 300 nm), primarily attributed to oxidative stress-related cellular damage. In contrast, freshwater acidification (FA) appeared to promote the growth of C. reinhardtii, possibly by upregulating transcripts associated with energy metabolism. However, when C. reinhardtii was exposed to both FA and MNPs simultaneously, distinct toxic effects were observed. The co-exposure to FA and NPs induced the most severe oxidative stress, implying the greatest energetic cost. This stress resulted in the downregulation of pathways involved in fatty acid biosynthesis and protein folding, ultimately causing significant damage to cellular structure and function. The increased energy from the upregulation of the TCA cycle was mainly allocated for DNA damage repair and cell division, which induced an energy deficit necessary for stress resistance. In contrast, during co-exposure to FA and MPs, energy was redirected towards DNA replication and the synthesis of anti-stress substances, facilitating recovery and promoting growth. Our study highlighted the decisive influence of climate change and particle size in assessing the ecological effects and risks associated with MNPs.