Plastic leachates promote marine protozoan growth

Millions of tons of plastic enter the ocean annually, yet the effects of their leachates on the microbial loop are poorly understood. This study investigates how dissolved organic carbon (DOC) released from field-collected plastics and a bioplastic influences the growth of the protozoan Oxyrrhis marina and its associated bacterial community. Plastics increased dissolved organic carbon (DOC) concentrations in seawater by 5 to 34-fold, stimulating O. marina growth by up to an order of magnitude compared to the control. After exposure to conventional beach plastic leachates and bioplastic leachates, O. marina exhibited growth rates up to 0.3 d$^{-1}$ and 0.4 d$^{-1}$, respectively, even in the absence of microalgal prey. We estimated that each gram of microplastics could lead to daily assimilation of up to 0.7 g of carbon per gram of protozoan, indicating that plastic-derived carbon enhances heterotrophic metabolism in the microbial loop through osmotrophy. Given that autotrophic prokaryotes are negatively impacted by plastic leachates and that plastic pollution is expected to triple in the coming decades, plastic leaching could alter the balance between microbial primary production and heterotrophy in the ocean.