Warming coupled with elevated pCO₂ modulates microplastic inhibition in a commercial red alga Pyropia haitanensis

Ocean acidification, warming, and microplastics are pervasive stressors in coastal ocean, yet their combined effects on economically important seaweed Pyropia haitanensis remain unclear. To investigate how elevated pCO₂, warming, and microplastics interact to affect physiology of P. haitanensis, we cultured thalli at ambient (418 μatm, AC) and elevated (1000 μatm, HC) CO₂ levels with two temperatures (20 and 24 °C), and a gradient of microplastics (0.025, 2.5, 25, 50, 100 mg L^-1) in a controlled indoor experiment. Our results indicate that microplastics imposed a strong, concentration-dependent stress on P. haitanensis, consistently reducing relative growth rate (RGR), F_v/F_m, photosynthetic pigments (chlorophyll a, carotenoids, and phycobiliproteins), and cellular reserves (soluble protein and carbohydrates), with the strongest inhibition observed at concentration of 100 mg L^-1. However, while the increased temperature (24 °C) promoted the content of pigments and soluble protein of the thalli, it decreased the content of soluble carbohydrate among the microplastic concentrations regardless of pCO₂ levels. It is noteworthy that under ambient pCO₂ level, elevated temperature exacerbated the growth inhibition caused by microplastics, resulting in the highest inhibition rate of 57 % occurring at 100 mg L^-1. In contrast, this temperature-aggravated microplastic toxicity was mitigated by high pCO₂ levels, with the inhibition rate of 32 % at the highest microplastic concentration. These findings reveal that while elevated pCO₂ and warming can modulate microplastic stress via physiological reallocation, persistent declines in photochemical efficiency and light-harvesting pigments may constrain yield and nutritional quality of P. haitanensis where microplastics are high in coastal aquaculture area.