Polyhydroxybutyrate (PHB) nanoparticles modulate metals toxicity in Hydra viridissima

The use of bioplastics (e.g., polyhydroxybutyrate) emerged as a solution to help reduce plastic pollution caused by conventional plastics. Nevertheless, bioplastics share many characteristics with their conventional counterparts, such as degradation to nano-sized particles and the ability to sorb environmental pollutants, like metals. This study aimed to assess the potential impacts of the interaction of metals (cadmium - Cd, copper - Cu, and zinc - Zn) with polyhydroxybutyrate nanoplastics (PHB-NPLs; ~200 nm) on the freshwater cnidarian Hydra viridissima in terms of mortality rates, morphological alterations, and feeding behavior. The metal concentrations selected for the combined exposures corresponded to concentrations causing 20 %, 50 %, and 80 % of mortality (LC₂₀, LC₅₀, and LC₈₀, respectively) and the PHB-NPLs concentrations ranged from 0.01 to 1000 μg/L. H. viridissima sensitivity to the metals, based on the LC₅₀'s, can be ordered as: Zn < Cd < Cu. Combined exposure to metals and PHB-NPLs yielded distinct outcomes concerning mortality, morphological changes, and feeding behavior, uncovering metal- and dose-specific responses. The interaction between Cd-LC_x and PHB-NPLs progressed from no effect at LC_20,96h to an ameliorative effect at Cd-LC_50,96h. Cu-LC_x revealed potential mitigation effects (LC_20,96h and LC_50,96h) but at Cu-LC_80,96h the response shifts to a potentiating effect. For Zn-LC_x, response patterns across the combinations with PHB-NPLs were like those induced by the metal alone. PHB-NPLs emerged as a key factor capable of modulating the toxicity of metals. This study highlights the context-dependent interactions between metals and PHB-NPLs in freshwater environments while supporting the need for further investigation of the underlying mechanisms and ecological consequences in forthcoming research.