Salt marsh zonation and substrate type modulation for plastisphere: an experimental assessment in the Lagoon Patos estuary in extreme south of Brazil

The accumulation of plastics in aquatic environments is a growing global concern, as biofouling on plastic debris leads to the formation of the plastisphere, an ecological niche for diverse microbial and macrofouling organisms. Although plastic characteristics such as size, color, and polymer type may influence plastisphere development, there is no consensus regarding their relative importance, and studies in estuarine environments remain scarce. Here, we investigated plastisphere formation across three salt marsh zones (dry, intermediate, and flooded) in the Patos Lagoon estuary (southern Brazil), considering variations in plastic size (6 × 2, 30 × 10, and 60 × 20 mm), color (white, black, and red), and polymer type (Polypropylene—PP, Polystyrene—PS, and Ethylene Vinyl Acetate—EVA). Three independent 21-day field experiments were conducted, and plastisphere development was assessed using multiple complementary approaches, including biomass (weight), bacterial density, photosynthetic pigment composition, macrofouling abundance, and DNA metabarcoding of bacterial (16S rRNA) and fungal (ITS) communities. Plastisphere development consistently followed the flooding gradient, with higher biomass, microbial density, and photosynthetic pigment concentrations in flooded and intermediate zones compared to the dry zone. Smaller plastic substrates favored microbial colonization, whereas larger substrates supported higher macrofouling abundance. Polymer color and type modulated colonization patterns of specific taxa, with EVA substrates showing higher biofilm accumulation. Community-level analyses revealed that flooding regime was the driver structuring bacterial and fungal community composition, with higher richness, diversity, and evenness in the intermediate zone. Differences among zones were driven mainly by shifts in the relative abundance of shared taxa rather than taxonomic turnover. Overall, this study demonstrates that environmental context, particularly flooding regime, outweighs plastic characteristics in shaping plastisphere communities in salt marshes, providing new insights into plastisphere dynamics in understudied Neotropical estuarine ecosystems.