Threat beneath the surface: impact of micro(nano)plastics on aquatic microorganisms†

Micro(nano)plastics (MNPs) pose a significant threat to aquatic ecosystems, yet their impact on eukaryotic microorganisms within microbial flocs remains poorly understood. This study investigated the effects of MPs (8 μm) and NPs (80 nm) on aquaculture-related microbial communities over a 4 week exposure period. Microbial floc samples were analyzed on days 7 and 28 post-exposure using 18S rRNA sequencing and functional prediction tools. Key findings revealed that NPs and MPs induced distinct temporal shifts in microbial diversity and community structure. On day 7, both NPs and MPs significantly increased microbial abundance, with MPs showing higher Simpson's diversity than NPs. By day 28, MPs caused a marked decline in community richness and altered dominant taxa, while NPs maintained higher diversity. Functional analysis highlighted elevated roles of microorganisms in plastic degradation and ATPase-related growth pathways in MP-exposed groups during the early stages. Notably, MNPs reduced the relative abundance of potential animal pathogens in early-stage flocs, with NPs exerting stronger stress on microbial assembly than MPs. Furthermore, NPs preferentially influenced biofilm-forming taxa, whereas MPs promoted shifts toward parasitic fungi. These findings underscore that MNPs disrupt microbial floc ecosystems primarily during initial colonization, with long-term stability restored through community self-regulation.