Plastics are inexpensive and widely used organic polymers, but their high durability hinders biodegradation. Polystyrene, including extruded polystyrene (also known as styrofoam), is among the most commonly produced plastics worldwide and is recalcitrant to microbial degradation. In this study, we assessed changes in the gut microbiome of superworms (Zophobas morio) reared on bran, polystyrene or under starvation conditions over a 3 weeks period. Superworms on all diets were able to complete their life cycle to pupae and imago, although superworms reared on polystyrene had minimal weight gains, resulting in lower pupation rates compared to bran reared worms. The change in microbial gut communities from baseline differed considerably between diet groups, with polystyrene and starvation groups characterized by a loss of microbial diversity and the presence of opportunistic pathogens. Inferred microbial functions enriched in the polystyrene group included transposon movements, membrane restructuring and adaptations to oxidative stress. We detected several encoded enzymes with reported polystyrene and styrene degradation abilities, supporting previous reports of polystyrene-degrading bacteria in the superworm gut. By recovering metagenome-assembled genomes (MAGs) we linked phylogeny and functions and identified genera including Pseudomonas , Rhodococcus and Corynebacterium that possess genes associated with polystyrene degradation. In conclusion, our results provide the first metagenomic insights into the metabolic pathways used by the gut microbiome of superworms to degrade polystyrene. Our results also confirm that superworms can survive on polystyrene feed, but this diet has considerable negative impacts on host gut microbiome diversity and health.

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The rise of microbial species is associated with multiple genetic changes and niche reconstruction. While recombination, lateral gene transfer and point mutations can contribute to microbial speciation, acquisition of niche-specific genes was found to play an important role in initiating ecological specialization followed by genome-wide mutations. The critical step at the very early microbial speciation between ecologically distinct habitats, such as land and ocean, however, is elusive. Here we show that the divergence of archaea Poseidoniales between brackish and marine waters was triggered by rearranging a magnesium transport gene corA in a global geological background. The corA gene was inserted within a highly conservative gene cluster and possibly function in concert with the other genes in this cluster in osmotic stress response. It then went through sporadic losses and gains that were coincident with the Pangea tectonic activities and sea-level rising. Notably, metabolic adjustment and proteome-wide amino acid substitution were found after the change of corA. Our results highlight salinity adaptation as the primary factor in microbial speciation at the interface between land and ocean. Such a process can start from simply changing one gene but may need coherent gene cluster rearrangement and work in tune with strong selective forces such as global landform changes.