Microbial communities of polyhydroxyalkanoate (PHA)-based biodegradable composites plastisphere and of surrounding environmental matrix: a comparison between marine (seabed) and coastal sediments (dune sand) over a long-time scale

Biofilms in plastisphere from freshwater wetlands: Biofilm formation, bacterial community assembly, and biogeochemical cycles

Microorganisms can colonize to the surface of microplastics (MPs) to form biofilms, termed "plastisphere", which could significantly change their physiochemical properties and ecological roles. However, the biofilm characteristics and the deep mechanisms (interaction, assembly, and biogeochemical cycles) underlying plastisphere in wetlands currently lack a comprehensive perspective. In this study, in situ biofilm formation experiments were performed in a park with different types of wetlands to examine the plastisphere by extrinsic addition of PVC MPs in summer and winter, respectively. Results from the spectroscopic and microscopic analyses revealed that biofilms attached to the MPs in constructed forest wetlands contained the most abundant biomass and extracellular polymeric substances. Meanwhile, data from the high-throughput sequencing showed lower diversity in plastisphere compared with soil bacterial communities. Network analysis suggested a simple and unstable co-occurrence pattern in plastisphere, and the null model indicated increased deterministic process of heterogeneous selection for its community assembly. Based on the quantification of biogeochemical cycling genes by high-throughput qPCR, the relative abundances of genes involving in carbon degradation, carbon fixation, and denitrification were significantly higher in plastisphere than those of soil communities. This study greatly enhanced our understanding of biofilm formation and ecological effects of MPs in freshwater wetlands.

Plastic litter changes the rhizosphere bacterial community of coastal dune plants

The presence of plastic litter in coastal environments like beach-dune systems has been well documented, and recent studies have shown that this pollutant can influence sand properties as well as dune vegetation. However, the effects of plastics on rhizosphere bacterial communities of dune plants have largely been neglected. This is an ecologically relevant issue since these communities may play an important role in improving plant growth and resilience of dune systems. Here, we explored the impact of plastic litter made of either non-biodegradable polymers (NBP) or biodegradable/compostable polymers (BP) on the structure and composition of rhizosphere bacterial communities associated with two widespread species along coastal European dunes, Thinopyrum junceum and Sporobolus pumilus, by using a one-year field experiment combined with metabarcoding techniques. Both plastics did not affect neither the survival nor the biomass of T. junceum plants, but they significantly increased alpha-diversity of rhizosphere bacterial communities. They also changed rhizosphere composition by increasing the abundance of the phyla Acidobacteria, Chlamydiae, and Nitrospirae, and of the family Pirellulaceae, and reducing the abundance of the family Rhizobiaceae. NBP reduced drastically the survival of S. pumilus while BP increased its root biomass compared to controls. BP also increased the abundance of the phylum Patescibacteria of the rhizosphere bacterial communities. Our findings provide the first evidence that NBP and BP can change rhizosphere bacterial communities associated with dune plants and highlight the importance of investigating how these changes can affect the resilience of coastal dunes to climate change.

Effect of the Presence of Lignin from Woodflour on the Compostability of PHA-Based Biocomposites: Disintegration, Biodegradation and Microbial Dynamics

Poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) has gained attention as a possible substitute for conventional polymers that could be integrated into the organic recycling system. Biocomposites with 15% of pure cellulose (TC) and woodflour (WF) were prepared to analyze the role of lignin on their compostability (58 °C) by tracking the mass loss, CO₂ evolution, and the microbial population. Realistic dimensions for typical plastic products (400 µm films), as well as their service performance (thermal stability, rheology), were taken into account in this hybrid study. WF showed lower adhesion with the polymer than TC and favored PHBV thermal degradation during processing, also affecting its rheological behavior. Although all materials disintegrated in 45 days and mineralized in less than 60 days, lignin from woodflour was found to slow down the bioassimilation of PHBV/WF by limiting the access of enzymes and water to easier degradable cellulose and polymer matrix. According to the highest and the lowest weight loss rates, TC incorporation allowed for higher mesophilic bacterial and fungal counts, while WF seemed to hinder fungal growth. At the initial steps, fungi and yeasts seem to be key factors in facilitating the later metabolization of the materials by bacteria.

Controllable 3D Flower-Like Ag-CF Electrodes as Flexible Marine Electric Field Sensors with High Stability

Construction of three-dimensional (3D) flower-like nanostructures with controlled morphologies has emerged as an attractive tool by scientists in the marine electric field sensor research field due to their peculiar structural features. Herein, novel 3D flower-like Ag-CF capacitive composite electrodes have been created by an eco-friendly water-bath strategy via AgNO₃ as a sliver source and subsequently compounded with carbon fibers (CFs) pretreated by thermal oxidation. A series of electrode samples with various morphologies obtained by modulating different reaction times and temperatures bring about the dominant formation mechanism of these nanostructures and the influence behavior on the CF electrode in detail. Especially, the 3D flower-like Ag-CF electrode shows a large surface area acquired under the conditions of 80 °C and 15 min, which can provide more electroactive sites in electrochemical analysis and exhibit a maximum areal specific capacitance of 619.75 mF·cm^-2 at a scanning speed of 10 mV·s^-1. This is mainly due to the synergistic behavior of the unique 3D flower-like morphology and the large specific surface area of CFs. Furthermore, a cylinder-shaped Ag-CF sensor is designed, which delivers a superior potential difference of 33.08 μV, a potential difference drift of 18.62 μV/24 h for 30 days, and a self-noise of 0.92 nV/rt (Hz)@1 Hz. In this work, the intriguing synthesis strategy can be a promising facile approach to manufacture the controllable 3D flower-like Ag-CF electrode for electric field sensor applications.

Time-course biofilm formation and presence of antibiotic resistance genes on everyday plastic items deployed in river waters

The plastisphere has been widely studied in the oceans; however, there is little information on how living organisms interact with the plastisphere in freshwater ecosystems, and particularly on how this interaction changes over time. We have characterized, over one year, the evolution of the eukaryotic and bacterial communities colonizing four everyday plastic items deployed in two sites of the same river with different anthropogenic impact. α-diversity analyses showed that site had a significant role in bacterial and eukaryotic diversity, with the most impacted site having higher values of the Shannon diversity index. β-diversity analyses showed that site explained most of the sample variation followed by substrate type (i.e., plastic item) and time since first colonization. In this regard, core microbiomes/biomes in each plastic at 1, 3, 6 and 12 months could be identified at genus level, giving a global overview of the evolution of the plastisphere over time. The measured concentration of antibiotics in the river water positively correlated with the abundance of antibiotic resistance genes (ARGs) on the plastics. These results provide relevant information on the temporal dynamics of the plastisphere in freshwater ecosystems and emphasize the potential contribution of plastic items to the global spread of antibiotic resistance.

Bacterial colonization dynamics of different microplastic types in an anoxic salt marsh sediment and impact of adsorbed polychlorinated biphenyls on the plastisphere

Plastic debris dispersed into the environment provide a substrate for microbial colonization, constituting a new human-made ecosystem called "plastisphere", and altering the microbial species distribution in aquatic, coastal and benthic ecosystems. The study aims at exploring the interaction among microplastics (MPs) made of different polymers, a persistent organic contaminant (polychlorinated biphenyls, PCBs), and the environmental microbial communities, in an anoxic marine sediment. Plastic pellets were incubated in the field in a salt marsh anoxic sediment, to observe the stages of plastisphere formation, by quantitative PCR and 16S rRNA gene sequencing, and PCB dechlorination activity on the MPs surface. Microbes from the sediment rapidly colonized the different microplastics types, with PVC recruiting a peculiar community enriched in sulfate-reducing bacteria. The composition of the plastisphere varied along the 1-year incubation possibly in response either to warmer temperatures in spring-summer or to microhabitat's changes due to the progressive plastic surface weathering. Even if PCB contaminated MPs were able to recruit potentially dehalogenating taxa, actual dechlorination was not detectable after 1 year. This suggests that the concentration of potentially dehalorespiring bacteria in the natural environment could be too low for the onset of the dechlorination process on MP-sorbed contaminants. Our study, which is among very few available longitudinally exploring the plastisphere composition in an anoxic sediment context, is the first exploring the fate and possible biodegradation of persistent organic pollutants sorbed on MPs reaching the seafloor.

Insights into microbial diversity on plastisphere by multi-omics

Plastic pollution is a major concern in marine environment as it takes many years to degrade and is one of the greatest threats to marine life. Plastic surface, referred to as plastisphere, provides habitat for growth and proliferation of various microorganisms. The discovery of these microbes is necessary to identify significant genes, enzymes and bioactive compounds that could help in bioremediation and other commercial applications. Conventional culture techniques have been successful in identifying few microbes from these habitats, leaving majority of them yet to be explored. As such, to recognize the vivid genetic diversity of microbes residing in plastisphere, their structure and corresponding ecological roles within the ecosystem, an emerging technique, called metagenomics has been explored. The technique is expected to provide hitherto unknown information on microbes from the plastisphere. Metagenomics along with next generation sequencing provides comprehensive knowledge on microbes residing in plastisphere that identifies novel microbes for plastic bioremediation, bioactive compounds and other potential benefits. The following review summarizes the efficiency of metagenomics and next generation sequencing technology over conventionally used methods for culturing microbes. It attempts to illustrate the workflow mechanism of metagenomics to elucidate diverse microbial profiles. Further, importance of integrated multi-omics techniques has been highlighted in discovering microbial ecology residing on plastisphere for wider applications.

Evolution of prokaryotic colonisation of greenhouse plastics discarded into the environment

Current knowledge on the capacity of plastics as vectors of microorganisms and their ability to transfer microorganisms between different habitats (i.e. air, soil and river) is limited. The objective of this study was to characterise the evolution of the bacterial community adhered to environmental plastics [low-density polyethylene (LDPE)] across different environments from their point of use to their receiving environment destination in the sea. The study took place in a typical Mediterranean intermittent river basin in Larnaka, Cyprus, characterised by a large greenhouse area whose plastic debris may end up in the sea due to mismanagement. Five locations were selected to represent the environmental fate of greenhouse plastics from their use, through their abandonment in soil and subsequent transport to the river and the sea, taking samples of plastics and the surrounding environments (soil and water). The bacterial community associated with each sample was studied by 16S rRNA metabarcoding; also, the main physicochemical parameters in each environmental compartment were analysed to understand these changes. The identification and chemical changes in greenhouse plastics were tracked using Attenuated Total Reflection Fourier Transform Infra-red spectroscopy (ATR-FTIR). Scanning Electron Microscope (SEM) analysis demonstrated an evolution of the biofilm at each sampling location. β-diversity studies showed that the bacterial community adhered to plastics was significantly different from that of the surrounding environment only in samples taken from aqueous environments (freshwater and sea) (p-value p-value > 0.05). The environmental parameters (pH, salinity, total nitrogen and total phosphorus) explained the differences observed at each location to a limited extent. Furthermore, bacterial community differences among samples were lower in plastics collected from the soil than in plastics taken from rivers and seawater. Six genera (Flavobacterium, Altererythrobacter, Acinetobacter, Pleurocapsa, Georgfuchsia and Rhodococcus) were detected in the plastic, irrespective of the sampling location, confirming that greenhouse plastics can act as possible vectors of microorganisms between different environments: from their point of use, through a river system to the final coastal receiving environment. In conclusion, this study confirms the ability of greenhouse plastics to transport bacteria, including pathogens, between different environments. Future studies should evaluate these risks by performing complete sequencing metagenomics to decipher the functions of the plastisphere.

Is coastal erosion a source of marine litter pollution? Evidence of coastal dunes being a reservoir of plastics

This baseline reports scientific evidence of marine litter items embedded in the dune volume at two study sites on the North Atlantic Portuguese coast. We described how stranded litter participate in the sand dune growth/erosion processes on a natural beach-dune system. From the storm-eroded foredunes on the urbanized beach, we documented exhumed plastics with age up to 38 years. Whether litter burial was due to beach-dune morphodynamic processes, or to irresponsible and/or illegal dumping in the past, this work emphasises the need of improving buried litter census and monitoring on coastal dunes. Coastal erosion processes may further exhume litter buried in dune volumes and on other coastal environments over short- and long-term, re-exposing items into the marine environment. Thus, coastal erosion can be accounted as a secondary diffuse source of littering pollution, beside the multiple sources already identified in the environment.

Contribution of stochastic processes to the microbial community assembly on field-collected microplastics

A growing body of evidence suggests that microplastics may be colonized with a unique microbiome, termed 'plastisphere', in aquatic environments. However, the deep mechanisms (deterministic and/or stochastic processes) underlying the community assembly on microplastics are still poorly understood. Here, we took the estuary of Hangzhou Bay (Zhejiang, China) as an example and examined the assembly mechanisms of bacterial communities in water and microplastic samples. Results from high-throughput sequencing showed that Proteobacteria, Firmicutes, and Actinobacteria were the dominant phyla across all samples. Additionally, microorganisms from plastisphere and planktonic communities exhibited contrasting taxonomic compositions, with greater within-group variation for microplastic samples. The null model analysis indicated the plastisphere bacterial communities were dominantly driven by the stochastic process of drift (58.34%) and dispersal limitation (23.41%). The normalized stochasticity ratio (NST) also showed that the community assembly on microplastics was more stochastic (NST > 50%). Based on the Sloan neutral community model, the migration rate for plastisphere communities (0.015) was significantly lower than that for planktonic communities (0.936), potentially suggesting that it is the stochastic balance between loss and gain of bacteria (e.g., stochastic births and deaths) critically shaping the community assembly on microplastics and generating the specific niches. This study greatly enhanced our understanding of the ecological patterns of microplastic-associated microbial communities in aquatic environments.