The review, led by researchers from Jiangxi Agricultural University with corresponding author Jingliang Shi, synthesizes current knowledge on the dual role of microplastics as vectors for both chemical and biological contaminants. Its central contention is that risk assessments focused solely on the plastic particles have systematically underestimated the environmental hazard, because they ignore the cargo these particles can carry and the ecological interactions they facilitate. “Microplastics should not be considered isolated particles in the environment,” Shi explains. “They can interact with chemicals and microorganisms, transport them between environmental compartments and, under certain conditions, amplify their ecological effects. Understanding when these processes become dominant is essential for realistic risk assessment.”

At the heart of the chemical dimension of this problem lies what toxicologists call the “Trojan horse effect.” Because plastic particles present large, often hydrophobic and chemically reactive surfaces, they readily adsorb persistent organic pollutants, heavy metals and a wide range of other contaminants from their surrounding environment. Once these loaded particles are ingested by organisms—whether filter-feeding mollusks, plankton, fish or grazing livestock—the physiological conditions of the digestive tract can alter the chemistry at the particle surface, causing pollutants to desorb precisely where the organism is most vulnerable to absorption. In effect, the microplastic delivers a concentrated dose of toxins that the surrounding environment alone might never have supplied.

The review also emphasizes a crucial and often overlooked size dependence in how this delivery occurs. Conventional microplastics, those particles larger than roughly one micrometer, generally deliver their chemical cargo through the gastrointestinal tract, releasing adsorbed pollutants into the gut where they may cross the intestinal lining. Nanoplastics, however—particles smaller than about one micrometer—present a fundamentally different and more troubling scenario. At these scales, the particles themselves may cross biological membranes, penetrating tissue barriers and distributing their associated pollutants directly to internal organs. This distinction matters for anyone attempting to model exposure, because it means that the same mass of plastic can produce qualitatively different toxicological outcomes depending on how finely it has been fragmented.

The biological dimension of the microplastic problem may prove even more consequential than the chemical one. When plastic particles enter the environment, they are rapidly colonized by microorganisms, forming dense microbial communities that scientists have dubbed the “plastisphere.” Far from being a random assemblage, this biofilm is a structured, functional ecosystem with its own chemical microenvironment. Within the protective matrix of the biofilm, pathogens can survive longer than they would in open water or soil, shielded from UV radiation, desiccation and predation. More alarmingly, the plastisphere can serve as a refuge for antibiotic resistance genes, and the extreme proximity of diverse microbial species packed into a biofilm creates ideal conditions for horizontal gene transfer—the process by which bacteria exchange genetic material directly, potentially accelerating the spread of antimicrobial resistance through the environment.

What makes the review’s analysis particularly compelling is its demonstration that the chemical and biological vector effects do not operate independently. Instead, they form what the authors describe as a bidirectional positive feedback loop. Pollutants adsorbed onto plastic surfaces can exert selective pressure on the microbial communities colonizing that surface, favoring tolerant or resistant strains and thereby enriching the biofilm in resistance determinants. In the other direction, the biofilm itself alters the physical and chemical properties of the plastic surface—adding extracellular polymeric substances and reactive functional groups—which can increase the particle’s subsequent capacity to adsorb further pollutants. Each process amplifies the other, meaning that a microplastic particle that has been in the environment for some time may be far more dangerous than a fresh one, accumulating both a richer chemical payload and a more hazardous microbial community.

Recognizing that the field has largely moved past the question of whether microplastics act as vectors and toward the question of when and how strongly they do so, the authors propose a three-tiered regulatory framework organized around physical, chemical and biological drivers. The physical tier concerns the particle itself: size, shape and degree of aging all influence how a particle travels through environmental compartments and how reactive its surface is. The chemical tier concerns the surrounding environment: polymer chemistry and ambient conditions such as pH, salinity and organic matter content govern the rates of pollutant adsorption and desorption. The biological tier concerns the living dimension: biofilm formation, ingestion by organisms and subsequent transfer through food webs determine how the particle’s cargo ultimately reaches and affects living systems. By structuring risk assessment this way, the authors argue, researchers and regulators can move beyond simplistic descriptions of microplastic abundance toward a mechanistic understanding of hazard.

The review goes further, identifying specific conditions under which the combined chemical and biological vector effects become particularly significant—and therefore particularly dangerous. These include situations of strong microbial selective pressure even at relatively low contaminant concentrations, which can drive resistance enrichment without any obvious chemical alarm signal; biofilms with high extracellular polymeric substance content, which provide both habitat stability and enhanced adsorption capacity; highly aged microplastics whose surfaces have accumulated oxygen-rich functional groups, making them substantially more chemically active than pristine particles; and prolonged exposure scenarios exceeding thirty days, over which time biofilms mature and pollutant loads can accumulate substantially. Each of these conditions offers a concrete, testable criterion that could inform monitoring priorities in real ecosystems.

This framework also exposes a fundamental weakness in how microplastic toxicity is currently studied. Most laboratory experiments rely on short-term exposures at concentrations far higher than organisms encounter in nature, producing results that the authors argue poorly represent chronic environmental conditions. The real hazard, they contend, lies not in acute toxicity from an overwhelming dose of plastic, but in the slow, cumulative effects of particles that have spent weeks or months in the environment, growing biofilms, adsorbing pollutants and shuttling genes between microbial communities. Addressing this gap will require long-term observations under environmentally realistic conditions, improved exposure models that track particle aging and cargo evolution over time, and a shift in the field’s basic assumptions about what a toxicity experiment should look like.

The practical implications extend into pollution management and governance as well. The authors call for targeted removal of high-risk aged microplastics—the particles most likely to have accumulated dangerous chemical and biological cargo—rather than undifferentiated cleanup efforts that treat all particles as equivalent. They also advocate for more unified approaches to global microplastic governance, a notable appeal given that plastic pollution, microbial communities and antimicrobial resistance all recognize no political boundaries. In an era when antimicrobial resistance is projected to become one of the leading causes of death worldwide, the possibility that plastic debris is quietly serving as an incubator and distribution network for resistance genes gives an entirely new urgency to what was once considered primarily a litter problem.

Perhaps the most significant contribution of the review is conceptual. By reframing microplastics as dynamic platforms that connect chemical pollution, microbial ecology and antimicrobial resistance across ecosystems, it dissolves the artificial boundary between plastic pollution research and the study of other environmental hazards. A fragment of plastic in a river is simultaneously a pollutant, a chemical sorbent, a microbial habitat and a potential vehicle for disease and resistance. Understanding and managing that multiplicity—and identifying the thresholds at which these vector effects tip from background noise into genuine ecological threat—is, the authors argue, the central challenge facing the next generation of microplastic research.

The crux of the problem lies in the highly heterogeneous nature of microplastic particles. These particles vary greatly in polymer type, size, shape, and chemical weathering status, factors that considerably influence detection, quantification, and toxicological assessments. Without positive controls that faithfully mimic this diversity, current methodologies often suffer from discrepancies between laboratories. This paucity of standardized materials impedes the harmonization of experimental protocols, rendering cross-study comparisons tenuous at best.

McIlwraith and colleagues present a compelling argument advocating for the design and deployment of positive control materials that encapsulate the physicochemical complexity of environmental microplastics. They propose creating reference materials that not only encompass a spectrum of polymer types such as polyethylene (PE), polypropylene (PP), and polystyrene (PS), but also include varied morphologies ranging from fragments to fibers. By incorporating such representative samples in analytical workflows, researchers can critically evaluate method sensitivity, specificity, and recovery efficiency.

A pivotal aspect of their discourse centers on the impact of polymer weathering and aging processes, which profoundly alter microplastic surface properties and environmental behavior. The study highlights that positive controls engineered to mirror these aged characteristics enable more accurate simulation of real-world detection scenarios. This approach addresses the pervasive issue of method calibration, as pristine laboratory-synthesized particles often fail to capture the complexity of environmental plastics.

The importance of validated positive controls extends beyond analytical chemistry into ecotoxicology, where understanding microplastics’ biological interactions hinges on exposure to realistic particle forms. Employing representative positive controls can thus refine dose-response models, facilitating better risk assessments. McIlwraith et al. emphasize that ecological relevance is paramount to ensure that experimental findings translate effectively into environmental policy and mitigation strategies.

Moreover, the study recognizes the critical role of international collaborations and standard-setting bodies, such as the International Organization for Standardization (ISO) and the Joint Research Centre (JRC) of the European Commission, in developing and disseminating these positive controls. Such cooperative efforts are poised to foster consensus and widespread adoption, ultimately propelling the microplastics field towards methodological consistency.

The practical challenges of fabricating universal positive controls are non-trivial. The authors discuss intricacies such as production scalability, long-term stability, and storage conditions, which must be meticulously addressed to maintain control material integrity. Furthermore, they underscore the need for open-access repositories, enabling researchers globally to obtain and utilize these standards without prohibitive costs.

An intriguing dimension of the study delves into analytical techniques employed in microplastics research, including Fourier-transform infrared spectroscopy (FTIR), Raman spectroscopy, pyrolysis-gas chromatography-mass spectrometry (py-GC-MS), and thermal extraction desorption gas chromatography-mass spectrometry (TED-GC-MS). The deployment of representative positive controls enhances the calibration of these instruments, fostering improved detection limits and classification accuracy across varied matrices like water, sediment, and biota.

McIlwraith et al. also tackle quality assurance and quality control (QA/QC) frameworks, emphasizing how positive controls form an integral backbone in verifying analytical reproducibility. Incorporating these standards into routine workflows minimizes false positives and negatives, which have historically plagued the field due to sample contamination and methodological biases.

The study’s implications resonate beyond environmental sciences, touching on public health domains concerned with human exposure to microplastics via ingestion and inhalation. More reliable data underpinned by representative positive controls will offer clearer insights into exposure pathways, aiding in the development of regulatory guidelines to curb potential health risks.

Crucially, the authors advocate for the development of layered control systems, encompassing negative controls (blanks), method blanks, and multiple positive controls with escalating complexity. This stratified approach would provide comprehensive performance benchmarking, enabling researchers to identify and troubleshoot analytical pitfalls holistically.

As microplastics research burgeons in scale and scope, McIlwraith et al. warn against complacency in methodological practices. They contend that without the foundational support of robust positive controls, scientific conclusions risk being marred by inconsistency and skepticism. Their research thereby serves as a clarion call, urging the community to prioritize standardization efforts urgently.

Looking forward, the integration of advanced materials science and nanotechnology offers promising avenues to fabricate synthetic control particles with finely tunable properties. These innovations could further mirror environmental microplastics’ heterogeneity, fostering next-generation reference materials that enhance methodological precision.

In summation, the work of McIlwraith and colleagues marks a pivotal turning point for microplastics investigations. By championing the indispensable role of positive controls that feature representative materials, they lay the groundwork for more rigorous, transparent, and comparable research practices. As this field confronts escalating environmental and health challenges posed by plastic pollution, their insights provide a vital roadmap to strengthen scientific foundations and catalyze impactful solutions.

Subject of Research: Standardization and development of positive control materials for microplastics research.

Article Title: Positive controls with representative materials are essential for the advancement of microplastics research.

Article References:
McIlwraith, H.K., Lindeque, P.K., Tolhurst, T.J. et al. Positive controls with representative materials are essential for the advancement of microplastics research. Micropl.& Nanopl. 5, 9 (2025). https://doi.org/10.1186/s43591-025-00115-y

Image Credits: AI Generated

DOI: https://doi.org/10.1186/s43591-025-00115-y