Microplastics as active mineralization platforms influencing calcium cycling in aquatic systems
Plastics are pivotal inventions in materials science of the 20th century and extensively used in packaging, textiles, construction, and healthcare owing to their advantageous properties. Unfortunately，plastic debris disintegrates into dispersed microplastics in aquatic ecosystems. Microplastic pollution has emerged as a major global environmental challenge owing to its adverse effects on ecosystem processes and biota. Although previous studies have primarily focused on microplastic occurrence, aging mechanisms, transport processes, and ecotoxicity assessments, the current understanding of their vector effects remains in its infancy. Such effects have raised global concern because microplastics can act as novel elemental carriers in ecosystems and may affect elemental cycling.1 Through complex interactions with minerals, microorganisms, and organic matter, microplastics can influence the migration, transformation, and ultimate fate of elements.1 Recent findings on microplastic-driven biocalcification and “plastic-rock” formation indicate that microplastics not only perturb biogeochemical cycling in the surface biosphere but also hold the potential to intervene in elemental cycling across the geospheres.2,3 This novel biogeochemical pathway challenges our understanding of the long-term environmental fate of microplastics. Nevertheless, the environmental implications stemming from microplastic element-carrying effects remain highly uncertain, rendering this field a burgeoning research hotspot.
The carbonate material-energy cycle sustains aquatic ecological and geological processes.4 Aquatic microbial mineralization of calcium carbonate (CaCO3) is an inherent part of biogeochemical cycles and regulates long-term calcium transport and cycling. Recent studies indicate microbial activity and surface “plastisphere” dominate microplastic fate.3 In fact, microplastics can inevitably alter microbial community structure in water bodies, with these effects driven by differences in polymer type, color, and morphology of microplastics.1,3 Therefore, we hypothesize that the calcium cycle in aquatic environments may be affected by the long-term persistence of exogenous microplastics. Accordingly, our viewpoint elaborates on microplastic surfaces as active platforms to improve the understanding of microplastic-driven microbial CaCO3 mineralization (Figure 1).
Microplastic-driven biocalcification is not an isolated reaction process but a synergistic coupling among environmental factors.3,4 Microplastic aging and surface interaction have been recognized as the core mechanism of this process.3 In natural water, environmental weathering processes can disrupt surface structural integrity of microplastics and markedly increase their surface roughness, chemical activity, and adsorption capacity. These changes enable microplastics to act as efficient carriers of dissolved organic matter, particularly by enriching their surfaces with extracellular polymeric substances (EPSs) secreted by algae and microorganisms.3 Dense biofilms on the microplastic surfaces generate a “microplastic-EPS composite interface,” which acts as the core microenvironment for mineralization.3 This interface should provide unique advantages for CaCO3 nucleation3: aging increases surface oxidation and roughness, supplying more active nucleation sites; polar functional groups within the EPS complex serve as nucleation templates that guide directional crystal growth; and biofilm microenvironments alter local pH and saturation state, creating thermodynamically favorable conditions for CaCO3 precipitation. Collectively, these three factors create a favorable physicochemical environment for the heterogeneous nucleation of CaCO3, thereby lowering the nucleation energy barrier.
Microplastic surfaces can act as active mineralization platforms that may further influence aquatic CaCO3 generation. Carbonate precipitation is often induced by the heterogeneous nucleation and biologically mediated processes on natural interfaces, including suspended particles, organic matter, microbes, biofilms, and sediment grains.3,4 Unlike traditional microbial mineralization, anthropogenic microplastics feature high persistence, increasing abundance, strong mobility, and chemical inertness, providing a novel active-mobile reaction platform for CaCO3 generation. They expand nucleation areas and alter the location, timing, and formation pathway of mineral precipitation3,5 while transporting biofilms and mineral coatings across the water column, shoreline, and sedimentary compartments. Together, these processes shift the mineralization pathways originally scattered across natural environments toward highly concentrated, readily transportable artificial interfaces with cross-medium migration potential, thereby markedly improving mineralization efficiency and specificity.
Bidirectional feedback between microplastics and CaCO3 further amplifies the disturbance inherent in the calcium cycle, creating a mobile cycling pathway different from traditional aquatic microbial mineralization. First, the formation of microplastic-CaCO3 complexes substantially alters microplastic vertical migration, which is governed by particle size and aging extent.3 CaCO3 coatings promote microplastic sedimentation from the water column through strong ballasting effects. Specifically, small-sized microplastics preferentially settle because their higher EPS adsorption capacity affords the formation of dense CaCO3 nucleation sites on their surfaces. Aging further enhances microplastic settling by increasing surface activity and strengthening selective microplastic-Ca2+ binding through polar components in EPS. This process accelerates the preferential sedimentation of small-sized aged microplastics and shifts their fate from horizontal dispersion to sediment enrichment.
