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Microplastics Disrupt the Oceanic Biological Carbon Pump: From Zooplankton Fecal Pellets to Radiocarbon Aliasing

Microplastics reduce fecal pellet sinking by 34%, alias radiocarbon dating by 4,000 years, and replace biogenic carbon with synthetic polymer at ocean depth.

By Dimitris Kyriacou, PhD Molecular Biology·05/06/2026·10 min read

Molecular Pathway Insights

Microplastics Disrupt the Oceanic Biological Carbon Pump: From Zooplankton Fecal Pellets to Radiocarbon Aliasing

The oceanic biological carbon pump transfers approximately 10 gigatonnes of carbon per year from the sunlit surface to the deep ocean through sinking organic particles. Microplastic contamination now disrupts every stage of this process: reducing copepod fecal pellet density and sinking velocity, accelerating microbial remineralisation of detritus in seagrass beds, replacing biogenic carbon with non-degradable synthetic polymer at depth, and aliasing the radiocarbon and elemental measurements used to monitor the entire system. This pathway analysis traces the mechanistic cascade from polymer ingestion through biogeochemical distortion.

This pathway analysis was generated from a verified BioSkepsis research thread View the research on BioSkepsis

Pathogenic origin of microplastic disruption to the biological carbon pump

The disruption of the oceanic biological carbon pump by microplastics originates from the convergence of three mechanistic forces: physical alteration of particle sinking dynamics (through density modification of fecal pellets and marine snow aggregates), biological restructuring of microbial decomposer communities (favouring polymer-degrading taxa over sulfur-cycling taxa in blue carbon sediments), and geochemical aliasing of carbon cycle measurements (through the introduction of 14C-depleted, nitrogen-free synthetic carbon into organic matter analytical pools) (PMID: 40153843; PMID: 40897048; PMID: 41082560). These three axes interact across spatial scales: from the micrometre-scale incorporation of polystyrene beads into individual copepod fecal pellets to the global-scale distortion of radiocarbon budgets and Earth system model parameterisations. The pathogenic origin is not a single pollution event but a systemic contamination of the biogeochemical machinery that connects surface photosynthesis to deep-ocean carbon storage.

Biological and chemical mechanisms of carbon pump disruption by microplastics

The biological mechanism centres on the copepod fecal pellet, the primary vehicle for vertical carbon export. The copepod Parvocalanus crassirostris incorporates nano- and microplastics (NMPs) into fecal material during feeding, with nanoplastics distributing uniformly and microplastics distributing non-uniformly within the pellet matrix (PMID: 40153843). Exposure to 5-micrometre polystyrene particles reduces fecal pellet volume by 65%, and at 5,000 micrograms per litre NMP concentration, pellet production drops by 52% due to feeding selectivity in the presence of diatoms. Fluid dynamic modelling shows that sinking rates of uncontaminated pellets range from 10.9 to 103.1 metres per day, but a 50% polystyrene admixture reduces sinking velocity by 34% (PMID: 40153843). Computational extension of this result predicts that such contaminated pellets would arrive at the seabed nearly 60 days later than clean pellets, a delay that vastly increases the window for microbial remineralisation in the mesopelagic zone. At the global scale, earth system modelling of biological microplastic uptake shows that for every two MP particles taken up at the surface in tropical waters, only one is effectively exported below 130 metres; the rest are released when the organic matrix degrades, re-entering the euphotic zone in an entrainment/release cycle (PMID: 33028852).

In coastal blue carbon ecosystems, the microbial mechanism is equally consequential. In 28-day mesocosm experiments with Zostera marina (eelgrass), microplastic exposure (320 mg per 100 g dry weight sediment of polyethylene and polypropylene) reduces leaf growth rate by 39%, net production by 57%, and rhizome elongation by 35% (PMID: 40897048). Critically, microplastics shift the sediment microbiome: polymer-degrading taxa including Flavobacteriaceae (adapted to plant-derived polysaccharides and lignin compounds) increase 1.7-fold, accelerating detritus decomposition by 1.5-fold and reversing the sequestration function of these habitats (PMID: 40897048). Concurrently, bacteria involved in the sulfur cycle (Arcobacteraceae) decrease by 0.45-fold under combined microplastic and nutrient enrichment, disrupting the REDOX processes that maintain the reducing sediment environment essential for seagrass root survival.

The chemical mechanism operates through the physical properties of synthetic polymers themselves. High-density polymers such as polyethylene terephthalate (PET, density approximately 1.33 g/cm3) can ballast marine snow aggregates when embedded in transparent exopolymer particles (TEP), promoting sinking (PMID: 36302504). However, buoyant polymers like polyethylene and polypropylene retard sinking unless sufficiently ballasted by biogenic minerals (calcite, biogenic silica). Biofouling by algal communities increases the effective density of buoyant particles, initiating settling, but this process is reversible: at depth, insufficient light causes biofilm loss, particles regain buoyancy, and oscillate vertically rather than reaching the seafloor (PMID: 28613852). This oscillation creates a persistent mid-depth reservoir of synthetic particles that neither sinks to permanent sequestration nor returns fully to the surface.

Ecosystem and biogeochemical damage from microplastic contamination of the carbon pump

The geochemical damage manifests as systematic aliasing of the measurements used to monitor global carbon cycling. Microplastics are carbon-rich (polyethylene is 86% carbon by mass) but derived from 14C-depleted petroleum feedstocks and contain no nitrogen. Even a 1% mass contamination of a sedimentary organic matter sample (1% organic carbon by mass) by polyethylene would mean that microplastic-derived atoms comprise approximately 40% of all carbon atoms measured during elemental analysis (PMID: 41082560). This contamination lowers the sample's radiocarbon abundance (Delta-14C) by 258 per mille, producing an artifactual age error of approximately 4,000 years. It also shifts the stable carbon isotope ratio (delta-13C) by negative 3.65 per mille and inflates the apparent C:N ratio from the natural Redfield value of approximately 8 to approximately 18, distorting source apportionment models to overestimate terrestrial contribution from 20% to 60% (PMID: 41082560). Even 0.1% contamination by mass introduces a Delta-14C error of negative 30 per mille, a subtle but significant bias that would likely go undetected in routine analysis.

The structural damage to the particulate organic carbon (POC) pool is quantified by depth-resolved measurements across 1,885 ocean stations sampled between 2014 and 2024. The microplastic-carbon to POC ratio increases from 0.1% at 30 metres to 5% at 2,000 metres depth in the North Pacific, because biogenic carbon is progressively remineralised by microbial activity while non-degradable synthetic carbon persists (PMID: 40307520). In the North Atlantic Gyre, drifting sediment traps deployed from 50 to 600 metres depth measured plastic-bound carbon contributing up to 3.8% of the total downward POC flux, confirming that the "biological plastic pump" is actively embedding synthetic material into the ocean's vertical carbon transport system (PMID: 36302504). Small microplastics (1 to 100 micrometres) distribute more evenly throughout the water column than large microplastics (100 to 5,000 micrometres), which tend to accumulate at pycnoclines where density stratification impedes their descent (PMID: 40307520). This size-dependent behaviour means that the smallest fraction, which is also the hardest to detect analytically, creates the most persistent contamination throughout the entire water column.

Downstream outcome: a self-amplifying disruption of the oceanic carbon cycle

The microplastic disruption of the biological carbon pump operates as a self-amplifying circuit. Plastic contamination reduces fecal pellet sinking efficiency, which increases the residence time of organic carbon in the mesopelagic zone, where microbial remineralisation converts it back to dissolved CO2 rather than sequestering it at depth. Simultaneously, the synthetic carbon that does reach the deep ocean persists indefinitely, progressively replacing biogenic POC and raising the plastic-C to POC ratio with each passing year. In blue carbon habitats, microplastic-driven shifts toward Flavobacteriaceae-dominated microbial communities accelerate detritus turnover while sulfur-cycle disruption (Arcobacteraceae suppression) undermines seagrass resilience, reducing the autochthonous carbon input available for the pump at source (PMID: 40897048). At the measurement level, the accumulating geochemical aliasing (radiocarbon age inflation, C:N ratio distortion, delta-13C bias) means that our capacity to detect and quantify this disruption degrades in proportion to its severity (PMID: 41082560). The net result is a system in which microplastics simultaneously reduce the efficiency of biological carbon sequestration, replace biodegradable carbon with inert synthetic material in the deep ocean, and compromise the analytical tools needed to track the damage.

Frequently asked questions

How do microplastics disrupt the oceanic biological carbon pump?

Microplastics reduce the size and density of zooplankton fecal pellets (the primary vehicle for vertical carbon transport), slow their sinking rates, and increase residence time in the upper water column where microbial remineralisation converts organic carbon back to CO2. In seagrass beds, microplastics reduce net production by 57% and accelerate detritus decomposition by 1.5-fold, undermining blue carbon sequestration.

What is the biological plastic pump?

The biological plastic pump describes the process by which marine organisms (zooplankton, filter feeders) package microplastics into sinking particles such as fecal pellets and marine snow. This transports plastics from the surface to depth, but the process is inefficient: for every two particles taken up at the surface in tropical waters, only one is exported below 130 metres.

Can microplastics distort radiocarbon dating of ocean sediments?

Yes. Microplastics are carbon-rich (over 70% by mass) but derived from 14C-depleted petroleum. A 1% mass contamination of sedimentary organic matter by polyethylene produces an artifactual age error of approximately 4,000 years and shifts apparent C:N ratios from 8 to 18, distorting interpretations of carbon source and decomposition dynamics.

How does biofouling affect microplastic sinking behaviour?

Algal biofilm accumulation on buoyant microplastics increases their density beyond that of seawater, initiating settling. However, this process is size-dependent and oscillatory: particles may sink, lose their biofilm at depth (where light is insufficient for algal growth), become buoyant again, and return to the euphotic zone. This vertical oscillation creates a persistent mid-depth reservoir of synthetic particles.

What fraction of deep-sea particulate organic carbon is now plastic?

Synthesis of 1,885 depth-profile stations shows that the microplastic-carbon to particulate organic carbon (POC) ratio increases from 0.1% at 30 metres to 5% at 2,000 metres, because biogenic carbon remineralises with depth while non-degradable synthetic carbon persists. In the North Atlantic Gyre, plastic-bound carbon contributes up to 3.8% of the total POC flux measured by sediment traps.

How do microplastics affect seagrass blue carbon ecosystems?

In Zostera marina (eelgrass) mesocosms, microplastic exposure reduces leaf growth rate by 39%, net production by 57%, and rhizome elongation by 35%. Microplastics also shift microbial communities toward polymer-degrading taxa like Flavobacteriaceae, which accelerate detritus decomposition by 1.5-fold. A notable decrease in sulfur-cycling bacteria (Arcobacteraceae, 0.45-fold lower) disrupts the REDOX balance essential for seagrass survival in reducing sediments.

Why are small microplastics more persistent in the water column than large ones?

Large microplastics (100 to 5,000 micrometres) concentrate at stratified layers (pycnoclines) where density gradients impede their descent, or sink rapidly if sufficiently ballasted. Small microplastics (1 to 100 micrometres) distribute more evenly throughout the water column because their low settling velocities and susceptibility to turbulent mixing create a persistent reservoir from surface to abyssal depths.

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Sources and further reading

  1. Dong Z, Wang WX. Modeling the vertical transport of copepod fecal particles under nano/microplastic exposure. Environ Sci Technol. 2025;59(13):6610-6622. PMID: 40153843. DOI
  2. Medina Faull LE, Taylor GT, Beaupre SR. Microplastic contaminants potentially distort our understanding of the ocean's carbon cycle. PLoS One. 2025;20(10):e0334546. PMID: 41082560. DOI
  3. Egea LG, Jimenez-Ramos R, Rodriguez-Arias L, Infantes E. Microplastics threaten seagrass carbon sinks through microbial changes. Mar Pollut Bull. 2025;222(Pt 1):118638. PMID: 40897048. DOI
  4. Galgani L, Gossmann I, Scholz-Bottcher B, et al. Hitchhiking into the deep: how microplastic particles are exported through the biological carbon pump in the North Atlantic Ocean. Environ Sci Technol. 2022;56(22):15638-15649. PMID: 36302504. DOI
  5. Zhao S, Kvale KF, Zhu L, et al. The distribution of subsurface microplastics in the ocean. Nature. 2025;641(8061):51-61. PMID: 40307520. DOI
  6. Kvale K, Prowe AEF, Chien CT, Landolfi A, Oschlies A. The global biological microplastic particle sink. Sci Rep. 2020;10(1):16670. PMID: 33028852. DOI
  7. Kooi M, van Nes EH, Scheffer M, Koelmans AA. Ups and downs in the ocean: effects of biofouling on vertical transport of microplastics. Environ Sci Technol. 2017;51(14):7963-7971. PMID: 28613852. DOI