01 · Sources
Where microplastics come from
Plastic enters the ocean via rivers, stormwater drains, coastal littering, and loss from fishing gear and shipping. The largest river contributors are in South and Southeast Asia, where waste management infrastructure lags plastic consumption growth. But even in countries with high recycling rates, tyre wear particles — the most abundant primary microplastic in road runoff — enter waterways with every rain event.
PRIMARY
- Nurdles — pre-production resin pellets lost in shipping
- Microbeads — used in cosmetic exfoliants (now banned in many countries)
- Synthetic textile fibres — shed during washing (est. 700,000 fibres per wash cycle)
- Tyre wear particles — washed off roads into drains
SECONDARY
- Fragmented bottles, bags, and packaging
- Broken-down fishing nets (abandoned, lost, or discarded)
- Construction foam and agricultural mulch films
- Paint microparticles from ships and offshore structures
02 · Chemistry
Why plastic does not just disappear
Most commodity plastics — polyethylene (PE), polypropylene (PP), polystyrene (PS) — are addition polymers built from carbon–carbon backbone chains. These C–C bonds are chemically inert: there is no hydrolysable ester or amide linkage for water or enzymes to attack. See the Polymer glossary entry for the structural detail.
UV photodegradation — absorbing high-energy photons — breaks the polymer chains into shorter fragments but does not mineralise them to CO₂ and H₂O. The result is ever-smaller pieces with the same chemical composition. The rate of fragmentation depends on the activation energy of the photo-oxidation reactions and the UV flux at the sea surface.
In contrast, polyesters like PET (drinks bottles) contain ester linkages that can be hydrolysed enzymatically. The discovery of PETase in Ideonella sakaiensis opened the door to biological degradation — see the plastic-eating enzymes article for how engineered variants are being developed.
Chemical pollutants hitching a ride
Microplastics act as sponges for hydrophobic organic pollutants — PCBs, DDT metabolites, polycyclic aromatic hydrocarbons — concentrating them at levels orders of magnitude above the surrounding water. When marine organisms ingest particles, these accumulated contaminants transfer into tissue. The process mirrors bioaccumulation in food chains that the pH and ocean chemistry principles discussed in the ocean acidification article already make more complex.
03 · Ecology
Impact on marine ecosystems
Marine organisms ingest microplastics at every trophic level. Filter feeders — mussels, oysters, whale sharks — cannot distinguish particles from plankton. Seabirds mistake pellets for fish eggs. Zooplankton confuse nanoplastics with phytoplankton cells. The consequences range from physical blockage and reduced feeding to endocrine disruption from adsorbed plasticisers.
A 2023 study in Nature Communications found that polystyrene nanoplastics at environmentally relevant concentrations suppressed photosynthesis in marine microalgae by up to 27 %, reducing the primary productivity on which the entire ocean food web depends. Given that marine phytoplankton produce roughly half of Earth's atmospheric oxygen, even a modest productivity decline has global consequences. For the biochemistry of how photosynthesis can be disrupted, see the Photosynthesis guide.
04 · Solutions
What chemistry offers
Three chemical approaches are attracting the most research funding:
- Biodegradable polymers — polylactic acid (PLA) from fermented plant sugars, polyhydroxyalkanoates (PHAs) from bacterial fermentation. Both contain hydrolysable ester bonds that micro-organisms can break down. Cost and mechanical properties remain barriers to replacing PE/PP at scale.
- Enzymatic degradation — engineered PETase/MHETase cocktails can depolymerise PET at 72 °C in days rather than centuries. The challenge is delivering enzymes to dispersed particles in open water.
- Advanced oxidation processes (AOPs) — UV/H₂O₂, ozone, and Fenton reactions generate hydroxyl radicals that attack C–C bonds non-selectively, fragmenting and eventually mineralising plastic. Practical mainly in wastewater treatment rather than open ocean.
The consensus in materials chemistry is that no removal technology can match the scale of the problem: prevention at source — reducing plastic production, improving waste management, and replacing single-use items with durable or truly biodegradable alternatives — is the only viable long-term strategy.
Microplastics — common questions
- A microplastic is any plastic particle smaller than 5 mm. Nanoplastics are below 1 µm. Primary microplastics include nurdles (pre-production pellets) and microbeads in cosmetics. Secondary microplastics form when larger items fragment under UV and mechanical stress. See the Polymer glossary entry for the chemistry of plastic degradation.
- Microplastics have been detected in human blood, lungs, placenta, and breast milk. They can adsorb chemical pollutants into tissues. The WHO and EFSA are reviewing the evidence. Long-term in vivo studies are ongoing. The chemical contaminants they carry — including endocrine-disrupting phthalates — are the most immediate concern.
- Surface macroplastics can be collected, but microplastics dispersed through the water column and sediment are impractical to remove at scale. Research into enzymatic degradation (PETase and related enzymes — see plastic-eating enzymes) offers a possible in-situ breakdown pathway, but commercial viability is years away.
- Polymer chemistry explains plastic durability. Reaction rate analysis quantifies degradation half-lives. Spectroscopy (Raman, FTIR) identifies particle composition. Green chemistry is designing biodegradable alternatives — polylactic acid (PLA) from fermented corn starch, polyhydroxyalkanoates (PHAs) from bacterial fermentation.