Plastic pollution fosters more microbial growth in lakes than natural organic matter

Plastic pollution introduces dissolved organic matter (DOM) into freshwaters through mechanical, photochemical, and biological degradation of polymers. This leached DOM can fuel bacterial growth and transfer energy up food webs, but it can also impair microbial growth due to toxic additives in plastics. The composition and fate of plastic-derived DOM in freshwaters remain poorly understood compared with natural DOM. Common buoyant polymers like polyethylene and polypropylene are exposed to high photodegradation in surface waters, potentially yielding high concentrations of plastic leachate relative to natural DOM. If leachate contains more labile compounds, bacteria may grow more efficiently; conversely, toxic additives could inhibit growth. Responses likely vary among waters because lake DOM composition and bacterial community structure differ across systems, influencing substrate use and metabolism. The authors hypothesised that the molecular composition of pre-existing lake DOM controls how bacteria respond to plastic leachate. To test this, surface waters from 29 Scandinavian lakes spanning gradients in DOM concentration and composition were incubated with or without environmentally representative amounts of low-density polyethylene (LDPE) bag leachate. Using FT-ICR-MS, 3H-leucine-based bacterial protein production (BPP), bacterial respiration, and calculated bacterial growth efficiency (BGE), the study evaluated whether leachate increased biomass production due to added carbon alone or also due to higher bioavailability. They further assessed how responses varied with lake DOM properties and bacterial diversity using 16S rRNA amplicon sequencing.

Prior work shows plastics act as substrates for biofilms (plastisphere) and leach DOM during degradation. Plastics are often considered non-biodegradable, yet they contain many labile additives (plasticizers, colorants, antioxidants) that can constitute a large fraction of plastic mass and can leach into waters. Polyethylene and polypropylene are buoyant and prone to photodegradation in surface waters, potentially elevating leachate concentrations. Earlier studies reported that plastic-derived DOC can stimulate microbial activity in oceans, while others showed impairment at high leachate concentrations due to toxic additives (e.g., UV filters like oxybenzone). Natural DOM across lakes is typically dominated by recalcitrant compounds; leachates with more labile molecules could therefore be readily assimilated, whereas in systems with already labile DOM, benefits might be smaller or similar if bacteria are preadapted. Microbial community composition and diversity vary geographically and by environment, affecting DOM utilisation. Advances in ultra-high-resolution mass spectrometry (FT-ICR-MS) enable detailed comparison of plastic leachate and natural DOM molecular signatures and their implications for microbial metabolism.

Study design and sampling: The authors sampled 29 Scandinavian lakes (59.1–70.3°N) in Aug–Sep 2019 spanning gradients in depth, area, surface temperature (9.4–20.6°C), pH (5.81–6.95), DOC (0.55–7.97 mg L−1), and DOM functional diversity (FD 6.12–6.96). Ten liters of surface water were collected at each lake’s deepest point. For 20 lakes, microbial community DNA was preserved on 0.2 µm Sterivex filters. For 22 lakes, samples for DOM/DOC/TN and FT-ICR-MS were collected (0.5 µm pre-combusted GF filters; acidified to pH 2). Plastic leachate preparation: LDPE shopping bags from four retailers were cut into 1 cm2 pieces; 240 pieces were incubated in 150 mL distilled water at 25°C for 7 days under LED light (395–530 nm; 100 µmol photons m−2 s−1) with agitation to simulate UV exposure and transport. A distilled water control was run in parallel. After incubation, leachate was 0.2 µm filtered into combusted glass vials, preserved for DOM/DOC analysis. DOM characterisation: Solid-phase extraction of DOM (PPL cartridges) followed by FT-ICR-MS (15 T Solarix XR, negative ESI) was used to assign molecular formulas (150–1000 m/z) via ICBM-OCEAN. Functional diversity (FD) was computed as the expected mass difference among formulas; formulas with H:C ≥ 1.5 were classified as high lability index. DOC and TN were measured on a Shimadzu TOC-L TNM-L analyzer. Experimental incubations: For each lake, nine 125 mL bottles were filled with lake water. Treatments (n=3 each): (i) leachate addition to achieve 0.1 mg C L−1 (4.6 mL), (ii) equal volume of distilled water, (iii) time-zero lake water (no addition). Incubations were performed in the dark for 72 h at ambient temperature. Parallel gas-tight 25 mL vials (triplicate) for each treatment were prepared for oxygen-based respiration measurements. Bacterial activity measurements: Bacterial protein production (BPP) was measured using 3H-leucine incorporation to carbon uptake after 72 h, with TCA-killed controls and scintillation counting; CPMs were converted to carbon uptake. Respiration was determined from dissolved oxygen decline measured by fiber-optic optodes (OXY-1 ST), correcting for conditions; assuming respiratory quotient of 1, BGE was calculated as BPP/(BPP+respiration). Bacterial community analysis: DNA was extracted from Sterivex filters; V6–V8 regions of 16S rRNA were amplified with bacteria-specific primers and sequenced (Illumina MiSeq 2×300 bp). Reads were processed with cutadapt and DADA2; taxonomy was assigned using SILVA v132. Shannon diversity index was computed from 2148 ASVs (1.7M reads classified; 75% of raw reads). Sequences are deposited under PRJEB49321. Statistics: Linear mixed-effects models (lmer, R lme4) tested effects of treatment (plastic vs control) and interactions with predictors (lake DOM FD, bacterial Shannon diversity, DOC, TN, temperature, pH, latitude) on ln-transformed BPP and BGE, with lake ID as random effect. Model selection used AIC via backward stepwise elimination; confidence intervals via emmeans. Associations between ASV relative abundance changes and fold-changes in BPP or BGE were tested with DESeq2 negative binomial models on ASVs with ≥100 reads, adjusting p-values (Benjamini-Hochberg).

• Plastic leachate DOM was chemically distinct and more labile than lake DOM:
  • Plastic leachate had 855 molecular formulas vs 3684–7116 in lakes (150–2000 Da window). Functional diversity (FD) of leachate was 3.46, lower than any measured lake (6.12–6.96).
  • 18.6% of leachate formulas had a high lability index (H:C ≥ 1.5), exceeding any lake (10.3–12.5%). Highly labile compounds accounted for 82.2% of normalized peak intensity in leachate but only 5.4–10.6% in lakes.
  • 35% of leachate formulas were unique (absent from 22 study lakes), including known plastic additives (e.g., isophthalic acid, phthalates) and plastic-specific breakdown products.
• Bacterial metabolism responses to an environmentally relevant leachate dose (0.1 mg C L−1):
  • BPP increased 2.29× (95% CI: 1.92–2.73) relative to control: from 0.078 [0.058, 0.105] to 0.178 [0.132, 0.240] µg C L−1 hr−1.
  • BGE increased 1.72× (95% CI: 1.27–2.32): from 8.1% [5.8, 11.5] to 14.0% [10.0, 19.5]. To sustain a mean BPP increase of 7.31 µg C L−1 over 72 h at 14% BGE, bacteria would process 51.5 [37.0, 72.1] µg C L−1 (~half the added DOC), indicating high bioavailability of leachate carbon beyond mere carbon addition effects.
• Environmental context modulated efficiency gains (BGE):
  • Interaction with lake DOM FD: at low FD (−1 SD), BGE increased 2.31× [1.54, 2.31] from 2.57% [1.71, 3.86] to 5.93% [3.95, 8.89]; at high FD (+1 SD), no significant change (1.18× [0.50, 2.80]).
  • Interaction with DOC: at low DOC, BGE increased 3.43× [2.82, 4.15] from 1.69% [1.39, 2.05] to 5.77% [4.75, 6.99]; at high DOC, no effect (0.74× [0.27, 2.04]).
  • FD and DOC did not significantly interact with leachate for BPP.
• Microbial diversity influence:
  • Bacterial Shannon diversity interacted with treatment: at high diversity, BGE increased 2.93× [1.71, 5.03] from 6.59% [3.84, 11.3] to 19.3% [11.2, 33.1]; at low diversity, no effect (1.08× [0.58, 1.99]). No interaction with BPP.
  • ASVs whose relative abundance increased with leachate-driven rises in BPP and BGE were identified; strongest associations included Hymenobacter (BPP) and Deinococcus (BGE), among others potentially relevant to plastic compound utilisation.

Findings demonstrate that plastic-derived DOM is compositionally distinct from natural lake DOM and substantially more labile, leading to marked increases in bacterial biomass production and efficiency at realistic environmental leachate concentrations. The enhanced BGE indicates that bacteria process plastic-derived carbon more efficiently, consistent with its higher bioavailability, rather than merely responding to added DOC mass. Variation in response across lakes highlights the importance of local DOM context and community composition: efficiency gains were greatest in lakes with less functionally diverse and lower-concentration DOM, likely due to higher relative availability of thermodynamically favorable substrates when leachate is added and potentially lower metabolic costs. Higher bacterial diversity amplified BGE gains, suggesting diverse communities are more likely to include taxa capable of efficiently utilising plastic-derived compounds. These results reconcile mixed outcomes from prior studies by showing that responses depend on both DOM characteristics and microbial community structure. Ecologically, increased bacterial production at the base of food webs could enhance energy transfer to higher trophic levels, while identifying taxa associated with leachate use informs potential bioremediation strategies to remove plastic-derived chemicals from aquatic systems.

This study shows that LDPE plastic leachate contains a suite of highly labile organic molecules distinct from natural lake DOM and that adding environmentally relevant concentrations can more than double bacterial biomass production and significantly increase growth efficiency. The magnitude of these effects depends on lake DOM concentration and functional diversity and on bacterial community diversity. These insights identify when and where plastic pollution is most likely to stimulate microbial metabolism and where mitigation may be most needed—particularly in lakes with high DOC, high DOM functional diversity, and low bacterial diversity that may be less able to remove leachates. Future research should: (1) test a range of leachate concentrations and plastic types to bracket responses and potential toxicity thresholds; (2) assess roles of other microbial groups (algae, fungi) and community interactions; (3) isolate and functionally characterise taxa driving efficient leachate use to inform bioremediation; and (4) resolve molecular structures, not just formulas, to better link chemistry to microbial pathways.

• The study focused on bacteria; other microorganisms (microalgae, fungi) also interact with plastics and leachates and could alter ecosystem-level responses.
• Only LDPE was examined; leachate chemistry from other polymer types may differ, potentially altering bacterial responses.
• A single, environmentally representative leachate concentration (0.1 mg C L−1) was used; responses at higher concentrations, such as near waste sites, may differ and could be less positive due to accumulation of toxic additives.
• FT-ICR-MS resolves molecular formulas but not structural isomers; some shared formulas between lakes and leachate may represent different compounds, and unique percentages may be conservative given prior contamination of some lakes with microplastics.