Biodegradation of components from an oxidized polyethylene by a Rhodococcus strain isolated from the gut of Atlantic Salmon

This study demonstrates that *Rhodococcus* sp. ASF-10, a bacterium isolated from the Atlantic salmon gut, can biodegrade low molecular weight, oxidized polyethylene derivatives through specific enzymatic pathways, offering potential for sustainable bioremediation strategies.

The Gist

The Big Picture: A Fishy Detective Story

Imagine the ocean is a giant, messy living room filled with plastic trash. Over time, sunlight and heat act like a slow-motion blender, breaking big, tough plastic bags (polyethylene) into tiny, crumbly pieces. Some of these crumbs get eaten by fish, like Atlantic Salmon.

Scientists wondered: What happens inside the fish's stomach? Do the tiny microbes living there just sit there, or do they try to eat the plastic crumbs?

This paper tells the story of a specific microbe found in a salmon's gut named Rhodococcus sp. ASF-10. The researchers wanted to see if this little bacterium could act as a "plastic-eating superhero."

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The Experiment: The "Plastic Buffet"

The scientists set up a little experiment. They put the salmon bacteria in a test tube with two different types of food:

1. Fresh Plastic: A big, tough chunk of low-density polyethylene (like a plastic bag).
2. Weathered Plastic Crumbs: A model of plastic that has already been broken down by the sun and weather. This is like the "crumbs" found in the ocean.

The Result:

• Fresh Plastic: The bacteria looked at the big chunk of plastic and said, "Nope, too hard to chew." They didn't grow.
• Weathered Crumbs: The bacteria looked at the oxidized crumbs and said, "Yum!" They grew happily and multiplied.

The Takeaway: The bacteria can't eat the brand-new, tough plastic, but they can eat the plastic that has already been broken down by nature into smaller, simpler chemicals.

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The "Genome" Check: What's in the Toolbox?

Before watching the bacteria eat, the scientists looked at its instruction manual (its genome) to see what tools it had in its toolbox.

They compared this salmon bacteria to 20 other Rhodococcus bacteria from different places (soil, water, other animals). They found that:

• Size Matters (a little bit): Bacteria with bigger instruction manuals (larger genomes) generally had more tools. However, once the manual got a certain size, adding more pages didn't give them more tools, just different ones.
• The Core Skill: Almost every Rhodococcus bacteria, including the salmon one, had a specific set of tools designed to break down alkanes.
  • Analogy: Think of alkanes as the "fatty" parts of the plastic. Even though the bacteria can't chew the whole plastic sandwich, they have a special set of scissors (enzymes) that can snip off the fatty crumbs.

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The "Eating" Process: How They Do It

So, how does this tiny bacterium actually eat the plastic crumbs? The scientists used a high-tech camera (proteomics) to see which tools the bacteria pulled out of their toolbox while they were eating.

Here is the step-by-step process they discovered:

1. The "Soap" Strategy: The plastic crumbs are greasy and hate water (hydrophobic). To get a grip on them, the bacteria produce a natural soap (biosurfactant).
   • Analogy: Imagine trying to wash a greasy pan with just water; it doesn't work. You need dish soap to break the grease up. The bacteria make their own dish soap to make the plastic crumbs easier to grab.
2. Building a House (Biofilm): The bacteria stick together and build a tiny community (biofilm) right on top of the plastic crumbs.
   • Analogy: It's like building a house directly on top of the food source so you don't have to walk far to eat.
3. The Cutting Tools (Enzymes): Once they are close, they use special molecular scissors to cut the plastic crumbs:
   • Alkane Monooxygenase: Cuts the straight chains.
   • Baeyer-Villiger Monooxygenase: Cuts the oxidized (rusted) parts of the plastic.
   • The Digestion: Once cut, these pieces turn into fatty acids, which the bacteria then burn for energy, just like we burn food for energy.

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The Twist: What They Didn't Find

For a long time, scientists thought bacteria used a specific tool called a Laccase (a type of enzyme) to chew through the tough plastic chains.

However, in this study, the salmon bacteria did not use this tool. Even though the bacteria had the gene for it in their DNA, they didn't turn it on when eating the plastic.

• The Lesson: Just because a bacterium has the gene for a plastic-eating tool doesn't mean it actually uses it. We have to watch them work to know for sure.

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Why Does This Matter?

This research is a bit of a "reality check" for the plastic-eating field.

1. Good News: We found a bacterium that can eat the broken-down pieces of plastic (the microplastics) that are already floating in the ocean and inside fish. This is a crucial step in cleaning up the mess.
2. Bad News (for now): This bacterium cannot eat the big, tough plastic bags yet. It can only handle the "crumbs."
3. The Future: By understanding exactly how this salmon bacteria eats the crumbs (the soap, the biofilm, the specific scissors), scientists can try to engineer better bacteria or enzymes to eventually tackle the bigger, tougher plastic pieces.

In short: The salmon bacteria is a skilled "crumb cleaner" that uses soap and specialized scissors to eat the tiny, weathered bits of plastic. It's not a "plastic shredder" yet, but it's a vital piece of the puzzle for understanding how nature might eventually help us clean up our plastic pollution.

Technical Summary

1. Problem Statement

• Context: Polyethylene (PE) is the most produced synthetic polymer, leading to significant microplastic (MP) accumulation in aquatic ecosystems. Environmental factors (UV light, heat, oxidation) cause PE to depolymerize into low molecular weight PE (LMWPE), alkanes, and oxidized derivatives (e.g., ketones).
• Knowledge Gap: While fish like Atlantic salmon ingest MPs, the metabolic potential of their gut microbiota to interact with or degrade these plastic-derived compounds is poorly understood. Previous studies often lack rigorous chemical validation to distinguish between the degradation of actual polymer chains versus the consumption of pre-existing low-molecular-weight additives or degradation products.
• Objective: To investigate the ability of a specific gut isolate, Rhodococcus sp. ASF-10, to utilize oxidized LMWPE as a carbon source and to elucidate the enzymatic mechanisms and genomic basis for this process.

2. Methodology

The study employed a multi-omics approach combined with advanced analytical chemistry:

• Strain Isolation: Rhodococcus sp. ASF-10 was isolated from the gut of Atlantic salmon (Salmo salar).
• Growth Experiments: The strain was cultured in minimal media with three carbon sources: oxidized LMWPE (model substrate for abiotically degraded PE), pristine LDPE, and sodium succinate (control). Growth was monitored via OD600.
• Comparative Genomics:
  • Phylogenomic analysis of ASF-10 against 20 other Rhodococcus genomes and Corynebacterium antarcticum using 120 single-copy markers.
  • Calculation of Average Nucleotide Identity (ANI) and Average Amino Acid Identity (AAI).
  • Metabolic Capacity Index (MCI): Used the distillR package to quantify metabolic potential based on KEGG pathways, correlating genome size with metabolic complexity.
  • Homology Search: Screened genomes against a database of 593 proteins associated with plastic and hydrocarbon degradation (from PlasticDB, PAZy, HADEG).
• Substrate Characterization (Pre- and Post-Growth):
  • FT-IR: To detect functional group changes (carbonyls).
  • Size Exclusion Chromatography (SEC): To analyze changes in molecular weight distribution (polymeric vs. low molecular weight fractions).
  • Gas Chromatography-Mass Spectrometry (GC-MS): To quantify specific depletion of alkanes and ketones.
• Proteomics & Network Analysis:
  • Label-Free Quantification (LFQ): Compared proteomes of ASF-10 grown on LMWPE vs. succinate.
  • Weighted Gene Co-expression Network Analysis (WGCNA): Identified protein modules correlated with LMWPE growth to find co-regulated pathways (e.g., biofilm, surfactant production, degradation enzymes).

3. Key Results

A. Genomic Potential and Phylogeny

• Phylogeny: ASF-10 belongs to a clade with small genomes (<6 Mb) and is most closely related to R. trifolii CCM 7905, though they are distantly related (ANI ~79%).
• Metabolic Complexity: A strong positive correlation exists between genome size and metabolic complexity (MCI) up to a threshold of ~6 Mb. ASF-10 (small genome) retains core functions for lipid and fatty acid degradation but lacks some xenobiotic pathways found in larger genomes.
• Hydrocarbon Genes: ASF-10 possesses a conserved set of genes for alkane degradation, including alkane 1-monooxygenase (AlkB), cytochrome P450 hydroxylases, and Baeyer-Villiger monooxygenases (BVMOs).
• Plastic Genes: While ASF-10 has genes for degrading PUR and PA, it lacks confirmed genes for direct polymeric PE depolymerization. Notably, it contains homologs for Laccase-like multicopper oxidases (LMCOs), enzymes previously hypothesized to degrade PE, but their expression was not induced in this study.

B. Growth and Substrate Utilization

• Growth: ASF-10 grew robustly on oxidized LMWPE (OD600 ~0.9) but showed no growth on pristine LDPE.
• Chemical Analysis:
  • FT-IR: No significant changes in the carbonyl region (~1750 cm⁻¹) were observed, suggesting no new oxidation occurred on the surface.
  • SEC: The high molecular weight polymeric fraction of LMWPE remained unchanged, indicating the bacteria did not depolymerize the polymer backbone. However, the low molecular weight fraction (LogM 2–2.5) was depleted.
  • GC-MS: Significant depletion (P < 0.05) of alkanes (up to C29) and 2-ketones (up to C28) was detected in the LMWPE after incubation with ASF-10.
• Conclusion: The strain metabolizes the oxidized derivatives (alkanes/ketones) present in the LMWPE mixture, not the polymeric PE itself.

C. Proteomic Mechanisms

• Upregulated Enzymes: Growth on LMWPE induced high expression of:
  • AlkB (alkane 1-monooxygenase) and AlmA (long-chain alkane monooxygenase).
  • Cytochrome P450s (CYP153) and BVMOs (for subterminal oxidation of ketones to esters).
  • Esterases and enzymes for $\beta$-oxidation (acyl-CoA dehydrogenases, enoyl-CoA hydratases), funneling metabolites into the TCA cycle.
  • ABC Transporters: Likely involved in the uptake of hydrophobic substrates.
• Biofilm and Surfactants: The "green module" in WGCNA (highly correlated with LMWPE growth) contained proteins for biofilm formation and biosurfactant production (trehalose-containing glycolipids). This suggests the bacteria enhance substrate accessibility to hydrophobic compounds.
• LMCO Status: The LMCO (LMCO2) gene was present but not differentially abundant, and its activity did not result in polymer chain scission, challenging previous assumptions about its role in PE degradation.

4. Key Contributions

1. Differentiation of Mechanisms: The study rigorously distinguishes between the degradation of polymeric PE and the consumption of abiotically generated LMWPE derivatives. It demonstrates that ASF-10 consumes the latter but cannot depolymerize the former.
2. Enzymatic Pathway Elucidation: It provides a detailed proteomic map of how a fish gut bacterium processes oxidized alkanes and ketones, highlighting the central role of AlkB, P450s, and BVMOs in converting these compounds into fatty acids for $\beta$-oxidation.
3. Ecological Insight: It confirms that gut-associated Rhodococcus strains possess the metabolic machinery to utilize hydrocarbon derivatives found in the aquatic environment, potentially influencing the fate of microplastics in the food web.
4. Methodological Framework: The paper advocates for a combined approach (Genomics + Proteomics + Advanced Analytical Chemistry like SEC/GC-MS) to avoid false positives in plastic degradation studies.

5. Significance

• Bioremediation Potential: While ASF-10 cannot eat intact plastic, its ability to mineralize oxidized, low-molecular-weight PE derivatives (which are often the most toxic and bioavailable fraction of microplastics) suggests it could be harnessed for bioremediation in aquaculture and marine environments.
• Re-evaluation of "Plastic-Eating" Enzymes: The findings suggest that enzymes previously touted as "PE-degrading" (like LMCOs) may actually be acting on small oxidized fragments rather than the polymer backbone. This calls for a re-evaluation of claims regarding microbial plastic degradation.
• Aquaculture Safety: Understanding that gut microbes can metabolize plastic derivatives provides new insights into the potential health impacts of microplastic ingestion in farmed salmon and the natural detoxification capacity of their microbiome.