Non-enzymatic assimilation of organosulfur compounds at the interface of geochemistry and biochemistry

This study demonstrates that microbes can utilize non-enzymatic, geochemically driven oxidative pathways involving light and heat to assimilate dimethyl sulfide and dimethyl sulfoxide for growth, challenging the assumption that such metabolic processes strictly require enzymatic reduction.

The Gist

The Big Idea: Nature's "Pre-Enzyme" Kitchen

Imagine that for billions of years, life on Earth was trying to cook a very specific meal: sulfur. Sulfur is essential for life (it's in your hair and nails), but it often comes in a form that is hard to eat, like a giant, tough rock.

For a long time, scientists believed that living things (bacteria) needed special, complex tools called enzymes (think of them as microscopic chefs with fancy knives) to chop up these sulfur rocks and make them edible. The paper says: "Wait a minute. Maybe the chefs aren't the only ones who can cook."

The authors discovered that nature has a "back-up kitchen" that doesn't need chefs at all. It uses heat, light, and rust (iron) to break down sulfur compounds naturally. This is a "geochemical" process (rock chemistry) that happens right at the edge of where rocks end and life begins.

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The Main Characters: The "Sulfur Twins"

The study focuses on two specific sulfur compounds found in the ocean:

1. DMS (Dimethyl Sulfide): The gas that gives the ocean that "sea breeze" smell.
2. DMSO (Dimethyl Sulfoxide): A chemical often used in medicine and industry.

Usually, bacteria need to use their internal "enzymatic machinery" to turn these into food. But the authors found that if you add sunlight or heat, these chemicals break apart on their own, creating food that bacteria can eat without doing any work.

The Analogy: The "Magic Rusty Spoon"

Think of DMS and DMSO as a locked treasure chest containing sulfur.

• The Old Theory: You need a specific, complex key (an enzyme) to unlock the chest. If you don't have the key, you starve.
• The New Discovery: You don't need the key. If you leave the chest in the sun (light) or boil it (heat) with a rusty spoon (iron), the chest starts to rattle and eventually pops open on its own.

Once the chest pops open, it releases two things:

1. Sulfur: The food the bacteria need to survive.
2. Carbon (Methane/Methanol): A side dish that some bacteria can also eat.

How They Proved It (The Experiments)

The scientists set up three different "kitchens" to test this theory:

1. The "Light Kitchen" (The Sun & Rust)
They took bacteria and put them in a jar with DMSO, water, and iron. They shined a bright light on it.

• Result: The bacteria grew like crazy! The light and iron acted like a chemical blender, breaking the DMSO down into food (sulfite and methanesulfonic acid) that the bacteria could eat.
• The Catch: Bacteria that didn't have the "enzymatic key" (the ability to eat the broken-down pieces) couldn't grow. This proved the light was doing the heavy lifting.

2. The "Heat Kitchen" (The Hot Spring)
They took a heat-loving bacteria (a thermophile) and put it in a hot jar (65°C / 150°F) with DMSO and iron.

• Result: Even without light, the heat and iron broke the chemicals down. The bacteria ate the sulfur and grew.
• The Lesson: This suggests that in hot environments (like volcanic lakes or deep-sea vents), life could have started eating sulfur long before complex enzymes evolved.

3. The "Inside the Cell" Test (The Ghost in the Machine)
They wanted to know if this happens outside the bacteria or inside them. They used a special "labeled" version of DMSO (like putting a neon sticker on the food).

• Result: The bacteria ate the neon food and breathed out neon carbon dioxide.
• The Conclusion: This chemical breakdown happens inside the bacteria's own body, driven by the natural "rust" (iron) and heat stress inside the cell. The bacteria aren't just waiting for the environment to cook their food; they are cooking it themselves using simple chemistry.

Why This Matters: The "Pre-History" of Life

This paper changes how we see the history of life.

• Before: We thought life started with complex enzymes, and that was the only way to get energy.
• Now: We realize that simple chemistry (light + heat + iron) was probably the "starter motor" for life.
  • Imagine early Earth as a giant, warm, rusty soup. The sun was beating down on it.
  • In this soup, sulfur compounds were breaking open naturally, creating food.
  • Early microbes didn't need to invent complex tools immediately; they just had to learn to eat what the "geochemical kitchen" was already serving.
  • Over millions of years, evolution invented the "enzymes" (the fancy chefs) to make this process faster and more efficient, but the old, slow, non-enzymatic way is still happening today.

The Takeaway

Life is more flexible than we thought. We often think of biology as a strictly "machine-driven" process where every step needs a specific tool. This paper shows that nature is messy and creative. Sometimes, the environment itself (sun, heat, rust) does the work, and life just steps in to eat the leftovers.

It's like realizing that while you have a high-tech microwave (enzymes), you can still cook a meal by leaving it in the hot sun on a metal roof (geochemistry). Both work, but the sun method is ancient, simple, and still going strong.

Technical Summary

1. Problem Statement

Dimethyl sulfide (DMS) and dimethyl sulfoxide (DMSO) are critical organosulfur compounds in global nutrient cycling, climate regulation, and microbial metabolism. Traditionally, the assimilation of sulfur from these compounds by microorganisms was believed to require specific enzymatic pathways (e.g., DMS monooxygenase, DMSO reductase, and sulfonate monooxygenases) to reduce or oxidize the compounds into usable forms like methanethiol or sulfite.

However, recent findings indicated that reactive oxygen species (ROS) could mediate the oxidative demethylation of DMS and DMSO to produce methane, methanol, and formaldehyde. The central problem addressed by this study is whether this non-enzymatic, ROS-driven oxidation can generate sufficient sulfur intermediates (specifically methanesulfonic acid [MSA] and sulfite) to sustain microbial growth, effectively bridging geochemical processes with biological metabolism without direct enzymatic intervention.

2. Methodology

The researchers employed a multi-pronged approach combining in silico genomic analysis, chemical modeling, and microbial cultivation under controlled conditions.

• Genomic Analysis: The authors analyzed the NCBI database to determine the prevalence of genes encoding enzymes for DMSO oxidation ($sfnG$, $ssuD/msuD$) versus reduction ($dmsA$). This was used to hypothesize that non-enzymatic oxidation might be a common environmental precursor to enzymatic pathways.
• Chemical Model Systems (Geochemical Drivers):
  • Fenton Chemistry: DMS and DMSO were treated with $H_2O_2$ and $Fe^{2+}$ to generate ROS.
  • Light-Driven LMCT: Two geochemical models were created to simulate environmental conditions:
    1. Neutral pH (pH 7) with $Fe^{3+}$-citrate (simulating marine environments).
    2. Acidic pH (pH 3) with $[Fe(H_2O)_6]^{3+}$ (simulating volcanic lakes).
       These systems were illuminated to trigger Ligand-to-Metal Charge Transfer (LMCT), generating $Fe^{2+}$ and hydroxyl radicals ($\cdot OH$) to drive DMSO degradation.
• Microbial Growth Assays:
  • Mesophiles: Escherichia coli (harboring $ssuD$), Pseudomonas aeruginosa (harboring $sfnG$ and $ssuD$), and Aeromonas hydrophila (lacking both) were grown in sulfur-free minimal media supplemented with untreated or chemically/light-treated DMS/DMSO.
  • Thermophiles: The acidophilic thermophile Alicyclobacillus acidocaldarius (harboring $ssuD$) was cultivated at 45°C and 65°C with DMS/DMSO to test heat-driven non-enzymatic degradation.
• Metabolic Tracing: Methylobacterium hispanicum (a methylotroph) was grown with $^{13}C$-labeled DMSO. The researchers measured the $^{13}C/^{12}C$ ratio in respired $CO_2$ to determine if the carbon released from non-enzymatic DMSO degradation was being assimilated by the cell.
• Analytical Techniques: Growth was monitored via $OD_{600}$. Degradation products (MSA, sulfite) were quantified using HPLC-MS. Gas analysis ($CO_2$, $CH_4$) was performed via GC-MS.

3. Key Contributions

• Demonstration of Non-Enzymatic Assimilation: The study provides empirical evidence that microbes can grow on DMS and DMSO as sole sulfur sources without the specific enzymes required for initial activation, provided the compounds are pre-oxidized by geochemical factors (light, heat, iron).
• Dual Carbon-Sulfur Metabolism: It establishes that the non-enzymatic degradation of DMSO releases not only sulfur (as sulfite/MSA) but also C1 compounds (methanol/formaldehyde) that can be simultaneously utilized by microbes for carbon metabolism.
• Intracellular Mechanism: The research suggests that this non-enzymatic pathway is not limited to the extracellular environment but can occur intracellularly, driven by endogenous ROS and iron within the cell.
• Evolutionary Context: The paper proposes that enzymatic sulfur metabolism may have evolved to enhance and accelerate pre-existing, slow geochemical reaction networks driven by radical chemistry.

4. Key Results

• Genomic Prevalence: Genes for the later steps of DMSO oxidation ($sfnG$ and $ssuD$) are widespread, while the initial oxidation step ($DMSO \to DMSO_2$) is rare enzymatically, suggesting the initial step often occurs non-enzymatically.
• Light-Driven Growth:
  • E. coli and P. aeruginosa grew robustly on light-treated DMS/DMSO (with $Fe^{3+}$-citrate or acidic $Fe^{3+}$), reaching high cell densities ($OD_{600} \approx 2.2–3.5$).
  • Untreated DMS/DMSO supported negligible growth ($OD_{600} < 0.13$).
  • A. hydrophila (lacking oxidation genes) showed minimal growth, confirming that the geochemical treatment produced sulfite/MSA which the other strains could utilize.
  • HPLC-MS confirmed the formation of MSA and sulfite from light-treated DMSO.
• Heat-Driven Growth:
  • A. acidocaldarius grew on DMSO at 65°C, with growth rates comparable to sulfate controls, especially when supplemented with $Fe^{3+}$.
  • At 45°C, growth on DMSO was significantly delayed, and no growth occurred on DMS, indicating a temperature threshold for effective non-enzymatic degradation.
• Carbon Assimilation:
  • M. hispanicum cultures supplied with $^{13}C$-DMSO produced $^{13}CO_2$ with a ratio of 3.95%, significantly higher than natural abundance (1.13%).
  • This confirms that the carbon skeleton of DMSO is broken down non-enzymatically and the resulting C1 compounds are respired by the bacteria.

5. Significance

• Redefining Metabolic Paradigms: The study challenges the dogma that metabolic pathways are strictly enzyme-driven. It posits that "geochemical metabolism" (non-enzymatic reactions driven by environmental factors like light, heat, and iron) can sustain life and provide a metabolic foundation that enzymes later optimized.
• Origin of Life Implications: The findings support the hypothesis that early life on Earth, particularly in the iron-rich, ferruginous oceans of the Archaean and Proterozoic eons, relied on radical chemistry and ROS-driven reactions for sulfur and carbon cycling before the evolution of complex enzymatic machinery.
• Environmental Impact: This mechanism suggests a ubiquitous, non-biological pathway for the degradation of organosulfur compounds in aquatic environments (marine, volcanic, and hydrothermal), influencing global sulfur and carbon cycles independently of microbial enzymatic activity.
• Evolutionary Continuity: It illustrates a continuum between geochemistry and biochemistry, where modern enzymes (monooxygenases) may simply act as accelerators for reactions that were already occurring spontaneously in the environment.