Laboratory evolution of copper tolerant yeast strains

For decades, copper tolerance in microbes was treated as a single trait — you either had it or you didn't. Then, Adamo and colleagues fractured that assumption with a deceptively simple experiment. They took two yeasts, raised them under identical copper pressure, and watched them become tolerant by completely opposite biochemical routes. One species cranked up its defenses while the other dialed them down. Same destination, entirely different roads. To understand why that matters, you need to feel the central tension that copper creates in any living cell. Copper is not optional; it is a cofactor for key enzymes and electron-transport proteins. However, free copper is dangerous. It generates reactive oxygen species, or ROS, which peroxidize membrane lipids, oxidize proteins, displace other metal cofactors from signaling proteins, and cleave DNA and RNA. The damage has been implicated in aging and cancer. Therefore, the cell must import copper and deploy it precisely, while never allowing it to run loose. That balancing act is what Adamo and colleagues set out to probe. Their two protagonists are Saccharomyces cerevisiae — the well-studied baker's yeast — and Candida humilis, a less familiar sourdough species. Both are yeasts, and both face the same copper chemistry. But their starting positions could not be more different. When Adamo and colleagues exposed both species to increasing concentrations of copper sulfate, Saccharomyces showed growth inhibition at low concentrations. In contrast, Candida humilis tolerated up to one gram per liter of copper sulfate and, at that concentration, accumulated more than seven milligrams of copper per gram of biomass. It didn't just survive; it stockpiled the toxin. It escaped oxidative damage because it was already running high basal levels of superoxide dismutase, or SOD, which neutralizes the superoxide radical, and catalase, which breaks down hydrogen peroxide. Those enzymes were constitutively active — always on, not waiting for a copper signal to switch them up. The numbers are vivid. In non-evolved Candida humilis, SOD activity measured roughly 133 units per milligram of protein. In non-evolved Saccharomyces, it was about 21. Catalase in Candida measured 81.5 units per milligram, while in Saccharomyces, it was 3.2. Candida entered the copper environment with an antioxidant arsenal six to eight times larger than its competitor, and that alone explains much of its natural edge. Consistent with this, ROS production under copper exposure was roughly threefold higher in Saccharomyces than in Candida. Then, Adamo and colleagues pushed both species further. The evolutionary engineering protocol was straightforward in design: serial passaging in progressively higher copper concentrations, raising the copper sulfate dose by half a gram per liter each round, selecting survivors, and repeating for seventy-two hour cultivation intervals. It mimics natural selection, compressed into a lab timeline. Both yeasts reached the same hyper-resistant endpoint — growth in two and a half grams per liter of copper sulfate. For Candida, that was a two and a half fold increase over its natural ceiling. For Saccharomyces, it was a dramatic leap from a species that struggled at concentrations far below Candida's natural limit. Critically, when the team removed the copper pressure, the evolved phenotypes held. The tolerance was stable in the absence of copper. That stability, combined with evidence of amplification of the CUP1 metallothionein gene in evolved Saccharomyces, suggests that real, lasting molecular changes had occurred — not just a transient physiological adjustment. Here is where the story becomes genuinely surprising. Both yeasts reached the same phenotypic ceiling, but the paths they took were biochemically reversed. In Saccharomyces, evolution toward copper tolerance followed a classical "fight the oxidant" logic. SOD activity rose from about 21 units per milligram in non-evolved cells to 39 in evolved cells grown in normal medium, and to 64 when those evolved cells were challenged with copper. Catalase climbed from 3.2 to about 12 units per milligram in copper medium. Cell death fell from sixty percent propidium-positive cells in non-evolved Saccharomyces under copper to just eleven percent in the evolved strain. Saccharomyces, which started weak, learned to fight. Candida did the opposite. Its antioxidant activities, already sky-high in the natural strain, dropped sharply after evolution. SOD in evolved Candida fell to about 32 units per milligram in normal medium and 41 in copper medium — a collapse from the 133 baseline. Catalase in evolved Candida grown in copper landed at about 28 units per milligram, down from 74 in non-evolved cells under the same conditions. And yet, evolved Candida was healthier: propidium-positive cells fell from twenty-eight percent in non-evolved Candida under copper to eight point five percent in the evolved strain. Less antioxidant activity, less cell death. That seems contradictory — until you look at the copper accumulation data. Non-evolved Candida accumulated 7.6 milligrams of copper per gram of biomass under high copper conditions. Evolved Candida accumulated 6.5. The kinetics tell the story even more sharply: within the first twenty-four hours, non-evolved Candida imported copper at roughly three times the rate of the evolved strain. Evolved Candida was controlling the gate. It was letting in less copper to begin with, and if less copper enters, less oxidative damage occurs, which means you need less antioxidant machinery running at full power. The strategy is not to fight the fire; it is to reduce the fuel. The copper-binding proteome — the suite of proteins that physically interact with copper inside the cell — changed in both species after evolution, but again in different directions. Evolved Saccharomyces showed induction of a broad range of copper-binding proteins, including pentose phosphate enzymes like transketolase, amino acid metabolism enzymes such as glutamate dehydrogenase and cystathionine-gamma-lyase, glycolytic enzymes like glyceraldehyde-3-phosphate dehydrogenase and triose phosphate isomerase, and the copper-containing form of SOD itself. This paints a picture of a cell that has integrated copper into more of its metabolic machinery. Evolved Candida, by contrast, showed a massive enrichment of a specific glycolytic isoform, glyceraldehyde-3-phosphate dehydrogenase three, and an increase in the peroxiredoxin Tsa1. Non-evolved Candida under copper actually repressed several copper-binding proteins, including ribosomal and translation components. The proteome reconfigurations were different in kind, not just degree. Adamo and colleagues frame the overall picture as a system with three levers: control of copper uptake, sequestration of intracellular copper by copper-binding proteins, and enzymatic antioxidant defense. Different species — and different evolutionary trajectories within the same species — reach tolerance by pulling different combinations of those levers. Saccharomyces pulls hard on the enzymatic defense lever and adjusts its copper-binding proteome broadly. Candida tightens the uptake valve, enriches specific sequestering proteins, and allows the antioxidant system to relax because there is simply less free copper to deal with. That divergence matters beyond cell biology for a specific reason: it means there is no single molecular signature of metal tolerance. If you wanted to identify tolerance in an unknown organism, you could not just measure SOD and call it. You would need to understand which combination of strategies that organism had selected for. Adamo and colleagues close with two translational possibilities. The first is dietary: yeast that accumulate high intracellular copper in a biologically available form could serve as functional food supplements, delivering copper as an organic micronutrient. The paper notes that edible microorganisms enriched in micronutrients are relevant for functional food production, and that these organisms that are generally recognized as safe can be valuable sources of microelements. With both Candida and evolved Saccharomyces accumulating copper well above baseline and remaining viable, the raw material for that application is there. The second possibility is medical: these evolved strains, with their tunable copper metabolism, offer experimental models for studying human diseases of copper handling, including Wilson disease and Menkes disease, both of which involve failures of copper transport and localization. Copper-accumulating yeasts have also been proposed for bioremediation, to clean copper from contaminated solutions. What makes this work resonate beyond the immediate findings is what it says about adaptation in general. Two organisms face the same toxic pressure. Both survive. But survival, it turns out, is not a single answer; it is a space of solutions. Evolution finds different paths through that space depending on where each organism started, what molecular tools it had ready, and which changes were easiest to make. The yeast flask turns out to be a surprisingly clear window onto that universal logic. This lecture was created by ennepō. Go to https://ennepo.ai to Discover, Create and Follow the latest research in your field. Read when you can. Listen when you want to.