forage · evolve · divide · endure
One tiny cell. One living ocean. Chase food, capture new genes, evade hungry protists, and multiply into a lineage strong enough to survive from sunrise to sunrise.
Scenario ·
A particle is far too big to swallow, so you take it apart from the outside: an enzyme dissolves it, and you absorb what comes loose. Respiration burns energy every second, so feeding never stops. Feed well and you elongate and divide — each daughter forages on its own and inherits everything you've evolved. Feed badly and you seal yourself into a cyst, dormant until the sea turns generous again.
Every particle is a patchy mix of lipid, protein and carbohydrate, and each one is locked behind its own enzyme. You start with carbohydrase alone. Lipid is the richest fuel in the sea — more than twice the calories of carbohydrate — so lipase is the gene that transforms your score.
The gold phage is rare, drifts slowly, and often lies buried inside a particle. It marks the minimap with a star. Catch one and it rewrites your genome at random: a new enzyme, stronger expression, chemotaxis, an antibiotic, CRISPR, twitching motility, or EPS production. Nothing you gain leaves your own body until you divide — so adapt early.
Twitching motility lets a bacterium crawl across solid food particles at half its ordinary speed. While attached, it drifts with the particle beneath it; in open water, it swims at full speed. EPS production adds EPS to the loaded-gene cycle; release it to place a non-degradable block that stops bacteria, protists and viruses. Level 1 lasts 4 seconds, and every further EPS upgrade adds another 4 seconds.
Phages drift, cling to particles, and infect the cells that arrive to feed. Each is tuned to a narrow band of adaptation, so they are colored against the cell you are steering: red can infect it, green cannot. Every gene you gain carries you out of one cohort's range and into another's. Not every contact lands — adsorption is a matter of chance — and a cell can take on several virions at once (a high multiplicity of infection), bursting all the larger for it. An infected cell lyses, and the burst seeds more phages.
A full day passes in about 4 minutes; last all twenty-four hours of it and the day is yours. Dawn is cold, dim and lean. By noon the sunlight has driven a phytoplankton bloom, and the warm water that feeds you also makes you burn through it faster. Through the afternoon the bloom is grazed into fecal pellets and ages into marine snow. At night the sea empties, and the grazers hunt hardest.
Extinction is a change of role, not an ending. When your last bacterium dies you become a protist — one trophic level up, grazing the bacteria instead of the particles. A grazer carries no enzymes, so Space becomes a sprint, and every gold phage you catch lengthens it. Prey keeps drifting in for you to hunt. So do rival grazers, and there is no way back down.
The chart stacks your bacteria by generation: one color per lineage, a fresh shade each time one adapts, so you can watch lineages rise and fall as they happen. Dormant cysts stay counted in their band. Lines above it track protists and viruses, and a second panel follows the food supply. When the run ends you get the whole history back, with every adaptation marked where it fell.
Bacteria! is a simplified model of marine microbial ecology — the invisible world of bacteria, viruses and single-celled hunters that runs the ocean. Every mechanic in the game is drawn from real biology, and what follows is the science behind each one. Underlined words link to Wikipedia.
A single milliliter of seawater — a few drops — holds roughly a million bacteria and ten million viruses. These bacterioplankton are the ocean's recyclers: they break down dead material and return its nutrients to the food web, a cycle called the microbial loop. They are invisible, and they run the planet's carbon and nutrient cycles.
You play one bacterium in that world — a single cell trying to eat, grow and survive.
Water does not feel the same to a microbe. At a bacterium's size, viscous forces overwhelm momentum — a regime of very low Reynolds number, described in Edward Purcell's classic lecture Life at Low Reynolds Number. A swimming cell that stops pushing coasts less than the width of an atom before it halts. There is no gliding, no drifting to a stop: thrust is the only thing holding you in motion, and the moment it ends, so does the motion.
Release the keys and your cell stops dead — no coasting, no glide, because there is no momentum to spend. Water thins as the sea warms, so the same effort drives you a little faster in the afternoon than it did at dawn.
The big drifting particles you eat are marine snow — clumps of dead plankton, fecal matter and mucus that sink through the ocean, carrying organic matter to the deep in what oceanographers call the biological pump. Real particles come in every size, with small ones vastly outnumbering large — a power-law size distribution. To a bacterium each particle is an island: a rich, crowded, short-lived hotspot in an otherwise dilute sea.
Particles spawn on that same power law: many small, a few large. All four types are real — marine snow, fecal pellets, chitin from crustacean shells, and diatom frustules, which leave their glass behind once the flesh is gone.
A bacterium is far too small to swallow a particle, so it digests its food externally: it secretes extracellular enzymes that cut large molecules into small ones by hydrolysis, then absorbs the fragments. Each enzyme is specific to one class of molecule — a lipase will not touch a protein — so what a cell can eat is decided entirely by which enzyme genes it carries.
Space releases your loaded enzyme, dissolving matching blocks into absorbable nutrients. A particle you have no enzyme for is just an obstacle.
Living matter is built from three macronutrients: lipids (fats), proteins and carbohydrates (sugars). They do not carry equal energy: fat yields about 9 Calories per gram against roughly 4 for protein and carbohydrate, because its carbon is more reduced and there is more of it to oxidize. That is why fatty food is so calorie-dense — for a cell as much as for you.
Your score is Calories absorbed, weighted by composition on that 9:4:4 ratio. A lipid block is worth more than twice a carbohydrate one, so it pays to evolve lipase and hunt the fatty particles.
Many bacteria swim with a corkscrew flagellum. They are too small to steer or to sense a gradient across their own length, so they navigate in time instead of space, by chemotaxis: swim straight (a "run") while conditions improve, reorient at random (a "tumble") when they stop improving. Neither move is aimed at anything, yet the biased random walk that results reliably carries the cell toward food.
Run-and-tumble is what your autonomous cells do — every daughter you aren't steering navigates exactly this way, and the chemotaxis adaptation is the bias: they hold a run for longer while food is drawing closer, and hold it harder the higher its expression. You get a privilege no real bacterium has: you steer. (Your cell still wobbles off-heading when you let go of the keys, which is only a reminder of what it would be doing without you.)
Bacteria reproduce by binary fission: a cell grows, copies its DNA and splits in two. Because every cell becomes two, populations grow exponentially — 1, 2, 4, 8, 16 — and a well-fed colony explodes astonishingly fast. What stops it is never the arithmetic; it is running out of food, or being eaten.
Eat enough and you elongate and divide. Daughters forage on their own while you keep steering one cell, and each division counts as a new generation.
Bacteria do not only inherit genes from a parent. They also acquire entirely new ones from their surroundings — horizontal gene transfer — and one route is transduction, in which a virus ferries DNA from one cell to the next. A cell can also turn up a trait it already has by carrying extra copies of the gene (amplification), raising its expression. This is why bacterial evolution can outpace our intuitions — and how antibiotic resistance spreads between species.
The rare gold phage transduces a random heritable adaptation into your cell. The genome strand shows which genes you carry, amplification appears as ×2, ×3…, and — as in life — an adaptation only spreads by dividing.
The deadliest killers of bacteria are viruses called bacteriophages — and they chase nothing. A phage has no motility whatsoever: no flagellum, no senses, no way to steer. It is a passive particle, shoved about by Brownian motion until it happens to collide with a cell whose surface it can grip. Only then does the lytic cycle begin: it injects its DNA, hijacks the cell's machinery to build copies of itself, and bursts the cell open to release them. What makes blind chance sufficient is sheer number — phages are the most abundant biological entities on Earth, and there are more of them in the sea than there are stars in the observable universe.
Red phages drift; they never home in on you. They stick to particles and wait, which is exactly where the collisions happen — the crowd around a particle is the danger, not the virus's intent.
Phages are specialists: each infects only bacteria closely related to the cell it came from. So whichever strain becomes most abundant is precisely the one its phages find most easily — and it gets cut down. Ecologists call this the Kill the Winner hypothesis. It prevents any single type from monopolizing the ocean and keeps the community diverse, in a permanent arms race between hosts and their viruses.
A phage reads red if it can infect the cell you're steering and green if it cannot. Every adaptation shifts you out of reach of the current cohort — and a lineage that booms will, in time, draw its own plague.
Phages kill an estimated 20–40% of ocean bacteria every single day. Each lysed cell spills its contents back into the water as dissolved organic matter, which other bacteria promptly take up — a huge flux of carbon diverted away from the larger animals that would otherwise have eaten those cells. This is the viral shunt, and it is one reason the microbial loop retains so much carbon near the surface instead of sending it to the deep.
Nothing is wasted: cells that lyse or starve return their matter to the sea, and spent particles recycle into new ones. Watch the food graph — a viral outbreak in a booming lineage feeds the next generation.
CRISPR is a genuine adaptive immune system: a bacterium files away snippets of the DNA of viruses that have attacked it, then uses those stored sequences to recognize and cut apart the same invaders on their return. It is inheritance of acquired immunity, and it evolved long before we noticed it — the gene-editing tools that won the 2020 Nobel Prize in Chemistry are borrowed bacterial defense machinery.
With CRISPR, viruses that cannot infect you stop being scenery and become food — you dismantle the harmless ones for energy.
Microbes are chemists. Many manufacture antibiotics and other secondary metabolites to poison their competitors or deter their predators — most of the antibiotics in our own medicine cabinets were first isolated from soil and marine microbes fighting each other. The targets tend to be distant relatives; close kin, which share the resistance genes, are spared. But the poisoned don't stay defenceless: relentless chemical pressure selects for resistance, and the grazers evolve their way back.
The antibiotic adaptation releases a toxin cloud that kills nearby protists and any genetically distant bacteria — three or more mutations from you — while your close kin walk through it unharmed. Higher expression widens the cloud. But every time a colony gasses its grazers to extinction, the protists come back more resistant: keep poisoning them and you breed a population your toxins barely touch, and the grazing pressure returns.
Twitching motility is a crawling movement driven by type IV pili. A pilus extends from the cell, attaches to a surface and retracts, pulling the bacterium forward. It lets surface-associated cells travel without a flagellum and helps them colonize particles and assemble into communities. But those same pili are the doorway many phages use to attach — so a lineage that evolves grappling hooks also evolves a receptor for viruses.
The twitching motility adaptation lets your lineage move directly across solid food particles at half speed while surface-bound. A cell on a particle rides with that drifting surface, then resumes full-speed swimming when it leaves. EPS remains a true barrier: twitching crosses particles, not walls. The catch: pili are the phage receptor, so a twitching cell is easier for green phages to infect — each encounter is far more likely to land a virus, and it shrugs off the kill-the-winner advantage of upgrading. Reach the food buried in the particle, but carry a bigger target.
Many bacteria secrete extracellular polymeric substances (EPS): a hydrated matrix of polysaccharides, proteins and DNA that holds cells together in a biofilm. The matrix changes diffusion and creates a physical refuge from grazers and viruses.
EPS production becomes a selectable gene with a visible expression count. Release it to place a non-degradable physical block ahead; bacteria, protists and viruses cannot pass through it. EPS level 1 lasts 4 seconds, with another 4 seconds added at each level.
Bacteria are prey as well as predator. Single-celled protists graze on them in enormous numbers, forming the link that carries microbial carbon up the food web to everything larger. Predator and prey then rise and fall in coupled cycles — abundant prey feeds a boom in predators, which crashes the prey, which starves the predators — the classic predator–prey dynamics, and each step up a trophic level passes on only about a tenth of the energy below it.
The big pink protists hunt your cells, and on the chart their numbers oscillate against your bacteria exactly as the equations predict. If your lineage is wiped out you become a protist yourself — one trophic level up, grazing the bacteria instead of the particles.
When conditions turn against them, many microbes simply stop. They enter dormancy, forming tough resting cysts or endospores that shrug off starvation, heat and desiccation, and revive when things improve. Endospores have been revived from sediments thousands of years old. Most of the microbial biomass on Earth, at any given moment, is dormant rather than dead.
A starving cell seals itself into a resistant cyst: dormant, unable to eat, immune to viruses and hard to kill — until food returns and it wakes. Cysts stay counted in their generation's band on the chart.
Everything in the surface ocean answers to the sun. Light drives primary production by phytoplankton, so food is manufactured by day and merely consumed by night. And at dusk the largest migration on Earth begins: grazers that sheltered in the dark deep rise toward the surface to feed under cover of night, then sink again at dawn — diel vertical migration.
Sunrise, sunset and twilight are not scripted. They are computed from real solar geometry — latitude, day of year, the tilt of the Earth — so push your sea far enough north and the sun simply stops setting. Food blooms with the light; the grazers hunt hardest in the dark.
A cell has no thermostat, so the sea sets its speed. Biological rates climb with temperature by a rule of thumb called Q10: for every 10 °C of warming, metabolism roughly doubles. Warmth is therefore double-edged — it accelerates your growth and your respiration alike, so a warm cell burns through its reserves faster and starves sooner without food. It is also why a warming ocean is a metabolically hungrier one.
Sea temperature trails the sun, peaking in the early afternoon, and every cell's metabolism scales with it at Q10 = 2 — yours, your daughters', and the protists' alike. The midday bloom feeds you and burns you at once.
Because different genes suit different conditions, a species fragments into varieties tuned to different circumstances — ecotypes, each occupying its own niche. That diversity is what makes a microbial community resilient: knock one type back and another is already positioned to take its place.
The colored bands on the chart are your lineages and generations over time — each band a trait group at an adaptation tier, not a true ecotype (convergent genomes can share a band). Watching them rise, split and fall is watching evolution happen — lineage by lineage, in a single run.
The soundtrack was not composed. It is sonified from an actual gene: the 16S ribosomal RNA sequence of E. coli — the very gene biologists read to identify and classify bacteria, and the one used to draw the tree of life. It is played base by base, each letter (A, C, G, T) mapped to a note of a minor scale. You are listening to a bacterium's DNA.
Press M to cycle the sound. The melody is one pass through the 16S gene; the drone and echo beneath it are only for atmosphere.
The real ocean is messier and stranger than any game can be — but if this has made a bacterium, a virus or the microbial loop feel a little more alive, it has done its job. Now go and evolve something.
Each scenario is a real microbial-ecology situation — a distinct ocean with its own lesson, environment, and enzymes. New ones are generated daily.
Beta — anything at all: what broke, what was confusing, what was boring, what you liked. No account needed.
Paste the DOI of a microbiology paper and it becomes a playable ocean — its organisms, its chemistry, its lesson. No account needed. It usually takes about twenty minutes to build, and then it lives in Scenarios for everyone.
10. and is on the paper's first page or in its URL —
doi.org/10.1038/nature12352 works too.