Eating Metal in the Dark


Microbes are everywhere. They are almost as old as the earth. Before earth had oxygen, there were no plants and animals, only metals in the crust. Later when life learned to do photosynthesis by trapping the energy of light and using water to make carbohydrates and carbon dioxide, life became more efficient and spread into every niche. What microbes learned about eating metal in the earliest days got incorporated into the machinery for plants to make carbohydrates. How did metal eaters eat and get energy to live without any sunlight or oxygen? Note: we still have metal eaters in West Marin.

What are microbes?


Today’s microbes are bacteria, protozoa, fungi and algae. They live everywhere all over the earth and are crucial for decomposing and recycling nutrients through all ecosystems. Microbes inhabit soil, hot springs, the atmosphere, rocks throughout the earth’s crust, and geothermal vents in the oceans. These guys are not the same as those extant when the earth was very young.


Microbes Evolve


Single-celled microorganisms were the first living things on the earth. They evolved about three to four billion years ago. Remember the age of the earth is about four and a half billion years, so they were present early on. Microbes have been identified in petrified sap (amber) that is 220 or so million years old. From such fossils we learn that the shapes and parts of microorganisms have changed very little since the Triassic, 250 million years ago. Once they appeared, further evolution of microbes seems to have proceeded very slowly. In those primordial times, all living things were microscopic in size.


For most of its history on Earth, life took very small forms. Animals and plants had only one cell. Even way back, nothing much could live on dirt, as there was no cellular equipment to use sunlight to make carbohydrates. Slowly, single cells evolved these abilities. They could acquire energy and food, make proteins and carbohydrates, grow, reproduce, excrete and avoid extinction. Early microbes diverged into today’s bacteria, algae and fungi. Microbes, being so small and compact, can evolve very quickly and exploit new niches and sources of energy very rapidly, almost as soon as these become available. Later microbes could reproduce very rapidly. When conditions are right, adult microbes can exchange genes and materials among their neighbors. Borrowing a neighbor’s genetic stuff ensures maximum genetic variability and permits a quick fit to new environments.


Contemporary sheets of microbes destroy metal

Microbes often form mat-like layers over surfaces. To the untrained eye these films appear as soft carpets of mucus slimed over stones, plants, soil and other objects. Think of the bacterial film on your teeth your dentist wants you to brush away. Microbes are tenacious and live everywhere, even in your mouth, one of the “dirtiest” places on earth if we define dirty as full of microbes. The oldest complete fossils of a microbial mat are about three and a half billion years old.


Energy from metals


How do they do it? Take iron for example. Microbes can use several metals for energy, but iron is the most common metal on the planet. Iron forms much of the earth’s outer and inner cores and was readily available in the environments of the earliest microbes. Fresh metallic iron is gray, but when oxygen is available in the air, iron combines with oxygen to make iron oxides, or rust. Iron oxides take up more space than the iron metal itself, so as rust forms, the rust swells and flakes off the metal, making more of the naked metal available to continue rusting.

Life from rust

Rust is two iron oxides together: ferrous and ferric oxide. Ferrous iron dissolves in water. Microbes can use this iron to get energy. What’s left after microbes get energy is ferric iron, ferric oxide. Ferric iron is not soluble in water and is excreted by the bacteria as little grains that are rust colored. But chemistry is terrific. Only when oxygen is available will the ferric iron react spontaneously with oxygen and become soluble ferrous iron again.

Iron bacteria also are hard at work over two miles down in the Atlantic where light doesn’t penetrate. They are eating the Titanic. This “unsinkable ship” lies on the sea floor where it sank on April 15, 1912, killing over fifteen hundred people. The bacteria eat the wreck’s metal and leave rusticles. Rusticles are icicle-like deposits of ferric oxide, hanging from the railings. Bacteria make the rusticles as they excrete the ferric iron. Each rusticle appears to be built of rings, similar to growth rings in a tree. About thirty percent of a rusticle is salts of iron: oxides, carbonates and hydroxides with some sulfur and phosphorus, but about seventy percent of a rusticle is just the working microbes. Rusticles are delicate and disintegrate into fine powder if disturbed, so in 20 years or so the Titanic may be an unrecognizable rust pile.


Tube worms from metal eaters.


Figure tube worm


Deep in ocean fissures, magma touches cold seawater under enormous pressure. Dissolved metals and chemicals from the magma, including iron, support complex biological communities. Bacteria use the energy from the iron and chemicals to grow in complete darkness. Snails, copepods, shrimps, crabs, tube worms, fish and octopuses are a food chain of predators and prey that live ultimately on the bacteria. Because vents release so much energy, vent critters live in dense populations, 10,000 to 100,000 times larger than populations on the ocean floor far away from the vents. The largest animals are the giant tube worms. These giants can grow over 80 inches long. They have no mouth or digestive tract. Tube worms absorb the nutrients from the bacteria directly into their tissues. About 285 billion bacteria are in one ounce of tubeworm.


Iron eaters of West Marin



In West Marin we have iron-eating bacteria in our springs and streams. The bacteria live underground where there is little oxygen, and the bacteria can stain spring and well water and block pipes with rust colored slime. Look for rusty patches in shaded fresh water. Bacteria deep in the ground use the ferrous iron for energy. They excrete the insoluble ferric iron. When the ferric iron contacts oxygen in the air it gains energy and cycles back to ferrous iron the bacteria can eat again. Recycling works for the bacteria of West Marin too.