A Garden of Marvels by Ruth Kassinger

Author:Ruth Kassinger
Language: eng
Format: epub, mobi, pdf
Publisher: HarperCollins
Published: 2013-03-10T16:00:00+00:00

eighteen

Once in a Blue-Green Moon

It is no accident that the chloroplasts in Pierce’s slugs—and chloroplasts in algae and plants—have their own DNA. The ancestors of chloroplasts are cyanobacteria (also known, confusingly, as blue-green algae), an ancient group of bacteria that live independently, floating around on the surface of the world’s oceans, subsisting on readily available sunlight and carbon dioxide and reproducing by fission.

About three billion years ago—or 1.5 billion years after Earth formed and 300 million years before the cyanobacteria appeared—the planet looked radically different than it does today. The new continents, far smaller than the ones we know, were mostly submerged, with just a few barren outcroppings. The ocean was a rich green color, thanks to massive amounts of dissolved iron, and it was the temperature of a hot bath. The sky was hazy and orange with carbon dioxide, ammonia, and methane that spewed from active volcanoes. There was no free oxygen on the planet, in either the water or the atmosphere. All atoms of oxygen were bound up with other elements, mainly with hydrogen in water and carbon in carbon dioxide. Deep in this strange ocean were single-celled bacteria and archaea that lived in the hot, mineral- and gas-rich vents on the ocean floor. They made the energy to survive by reacting elements together in a membrane and using some of the energy released for their own simple metabolic needs. Some of these single-celled beings (prokaryotes, pronounced pro-CAR-ee-oats) were bacteria that stole electrons released in the combination of sulfur and iron abundant in those primal waters. Others, especially the archaea, corralled a little energy in reacting the hydrogen escaping from the Earth’s core with carbon dioxide to make methane.

About 2.7 billion years ago, a new kind of bacteria evolved. These floated near the surface of the green ocean, and instead of collecting energy by reacting various chemicals together, they used the energy of sunlight—the photons streaming toward Earth—to strip off electrons from various chemical compounds in the waters around them. Some of these photosynthetic bacteria stripped electrons from hydrogen sulfide (H2S), others favored hydrogen molecules (H2), but the ones we care about—all the species of cyanobacteria—snagged electrons by splitting water (H2O).

Cyanobacteria converted that electron energy into ATP. They then used up the ATP in fastening hydrogen protons onto readily available molecules of atmospheric carbon dioxide, thereby making sugars. The sugars became the building blocks of the rigid wall that separated a cyanobacterium from its watery environment, as well as its thick, mucosal extracellular coating. That coating was critical for cyanobacteria living at the ocean’s surface. Ultraviolet light was as yet unfiltered by an ozone layer and would have otherwise fried their DNA. Whenever a cyanobacterium split H2O, it burped a tiny bubble, an infinitesimal O of oxygen into the water.

Cyanobacteria lived an easy life. They had no need to chase down prey: They floated at the boundary of their two “foods,” water and atmospheric carbon dioxide, and their energy supply was boundless. They divided and redivided and populations

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