A Garden of Marvels

by Ruth Kassinger

  Saussure published his masterwork, Chemical Investigations of Plant Growth, in 1804, the year that Joseph Priestley died. In a thirty-year span, the role of leaves had been transformed from incidental to essential. The remarkable process of photosynthesis—the creation of organic compounds using light energy—was revealed.

  Understanding exactly how sunlight transforms carbon dioxide and water into carbohydrates was another matter.

  seventeen

  The Vegetable Slug

  The inch-long sea slugs that I have come to see in Dr. Sidney Pierce’s lab at the University of South Florida are, unlike the usual brown slugs, lovely animals. Hovering in the clear water of a ten-gallon aquarium, they look like translucent scraps of bright green, ruffle-edged arugula. One lettuce bit ripples its edges on the front glass, and I can make out under the white light of a fluorescent bulb what looks like a pair of knobbed green horns. This slug is adorable. It’s slug à la Pixar.

  The aquarium is empty except for Pierce’s little herd. Or should I say little crop? For these creatures, perfectly healthy and presumably happy, live like plants by photosynthesizing. For the past eight months, they have eaten nothing but the photons pouring from the lightbulb above. Remember the borametz, the mythical vegetable lamb chimera said to live on the Caucasian steppes? This slug, Elysia chlorotica, and a few related species are its real-life cousins. They are genuine chimeras, part sea slug and part algae.

  I am entranced by the dance of the lettuce leaves, but I know Dr. Pierce has limited time. We adjourn to his office next to the lab. He tells me he’s thinking about retirement, but with a boyish mop of white hair, a rosy complexion, a restless physical energy, and a certain provocative attitude—taped to his office door is a warning to any students coming to inquire how they might raise their grade that the only way to do so is to actually do better on his exams—it’s hard to credit. A “biochemist by trade,” he is also a retired chairman of the biology department at the University of Maryland, an active professor at USF, and author of a multitude of articles on the biochemistry of invertebrates, as well as a textbook on invertebrate anatomy. He has also become a “sea monster” expert. From time to time, a huge gelatinous blob washes up on a beach somewhere in the world. When the discoverer inevitably wonders if this is evidence of some previously unknown creature of the deep, he or she is often directed to Dr. Pierce. Just as inevitably, Pierce pronounces the subject to be something less than marvelous, usually a piece of decomposing whale blubber. There is a certain irony, I think, in his fame as marine mythbuster, given that he has spent the past two decades elaborating the physiology of what would seem to be a scientifically impossible creature, a photosynthesizing animal.

  Pierce’s office is in a state of confusion. He is in the middle of packing up his papers so he can adjourn to Woods Hole Marine Biological Laboratory on Cape Cod, Massachusetts, for his annual summer research. It was at Woods Hole, he tells me, about twenty-five years ago that a couple of graduate students brought him the first specimens of the photosynthetic Elysia chlorotica.

  “Until these students brought them in, no one knew they lived off Cape Cod, in spite of the fact that this big fancy lab had been studying the local marine environment for something like a hundred years. I was studying salinity tolerance at the time. Oddly, these completely marine creatures turned out to be some of the most salt-tolerant creatures I’d ever encountered, able to survive in everything from fresh water to water as salty as the Dead Sea. This was pretty remarkable since a sea slug is basically an unprotected ball of slime, crawling around in a marsh. So, I featured them in the talks and slide shows I gave about our research on salinity tolerance. For years, after my talk, people would come up to me and say, ‘Very nice talk, but look at those slugs! Those slugs are green. Why aren’t you studying that, you fool?’ Finally, I listened.”

  Pierce wasn’t the first to work on green sea slugs. In the 1960s and ’70s, a group of scientists studied a European species, wondering if its coloration meant it used the sun’s energy to photosynthesize. (In general, green animals—fish, reptiles, amphibians, and birds—look green because blue light reflects through a layer of yellow pigment.) If the European sea slug did photosynthesize, its color must derive from green chloroplasts, which are the organelles (organs of a cell) in plants that use solar energy to transform carbon dioxide and water into glucose. The scientists found that the slugs did have chloroplasts, an unprecedented innovation in an animal. How these little engines of photosynthesis came to be inside an animal was an interesting mystery, and they started out to investigate. Their first step was to remove the chloroplasts from the slugs.

  “Of course, they immediately ran into the mucus problem. Sea slugs,” Pierce explained, “are enormously capable of making mucus. They’re practically nothing but damn mucus. Put a sea slug in a blender to grind it up, and when the little packets of anhydrous mucus that lie right beneath its skin hit water, they explode into this big wad of snot. Out of which you can centrifuge nothing.

  “Before they could solve the mucus problem, the team discovered Symbiodinium, which are algae that live symbiotically—the whole alga that is, not just its chloroplasts—inside some invertebrates, like coral and giant clams. Coral reef bleaching was becoming a big issue, and these guys figured out that when coral polyps become stressed, their symbiotic algae flee. The polyps then no longer have access to the carbohydrates and amino acids that the algae produce, and they bleach and die. The researchers got enormously famous for this work, and gave up on the sea slugs forever. The green sea slug business sat fallow for years until I backed into it.”

  The first thing Pierce and his graduate students had to do was to tame the mucus. Running through a list of compounds developed to treat cystic fibrosis, a human disease that causes thick, sticky mucus to build up in a sufferer’s lungs and digestive tract, they found one that also keeps the molecules of slug mucus from clumping together. They were then able to homogenize the sea slugs, centrifuge the goop, and isolate their chloroplasts.

  The bulk of a chloroplast is filled with thylakoids, which look like stacks of flattened, green sacs, and stroma, a gel-like substance that surrounds them.

  A chloroplast.

  The outer surface of the thylakoids is where the first phase, the light-dependent phase, of photosynthesis begins. Attached to the thylakoid membrane are pigments—most prominently, chlorophyll—that act like antennae tuned to receive the energy of visible light. (Chlorophyll absorbs red and blue wave frequencies and reflects those in the green range, which is why leaves look green to us.) During daylight hours, when photons hit these pigment antennae, they bang off a flow of electrons. Some of those electrons wind up in the chemical bonds of ATP, that temporary energy storage molecule. Others split water into its hydrogen and oxygen components. Some of that hydrogen is incorporated in NADPH, another energy storage compound. As for the oxygen, some is used by plant cells, just as it is in animal cells, to fuel the breakdown of stored sugars. Most slips off into the atmosphere, where, to our great good fortune, we can inhale it.

  Plant cells use ATP and NADPH for their immediate needs, like moving chemicals across cell membranes. But some of these molecules are used in the second half of photosynthesis, the light-independent phase, which takes place in the stroma. In this phase, which can occur both during the day and night, the energy molecules are used to combine hydrogen with carbon dioxide to make glucose, or C6H12O6. (You may recall this from high school biology as the Calvin cycle. I didn’t.)

  Some of the glucose is converted into another sugar, the less chemically reactive sucrose, or C12H22O11. Sucrose travels through the phloem headed for use in creating new cell components or for storage in, say, sugarcane stalks or fruit. String a hundred or so glucose molecules together and you have starch, which is stored in parenchyma throughout the plant—but most dramatically in tubers like potatoes and carrots—and is easily oxidized when needed for plant growth. In some plants, sucrose is converted into fatty acids
and stored in oil-rich seeds, including those of the sunflower, rapeseed, and peanut. Weave several hundred to thousands of glucose molecules together, and you have cellulose, the tough matter of cell walls and what the cereal box notes as “dietary fiber.”

  Animals can’t photosynthesize. We are missing two major components of the process. One, we have no chloroplasts. Two, we lack the genes that instruct the creation of the enzymes—most notably RuBisCO—that catalyze the chemistry of photosynthesis. Consider photosynthesis to be a pinball machine. Even if we animals somehow acquired the cabinet (the chloroplast with its stroma and thylakoids), we would still lack the flippers, bumpers, and springs (the enzymes) as well as any written instructions (the DNA) to play a game.

  Which brings us to the question: How do the chloroplasts in the cells of green sea slugs manage to function? First, Pierce tells me, his sea slugs must eat at least one meal of algae before they can photosynthesize. The algal species they prefer, Vaucheria litorea, looks like translucent green straws. A newly hatched, brown slug crawls up to a straw and, sipping on it like a child drinking from a juice box, siphons the Vaucheria’s chloroplasts into its mouth. The chloroplasts then take up residence in the slug’s gut cells, which form a network of ducts throughout its body. One meal of Vaucheria and the slug turns green and never needs to eat anything—except sunshine—again.

  If slugs lived for only a few days or even a few weeks, Pierce says, those ingested chloroplasts might survive long enough to capture all the solar energy they need. But the slugs live nine months or longer, and the light-harvesting apparatus of the thylakoids, constantly bombarded by photons and stripped of electrons, wear out and need regular repair. Chloroplasts in plants and algae like Vaucheria have some of their own DNA, DNA that is different than the DNA of the plant or alga itself. The chloroplast’s DNA directs the production of some of the chloroplast proteins. But a larger portion of the DNA that instructs chloroplast operation, including the repair instructions, resides in the DNA of the plant or alga. This is why chloroplasts cannot live outside a cell. They rely on their own DNA and the DNA of the organism in which they live. So, how do the thylakoids in sea slug chloroplasts manage to get repaired?

  One possibility Pierce had to consider was that no repair was needed. Maybe sea slugs don’t repair worn-out chloroplasts, but instead hold some chloroplasts in reserve to fill in for losses. To test this hypothesis, he set them swimming in a bath of radioactive amino acids, which are the building blocks of proteins and enzymes. After several weeks, he found the sea slugs were “hot.” The slugs had manufactured new spares from the radioactive amino acids using instructions encoded in its own DNA. Somehow, algal DNA has been incorporated into the slug genome. In fact, Pierce and his students ultimately proved that the slugs can manufacture the entire sixteen-enzyme pathway to make chlorophyll. He also demonstrated that unhatched sea slugs contain the genes for photosynthesis, which means the slugs’ ability to make chloroplast proteins is inherited. All they need is a first meal of chloroplasts to jumpstart the process.

  No one had ever demonstrated a transfer of functional, heritable genes between taxonomic kingdoms—Plantae and Animalia, in this case—before. Pierce and his team have painstakingly identified the specific algal genes permanently embedded in the slug genome, a process that Pierce says has been “like trying to find a needle in a haystack without knowing what the needle looks like.” Pierce’s work has been reproduced by other researchers and there is little question that, however improbable it may seem, his sea slugs are aquatic chimeras between plant and animal. It’s as if someone proved the existence of the borametz.

  It occurs to me that photosynthesizing might be a useful trait for humans as well as sea slugs. I have very pale skin, a liability in our era of ozone depletion, and I sunburn quickly. What if I could incorporate algal DNA into my genome and manufacture chlorophyll in my epidermis? I could bask in the sun without burning. Better yet, I wouldn’t have to bother with cooking dinner. Or, I could store those sugars, burn them on cold winter nights, and save on the gas bill. I would give “going green” new meaning. I pose the question to Pierce.

  “Dream on.” He laughs.

  Later, I look further into the question. Enough solar energy falls on the Earth in one hour that, if just we had the technology to capture it, we could fuel all human energy needs for a year. Plants turn only a tiny portion of that energy into biomass, in part because chlorophyll and other plant pigments trap light only within a narrow bandwidth of 400 to 700 nanometers. (More than half of solar energy arrives in the infrared and ultraviolet range and is useless to plants.) And although photosynthesis is a spectacular feat and powers almost all life on the planet, it is inefficient. Leaves convert only about 5 percent of the solar energy in its bandwidth into stored energy. Photosynthesis therefore means using a large, horizontal light-capturing surface area—that is, lots of leaves—to power a small volume of living cells. What about trees? you may wonder. Don’t be misled by their size. Only about 1 percent of their vast bulk is living, energy-consuming cells.

  I compared myself to a magnificent old oak down the street. Estimating that the tree has about a fifty-foot-wide canopy and using a formula suggested by Dr. Kim Coder of the University of Georgia, I estimate that the tree has about eight thousand square feet of leaves—nearly a fifth of an acre—to power its living cells. If I were to arrange for an algal transfer into my epidermis, I would have less than twenty square feet of photosynthetic skin, of which only half could I expose to the sun at any one time. On the other hand, I am composed of a relatively substantial volume of mostly living and hardworking cells, both my own and billions of resident microbes. If a green me sunbathed continuously between 9:00 A.M. and 3:00 P.M., I would collect about ninety kilocalories. Even given my sedentary existence, I figure I burn some 1,400 kilocalories a day. The bottom line is that for me to survive by simply soaking up rays I’d need a day with ninety hours of sunlight.

  So, there’s a good reason that plants have no brains, gather their water and food without moving a muscle, and don’t go in for Kama Sutra. If any animal could live by photosynthesis, it is not surprising that an inch-long, translucent slug—whose common name derives from sluggard—is the one to manage the trick.

  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. Thes
e 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 doubled and redoubled, all the while splitting water, fixing carbon dioxide into cell membranes, and popping out pinpricks of oxygen into the water. The numbers of cyanobacteria became incomprehensibly large, but know this: A single bacterium that starts dividing when the sun rises at six o’clock in the morning can, under perfect conditions, become a population of more than 34 billion by the time the sun sets. As the hundreds of millions of years passed, cyanobacteria became so abundant that they formed slimy, floating sheets in the open oceans. In shallow waters off the barren continents, the mats piled up and, interleaved with thin layers of mud and dead bacteria, formed pillows and domes and great reeflike structures called stromatolites that in shallow waters rose above the surface. If you had been able to scan the horizon 2.5 billion years ago, anything you saw that wasn’t water or barren rock was stromatolites. In deeper waters, the mats accumulated in cones and columns as much as a hundred feet high.