The following is an excerpt of Slime: How Algae Created Us, Plague Us, and Just Might Save Us by Ruth Kassinger.

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Today sushi has become a common menu item everywhere from upscale restaurants to food courts and college dining halls. People far beyond East Asia have become enamored of the nori-wrapped delicacies (or indelicacies, as traditionalists, dismayed by sushi with bacon, curry, mango, cream cheese, and other unconventional ingredients, might say). In the U.S., grocery store sales of sushi are increasing at an annual rate of 13% and sales of nori snacks are growing at more than 3% per year. Consequently, Porphyra farming and nori production are now multibillion-dollar businesses. How, I wondered, are fisherman-farmers keeping up with demand? How has a labor-intensive, family-based occupation managed to keep up in the 21st century?

In search of answers, I find myself, one February morning, being driven through the countryside of Jeolla province in southwestern South Korea. I am about as far from the capital of Seoul as one can travel in the country, in an area that few foreign tourists visit. With two thousand islands, nearly four thousand miles of coastline, and countless sheltered bays, Jeolla is home to South Korea’s seaweed farms.

Today, the sun is brilliant, but the wind is whippy and the temperature well below freezing. The rice paddies on either side of the road are fallow and mostly drained, but remnants of water have frozen into glittering ribbons in the channels between what were once mounded rows of plants. In other fields, miniature by the standards of American heartland agriculture, I see rows of the slender, flaring leaves of spring onions or pale green cabbages the size of basketballs. Farmhouses are scattered across the landscape; their roofs, either bright blue or fiery orange, are gently upswept at their very ends. This traditional architecture contrasts with the utilitarian white hoop houses—nurseries for rice seedlings—alongside most paddies.

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Dr. Eun Kyoung Hwang, my host and a senior researcher at the Seaweed Research Center of the National Fisheries and Research Development Institute in Mokpo, South Korea, is driving. She is in her mid-forties with short hair, rimless glasses, and a brilliant smile. Her English is very good, but I sense the effort it takes for her to summon up words in a language she doesn’t often use. She and two of her female colleagues are accompanying me on a visit to Aphaedo, one of the largest of the province’s islands, to visit a Porphyra farm and a nori-processing plant. Dr. Park, who is in the back seat with me, has opened a thermos of a fragrant hot drink, which she explains is made from Asian pear, spring onions, jujube, and ginger. I take a cup while Dr. Lee passes around sweet cookies dusted to a light green with seaweed.

I am not surprised by seaweed cookies; I have been eating seaweed in all forms since I arrived in South Korea a few days ago. My favorite dish, which Dr. Hwang and her colleagues introduced me to at a restaurant the night before, is a soup made with maesaengi, a seaweed that grows in the winter in Jeolla province. I was leery at first: The individual seaweeds looked alarmingly like silky green hairballs and they floated in the clear broth with pillowy, white blobs. But the blobs turned out to be dissolving rice cakes and plump, pale oysters, which I adore. The soup was delicious, with a rich, slightly nutty taste.

Now, as we drive along, Dr. Hwang asks me a question that I suspect she only just feels comfortable asking me, as it implies I may have made a mistake. Why, she puzzles, did I choose to come to South Korea instead of Japan to research nori? Everyone interested in sushi goes to Japan, where the tradition of making it is far older.

Indeed, from the 1960s into the 1980s, if you lived outside Southeast Asia and ordered sushi, you would have eaten Japanese nori. At the time, Japan was the only source of exported nori in the world. In the 1970s, however, South Koreans began to farm Porphyra in the wide, shallow waters off their country’s southwest coast. Growing conditions were good, and production increased every year. Around 1990, the Chinese entered the business, too, forming joint ventures with the Japanese, who contributed technology while the Chinese provided the labor and coastal locations. By 2013, the Chinese produced 1.14 million tons of nori, the Koreans 405,000 tons, and the Japanese only 316,000 tons.

In the U.S., grocery store sales of sushi are increasing at an annual rate of 13% and sales of nori snacks are growing at more than 3% per year. Consequently, Porphyra farming and nori production are now multibillion-dollar businesses.

Japanese production has stagnated in recent years and is unlikely to increase. Rising temperatures in offshore waters are shortening the growing season and encouraging parasites. Industrial pollution has limited the sites where the crop can be safely grown. Also relevant, the life of a seaweed farmer—motoring out in a small boat in the dark early-morning hours of winter to haul wet seaweed out of icy water—holds few attractions for Japan’s younger generation, and the country admits few foreign workers who could fill the labor gap.

In contrast, the South Korean government established the Seaweed Research Center and made the development of the industry a priority. Korea now exports 10 billion sheets of nori—40% of the country’s production—each year. Japanese seaweed is of higher quality and is correspondingly more expensive; upscale sushi restaurants buy most of it. (The highest-quality nori is smooth and uniform, dark green to black, sounds like a finger snap when folded, and then seems to melt away in your mouth.) Chinese nori is more likely to be sold into the domestic market or to find its way into fish and animal feed.* South Korean nori, more likely to be dimpled and have some small holes, falls between the two in quality. Because almost all the nori on the shelves of American grocery stores has been harvested from South Korean waters, I tell Dr. Hwang, it seemed most appropriate to visit her country.

Besides, she was so enthusiastic when I contacted her, I couldn’t resist.

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The Shinan Sea Cooperative Association, our destination, has its facilities on the shore of a wide and shallow bay. The tide is hundreds of feet out when we arrive, leaving acres of seaweed nets suspended in midair on skinny poles. Small boats are also stranded; the shining, wet sand mirrors their colorful hulls. When we enter the lobby of the cooperative to meet with Mr. Bae Chang Nam, we immediately exchange our boots for slippers. Mr. Bae, tall and handsome in a Marlboro-man sort of way, ushers us into his office, where we sit on the sofa and exchange introductions. Tea is offered and accepted. With Dr. Hwang as translator, I learn that Mr. Bae inherited this business from his father, and that his son, the stocky, broad-browed young man who has quietly entered the room, will one day take over from him. Farming Porphyra is very much a family and a village affair in Korea. Mr. Bae’s operation, though, is unusual in that it also processes its crop. Most fishermen-farmers sell their seaweed to processors at auction, but Mr. Bae has a more entrepreneurial bent. By controlling the downstream processes, he captures the profits from all stages of nori production.

Dr. Hwang and her colleagues speak in Korean with Mr. Bae, and the light tone of pleasantries gives way to a more serious conversation. Then our host stands up, smiles, and it’s off with the slippers and back into boots. We head into the cold to trace the voyage of Porphyra into sheets of nori.

I learn that Mr. Bae inherited this business from his father, and that his son, the stocky, broad-browed young man who has quietly entered the room, will one day take over from him. Farming Porphyra is very much a family and a village affair in Korea.

The first stop on the tour is an aluminum shed that shelters a half-dozen large vats. In each vat is a dark suspension, a mixture of freshly harvested Porphyra blades, which look like scraps of satin ribbons, and water. A giant, rotating daddy longlegs of a machine sits on top of the vats, stirring the contents. After several rinses to remove sand, parasites, and epiphytic algae, the blades go through a mincing machine, where they become a dense, nearly black slurry. The slurry is then channeled indoors, where it flows into an industrial nori-making machine, a clanking, whirring, thumping blue monster dozens of feet long. Running through the heart of the monster is a 16-foot-wide gray conveyor belt that is striped with rows of yellow mats. At the far left of the belt, spigots spray neat black squares of Porphyra slurry onto a row of mats so only a frame of yellow is visible around each. Every two seconds, as the rows proceed down the length of the machine, steel beams running perpendicular to the belt briefly thump down, pressing the water out of the patches of slurry. At the far end of the room, the belt enters a blue industrial oven the size and shape of a storage pod, where it folds like an accordion and pauses as the slurry squares are thoroughly dried. Then, the square sheets of nori are flipped off their mats and onto another belt that carries them through a series of other rooms, where they are toasted, inspected, bundled, wrapped, and boxed for shipment. From slippery seaweed to elegant packages of tissue-paper-thin nori takes only a few hours.

On the way back to Mokpo, Dr. Hwang explains why she has been meeting with Mr. Bae. A community seaweed research group has been offering farmers in Mr. Bae’s area a new variety of Porphyra it discovered. At first, the new variety seemed like a winner, developing bigger and more robust blades and significantly more mass. But the texture and the taste of the nori, Mr. Bae decided, are inferior. Although he has chosen not to culture it, his neighbors do. Now he finds that the new variety is aggressive. As soon as he puts his seeded nets into the water, the new seaweed colonizes them and outcompetes the traditional variety he has built his company on.

It is a difficult problem, Dr. Hwang says. She needs to tread carefully with the local group to make sure she doesn’t disparage its efforts or appear highhanded. Mr. Bae’s dilemma highlights one of the peculiarities of sea farming: farmers have little control over their “soil.” In that way, sea farming is more like English farming in the days of the commons. Nonetheless, Dr. Hwang’s research group is exploring solutions. She hopes that if Mr. Bae sets out his nets earlier in the season, his Porphyra variety will be sufficiently established to fight off the introduced one, but there may be other ways he can change his cultivation methods to control his crop.

*Some seaweed snacks in U.S. grocery stores are Chinese in origin, but unless they are USDA-certified as organic, I pass them by. Because seaweeds easily take up and assimilate minerals and metals, they must be grown in clean waters. Coastal water pollution is a serious problem in China, and the regulations on where seaweed can be cultivated are lax.

Excerpted from SLIME: How Algae Created Us, and Just Might Save Us by Ruth Kassinger. Copyright © 2019 by Ruth Kassinger. Used by permission of Houghton Mifflin Harcourt. All rights reserved.

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