Before the harvest alarm on the seabed farm could sound, Sujin was already zipping up the waterproof seal of her isolation suit. A notice flashed on the screen inside her helmet: the water-temperature sensor had dropped 3 degrees. Depth, 340 meters. Sector 7 of the Jinhae Bay Seabed Circulation Farm. The water pressure bore down on the shoulders of her suit. The compressed air from the regulator was cold. Sujin pushed off from the submersible's platform and set down onto the seabed floor. Sediment stirred up beneath her boots. Her headlamp's beam cut through the murky water only 2 meters ahead. Beyond that was a wall of water no light could reach. Sujin walked slowly, thrusting her sampling rod out in front of her. The boots sank into the sediment, and a heavy resistance dragged at each step.
The floor of Sector 7 was carpeted in mycelium—spread across the seabed soil like a thin white membrane. A circulatory structure: it broke down the organic waste piped down from the surface and the suspended matter carried in on the currents, then delivered the products of that decomposition to the seaweed of the seabed farm. Without this mycelium, the farm did not function. Discarded food from the surface, sewage-treatment residue, slaughterhouse waste, even industrial organic sludge—all of it came down to the seabed through the pipes. The pipe's diameter was 1.2 meters. The concrete conduit ran 3 kilometers from the shore to the seabed. The mycelium broke it all down, converting the nitrogen and phosphorus into forms the seaweed could absorb. The seaweed released oxygen and fixed carbon. The volume of organic waste a single Seabed Circulation Farm processed came to 40 tons a day. In Jinhae Bay alone there were 12 such farms. Counted across the entire southern coast, more than 40.
Sujin drove the sampling rod into the surface of Sector 7's mycelium. The sensor measured the mycelial density. A number came up on the screen. 62,000 strands per cubic centimeter. Eighteen thousand higher than last week. Sujin pulled the rod out and drove it in again 5 meters over. 71,000. Higher still. She tucked the rod into her belt and pressed the comm button on her helmet.
"Control. Sector 7 density between 60 and 70 thousand. Exceeding the 40,000 limit."
Two seconds later, Lee Jae-ho's voice came from control.
"60 thousand? It was 44,000 a week ago."
Sujin answered.
"The sediment reeks of organic matter. Feels like the input volume's gone up on the pipe end."
Lee Jae-ho went quiet for a moment. Over the comm she could hear his breathing, threaded with static. Then he said,
"Come up. I'm calling a meeting."
Sujin climbed into the submersible and began her ascent. As the depth lessened, the darkness beyond her headlamp thinned. Past 200 meters, a faint blue light filtered down from above. 100 meters. 50 meters. Near the surface, sunlight pierced through the water. When the submersible broke the surface, the wind of Jinhae Bay struck her wet helmet. A flock of gulls flew low over the water. They were pecking at something that had floated up to the surface. The air of August 2071. Humid and hot, the smell of the sea mixed with the smell of concrete. Sujin took off her helmet and climbed up onto the concrete of the pier. The management building stood 200 meters from the pier. Three prefabricated structures stood in a row. Sea wind blew between them and stirred Sujin's wet hair. Solar panels lay across the roofs.
Lee Jae-ho was already in the meeting room. A large screen on the wall displayed a seabed map of Jinhae Bay. The mycelial distribution was marked in colors. Sector 7 was a deep red. Lee Jae-ho pointed at the screen.
"Sector 7 isn't the only problem. The density's climbing in Sectors 5 and 6 too."
Sujin sat down, still in her wet isolation suit. Water spread across the chair. She asked,
"Did the pipe input go up?"
Lee Jae-ho nodded.
"Waste from the Nam-gu, Busan treatment plant got added this month. The daily input rose from 40 tons to 58."
Sujin looked at him.
"Who decided that."
Lee Jae-ho leaned back in his chair.
"The Circulation Committee. The Nam-gu plant went over capacity, so they rerouted it to the seabed."
The meeting room door opened and one more person came in. It was Supervisor Choi, the inspector sent by the Circulation Committee. He set his briefcase on the table and looked at Lee Jae-ho and Sujin.
"I've received the density report."
Sujin said,
"Sector 7 is between 60 and 70 thousand. At this rate of increase, it'll spread as far as Sector 8 within 2 weeks."
Supervisor Choi took a tablet from his briefcase.
"The Circulation Committee's position is this. A rising mycelial density also means the decomposition efficiency is going up. Look at the seaweed harvest data—Sector 7's harvest is up 23 percent over the previous month."
Sujin shook her head.
"When the density rises, the mycelium penetrates all the way into the seaweed's roots. It stops being symbiosis and turns into parasitism. Root discoloration has already shown up in 15 percent of Sector 7's seaweed."
Supervisor Choi set the tablet down on the table.
"We can't shut the farms down. Twelve farms in Jinhae Bay are processing the organic waste from Busan, Changwon, and Geoje. If even one goes offline, the surface treatment plants can't cope."
Lee Jae-ho asked,
"Can't we just cut the input back to 40 tons again?"
Supervisor Choi's expression hardened.
"The Nam-gu plant has already begun decommissioning. There's no turning back."
The conference room fell quiet. Air from the vent stirred the still-damp sleeve of Sujin's containment suit. Sujin asked,
"What if we partially remove the mycelium? Bring the density down by force."
Supervisor Choi crossed her arms and looked at Sujin.
"And where do we send what we remove? The mycelium is organic matter itself. Put it in another farm and the density there climbs; bring it up to the surface and there's nowhere to process it."
Sujin opened her mouth, then closed it. She had no answer.
That evening Sujin sat on the breakwater at the pier. The surface of Jinhae Bay was stained red in the evening light. The wind carried the smell of salt. Her containment suit still wasn't fully dry, and below the knee it was cold against her. Sujin had worked in Jinhae Bay for 6 years. She had majored in marine biology, earned her seabed-farm manager's certification, and come straight here. When she first arrived there had been 3 seabed farms. Now there were 12. Each time a waste-treatment facility on land shut its doors, another seabed farm went in. The mycelium was more efficient than the surface facilities, and it cost a fifth as much. In her first year here, Sujin had seen a goat die near one of the pipes. In two days there was nothing left but bone. The mycelium had decomposed the carcass. Back then it was something that happened only on the seabed floor. Now it was happening in the roots of the seaweed.
Before dawn the next day, Sujin took the submersible back down to Sector 7. When she switched on her headlamp, the mycelium on the floor had grown thicker than the night before. It was no longer a thin film but swollen, like cotton wool. Sujin drove in the sampling rod. 76,000. It had risen 5,000 in a single day. She pulled the rod out and went to check the seaweed stalks. Sector 7's seaweed grew on lines suspended from a steel frame. She followed a line down and looked at the roots. They had turned white. The mycelium had penetrated into the root tissue. Sujin took a root carefully in her gloved hand and felt it. It was soft. When she pressed with a finger, the tissue tore. A healthy seaweed root should be firm and springy. This one had already begun to break down.
Sujin took her hand from the root and looked up. The seaweed stalks stretched dozens of meters overhead. Within the reach of the headlamp's beam, the lower third of the stalks had discolored to brown. The upper part was still green. The mycelium was climbing from the bottom up. Sujin switched on her comm.
"Jae-ho. The bottom third of the Sector 7 seaweed has gone brown. Root decomposition is underway."
Lee Jae-ho's voice came through.
"Is there any harvestable part left?"
Sujin looked at the stalk again.
"The upper two-thirds are still fine. But at this rate it'll reach the halfway point within a week."
Lee Jae-ho was silent for a moment, then asked,
"Can you move the harvest up?"
Sujin answered,
"It's possible. But once we harvest, the seaweed is gone, and so is what the mycelium is decomposing. It'll go looking for the next organic matter and spread."
When Sujin came back up to the surface and stepped into the management building, Supervisor Choi was there. Two tablets lay on the conference-room table. Supervisor Choi said,
"The Circulation Committee has issued an emergency decision. Early harvest of Sectors 7 and 5 is approved. After harvest, we transplant new seaweed onto the empty frames and wait for the mycelium density to fall on its own."
Sujin shook her head.
"It won't fall on its own. Organic matter keeps coming down the pipes—there's no reason for the density to drop."
Supervisor Choi looked at her.
"You know we can't block the pipes, don't you?"
Sujin asked,
"Why not?"
Supervisor Choi swiped to a new screen on the tablet. Busan Waste-Treatment Status.
"Five of the 8 surface treatment facilities are closed. The remaining 3 are already over capacity. If the seabed farms stop, waste piles up across Busan. Within a week they'd declare a sanitation emergency."
Lee Jae-ho gripped the back of a chair and said,
"So you're telling us to wait until the mycelium has eaten all the seaweed? If the seaweed's gone, the farm's carbon-fixing function goes with it."
Supervisor Choi looked at him.
"The Committee knows that. They're reviewing alternatives."
Sujin asked,
"What alternatives?"
Supervisor Choi paused a moment, then answered,
"Introducing a new mycelium strain. One that decomposes more slowly than the existing mycelium and has a built-in density ceiling. The Korea Institute of Ocean Science is developing it."
Sujin asked,
"How long until it's ready?"
Supervisor Choi answered,
"Six months to a year."
Sujin leaned back in her chair and said,
"The Sector 7 seaweed could be wiped out within a month."
That night Sujin lay in the narrow room of her dormitory in the management building. She could hear water running through the pipes in the ceiling. Beyond the breakwater, the sound of the waves climbed up the wall. Sujin opened the portable terminal beside her pillow. She searched for papers on mycelium density limits. There was a report from the ocean research institute, published in 2068. Title: 'Patterns of Biological Penetration in High-Density Mycelium Environments.' According to the report, once mycelium density passed 80,000 per cubic centimeter, the hyphae began to pierce and penetrate the cell membranes of the organisms around them. Not just seaweed—crustaceans, mollusks, fish, even the tissue of whale carcasses had hyphae boring into them. 80,000. Sector 7's current density was 76,000. A gap of 4,000. At the rate it was climbing, 5,000 a day, it would cross the line by tomorrow.
Sujin switched off the terminal and looked at the ceiling. Six years ago, when she'd started this work, the Seabed Circulation Farm had been called the answer to the climate crisis. A complete cycle: waste broken down on the seabed, carbon fixed by seaweed, the harvested seaweed turned into feed and fertilizer. Sujin closed the terminal and rolled over. The water in the pipes wouldn't stop. Even now, organic matter would be sinking down the pipes to the seabed. And the mycelium would be feeding on it, swelling.
Two mornings later, coming up from Sector 7, Sujin noticed something stuck to the outer hull of the submersible. Depth: 200 meters. Something like white thread was snarled around the submersible's steel landing legs. Hyphae. Sujin brought the sub to a halt and looked out. The hyphae had worked into the seams of the submersible's joints. Risen from the mycelium on the seabed floor, they had climbed the water's drifting organic particles and latched onto the sub. Sujin peeled the hyphae off with a gloved hand. They were sticky. The white thread clung to the glove and stretched.
When Sujin surfaced and brought the submersible in to the dock, Lee Jae-ho was waiting, leaning against a mooring post. He lowered the tablet in his hand and looked at her. His face was set.
"A fisherman in Sector 8 filed a report. Says he pulled up a white lump instead of seaweed in his net. A clump of mycelium."
Climbing out of the submersible, Sujin asked,
"Sector 8? It spread over from Sector 7?"
Lee Jae-ho nodded.
"Looks like it rode the current across. Sector 8 isn't our jurisdiction. That's the Tongyeong farm."
Unzipping her containment suit, Sujin said,
"Have you called Tongyeong?"
Lee Jae-ho shook his head.
"Not yet. We confirm it ourselves first, then report."
Sujin and Lee Jae-ho took the management building's boat out to the waters of Sector 8. White debris floated here and there on the surface. Up close, the pieces ranged from the size of a fingernail to the size of a plate. Fragments of mycelium. Sujin scooped one up with a sampling tool and looked at it. About the size of a palm. The surface was smooth, and when she pressed it, it gave like a sponge. Lee Jae-ho looked over her shoulder.
"This came up from the seabed?"
Sujin nodded.
"When the density climbs, the mycelium clumps together and floats up. It makes gas, so it turns buoyant."
Lee Jae-ho scanned the surface. The white debris stretched across tens of meters.
"If this gets into the fish farms, we're finished."
Sujin looked at him.
"It may already have."
That afternoon, an emergency meeting of the Circulation Committee was called—not in the management building, but in a video conference room at Busan City Hall. Five members' faces came up on the screen, Supervisor Choi among them. Sujin and Lee Jae-ho connected from the management building's meeting room. Sujin switched on the building's aging camera, hit share screen, and reported the data from Sectors 7 and 8. Hyphal density trends, the state of seaweed-root decomposition, the spread of the surface debris. One of the members asked,
"Can't the mycelium just be removed chemically?"
Sujin answered,
"Spray antifungals in bulk and the mycelium dies. But the farm's whole cycle dies with it. It's the mycelium that feeds the seaweed, after all."
Supervisor Choi said,
"And if we kill the mycelium, what happens to waste processing?"
Sujin answered,
"The organic matter coming down the pipes just piles up on the seabed. Burning through oxygen as it does. A dead zone forms on the seafloor, and fast."
The room fell quiet. On the screen, the members' faces had hardened. Supervisor Choi asked,
"So in the end, what are our options?"
Sujin said,
"Three. One: cut the input through the pipes. But with no surface processing plants, the waste piles up in the city. Two: remove the mycelium. The farm stops and the seabed is poisoned. Three: hold the status quo. The mycelium eats through the seaweed and, in the end, breaks into the fish farms."
One of the members said,
"None of those are things we can choose."
Sujin looked at the monitor.
"That's why I'm proposing a fourth."
Lee Jae-ho looked at Sujin. Sujin said,
"We harvest all the seaweed in Zone 7, then dismantle the frame. The mycelium stays behind on the empty seabed, but once we cut off its supply of organic matter, its density drops. The organic matter from the pipes gets distributed into Zones 5 and 6—but we harvest on a cycle, before it crosses the density threshold, so we starve the nutrients feeding back to the mycelium. Zone 7 lies fallow for 6 months."
Supervisor Choi asked,
"And during those 6 months Zone 7 is fallow, who makes up that zone's carbon fixation?"
Sujin answered,
"No one. For 6 months, 8 percent of the annual carbon fixation target goes unmet."
Supervisor Choi's mouth opened, then closed. On the screen, the other members looked at one another. One of them said,
"Missing the carbon target means a penalty in the emissions trading. That hits the Busan city budget."
Sujin looked at the member's face on the monitor.
"If the seaweed dies off completely, it isn't 8 percent that goes unmet. It's 100."
After the meeting, Sujin went out to the pier. The sun was sinking below the western horizon of Jinhae Bay. The ridgeline of Geoje Island was cut into a black silhouette. The water was stained orange. The concrete of the breakwater still held the day's heat, so when you sat down it warmed your back. Lee Jae-ho came over with two cups of coffee. He handed one to Sujin.
"Will the committee go for it?"
Sujin took the coffee and said,
"They'll accept dismantling Zone 7. The distributed input into the other zones—I don't know."
Lee Jae-ho took a sip. The paper cup shivered in the wind coming off the sea. He said,
"The harvested seaweed we can send to the feed plant, but the roots the mycelium's gotten into are useless. That's waste too."
Sujin wrapped both hands around her cup.
"It just goes round and round."
Lee Jae-ho set down his coffee and looked at the water.
"The sea processes what we throw away, and what the sea can't process we have to take back—but there's nowhere to take it."
Sujin didn't drink; she looked down into her cup. The orange light of Jinhae Bay was reflected on the surface of the coffee. Sujin said,
"I'm going back down to Zone 7 tomorrow. I have to check the seaweed's condition one last time and set the harvest schedule."
Lee Jae-ho nodded.
"Be careful. Once the density passes 80,000, it can get into the seams of the isolation suit."
Sujin drank the rest of her coffee in one gulp and crushed the paper cup. She stood.
"I know."
The next day, Sujin stood on the seabed of Zone 7. She drove in the sampling rod. 81,000. Past the threshold she'd read about in a paper the night before. She pulled the rod out, tucked it at her waist, and walked toward the seaweed frame. The 10 meters took a long time, because she was walking over the thick mycelium covering the seabed. Half the stalks had gone brown. The mycelium had climbed from the roots to the middle of the stalks. Sujin touched the green part of a stalk with her hand. Smooth and cold. The part still alive. From this stalk, the seaweed to be harvested a month from now was supposed to grow. She took her hand off the stalk and looked down. Her headlamp lit the seabed floor. The mycelium had piled up thick, like sponge rather than cotton. With every step she sank to the ankle. When she pulled a sunken foot free, the mycelium clung and rose after it, wrapping the boot. Sujin lifted her foot. The mycelium clung and rose up the boot. White threads stretched between the boot and the floor, then snapped.
Sujin gripped the frame's steel column, stood, and looked up. In the darkness at a depth of 340 meters, the green stalks of seaweed reached upward. These stalks held carbon out of the sea, made oxygen, fed the fish, and gave people back feed and fertilizer. At the bottom of that cycle was the mycelium. What was being crushed under Sujin's boots was the same mycelium. She took her hand off the cold steel column of the frame and switched on the comms.
"Lee Jae-ho. It's 81,000. We have to move the harvest up to this week."
Lee Jae-ho's voice rang in the helmet speaker.
"Understood. I'll assemble the harvest team."
Sujin cut the comms. She let out a breath inside her helmet and went back to the submersible. In every footprint stamped on the seabed floor, mycelium followed up like white thread. She settled into the cockpit and pressed the ascent button. The craft began to rise slowly off the seabed. Beyond the window, Zone 7's seaweed drew away below her. Brown was spreading between the green stalks. Beneath that, the mycelium on the seabed floor shone white in the headlamp's light. Sujin tightened her grip on the control stick and climbed toward the surface. The depth reading fell. 340, 300, 250, 200. Beyond the glass the color of the water shifted from black to deep indigo, from indigo to blue. Sunlight was coming down from above.
