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ALGAE. READ THE word and the bright green slime around an outdoor drain may leap to mind. Or maybe it’s the fuzz on the glass of a fish tank.

What you probably don’t think of is gasoline, diesel, and jet fuel. But the world’s petroleum reserves are made in large measure of dead algae, compressed on the ocean floor into crude oil millions of years ago.

Today, we’re burning that oil at the rate of 100 million barrels per day, chiefly in our cars, trucks, ships, and planes. The carbon dioxide we’re releasing is heating our atmosphere, threatening our well-being and even our lives.

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And it’s not just the algae that sank to the bottom of the ocean so many years ago that imperil our environment.

It’s the algae that float atop our waterways now.

All the sewage and fertilizer we send into our rivers, streams, and bays has created enormous algae blooms. Every summer, pea-soup algae scum chokes more ponds and lakes around the world. And dead zones, where no life at all survives in the water, grow larger by the year; this year, the Gulf of Mexico zone is expected to reach record size, as large as Massachusetts.

We’ve turned algae into killers. But we can change their future and ours. We can harness them to help save the environment we’ve been ravaging.

AFTER THE OPEC oil embargo of the mid-1970s caused gas shortages and price hikes, government-funded biologists and engineers across the country began to study an unlikely alternative: algae fuel.

Algae, the world’s first photosynthesizers, are full of carbon-based molecules, including oils. The researchers’ goal was to grow them in concrete ponds, harvest them, and either extract their oils or, with pressure and heat, liquify the entire biomass into crude oil.

They made progress, but only on an experimental scale. When the embargo lifted and prices fell, research funding dried up.

Interest reemerged about 2005 as the price of fossil oil began a spectacular rise from $20 to more than $150 per barrel. Entrepreneurs and venture capitalists, spurred by the belief that prices would remain high because we were at “peak oil” (the moment when world production would only decrease), leapt into the algae oil business.

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The environmental benefits were clear. Algae pull carbon dioxide out of the atmosphere because they need it to grow, and that makes them a nearly carbon-neutral source of fuel.

By 2012, algae fuel was powering US Navy planes and ships, UPS delivery trucks, and other vehicles, in demonstration projects. It was far too expensive, at roughly $8 per gallon, but everyone understood that the technologies involved were still at an embryonic stage.

A year later, though, with the advent of fracking, petroleum supplies went up and the price of fossil oil began a precipitous fall to $30 per barrel. Some algae oil companies managed to survive by pivoting to high-value products like cosmetics and cooking oil. But others went out of business. The industry died before its promise could truly be assessed.

It may be time, though, to pay attention to algae fuel again.

Why? For one, new technologies like CRISPR have the potential to improve algae’s ability to create and store oil, use sunlight more efficiently, and tolerate varied growing conditions. Scientists have identified at least 50 genes that affect oil production. Already, various genetic tweaks have substantially increased oil yields in the lab.

Other technologies have advanced in the last seven years, too. We are able to screen species and varieties, filter massive quantities of water, and liquify algae biomass much faster and less expensively.

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“Algae oil is all a matter of economics,” explains Phillip Savage, head of chemical engineering at Penn State. In other words, it’s not a question of whether it can be done, but whether it can be done at a price competitive with fossil oil.

Electric vehicles may obviate the need for an environmentally friendly liquid fuel for cars and trucks (if the electricity they use is generated by renewable sources). But airliners will not be flying on electric batteries anytime soon. And if the aviation sector were a country, it would be the world’s seventh-largest producer of carbon dioxide emissions. Supplementing or replacing jet fuel alone with algae fuel would be valuable.

If we actively pursued competitively priced algae oil, how soon could we get there? Stephen Mayfield, codirector of the California Center for Algae Biotechnology and professor at the University of California, San Diego, told me it would take 10 years and $10 billion.

Sound like too much time and money? Let’s assume it takes twice as long and twice the funds. That still looks like a good investment, measured against the broader cost of climate change, which is projected to be between $54 trillion (according to the UN Governmental Panel on Climate Change) and $69 trillion (according to Moody’s Analytics) by 2100.

And fuel is just one way that algae may help save the planet.

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Plastics are made chiefly from natural gas, which is methane, a powerful greenhouse gas. Ultimately, plastics degrade and release the gas into the atmosphere. Algix, a Mississippi company, is now making plastics from algae — from pond scum, to be precise. Last year, the company turned 12 million pounds of slime into shoe soles; many other products, from flooring to backpacks, are possible.

Seaweeds, known as macroalgae, can also make a difference in fighting climate change.

About 10 years ago, two scientists, Robert Kinley and Rocky de Nys at James Cook University in Australia, discovered that adding a soupçon of a seaweed called Asparagopsis taxiformis — a lovely pink, fernlike seaweed — to sheep’s feed reduces their belches by 85 percent. Recently, researchers at the University of California, Davis, proved that the seaweed works in cows, too.

And belching matters.

The methane burps of our 1.5 billion sheep, cows, and goats account for about 15 percent of greenhouse gas emissions.

It will take a lot of Asparagopsis to supplement livestock feed. But fishermen in East Asia have long farmed seaweeds on a large scale, growing them on floating nets for food and on lines stretched in shallow waters for carrageenan, the thickener and emulsifier found in everything from shampoo to chocolate milk to toothpaste. There’s every reason to believe we can grow this gut-friendly seaweed commercially.

SOME ALGAE CAN double every two hours, which means a one-foot square of algae can cover six football fields in two days. That astonishing fertility creates noxious blooms, but could also help save our climate.

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Since algae draw down carbon dioxide from the atmosphere, they serve as important “carbon sinks,” even more powerful than our largest forests.

The Southern Ocean, also known as the Antarctic Ocean, has markedly fewer algae than other oceans, due to its low iron content. Some scientists have proposed spreading iron dust on the Southern Ocean to trigger large algae blooms — a massive geoengineering scheme to capture CO2 and send it to the ocean floor when the algae sink.

A handful of experiments have demonstrated that iron seeding does produce more algae, but no one knows if induced blooms would harm the ocean’s ecology. And no one is sure how much carbon actually makes it to the bottom.

Deep history, though, gives us some reason for hope.

Forty-nine million years ago, CO2 levels were 100 times greater than they are today. The Arctic Ocean, nearly enclosed by the continents, was as warm as bath water. On its northern shores ancestors of hippos and crocodiles trundled along under palm trees.

And during the long summer days, the ocean was covered shore to shore in a thick green carpet of azolla, a tiny fern that harbors symbiotic algae. Every winter, when the daylit hours were few, the entire carpet of azolla died and sank to the sea floor. Each spring a new crop took over the surface.

This Azolla Event, as it is called, lasted nearly a million years. By the end, the fern/algae hybrids had sucked out and sequestered 80 percent of the ambient CO2, and cooled the atmosphere so much that the ice caps returned.

There is no single, quick fix to our intertwined climate problems, but algae can be an important tool. And although it’s no solace, we can say this: If we fail in our efforts, then some day, long after our species has gone extinct, algae may slowly clean up the mess we’ve left behind.


Ruth Kassinger is the author of “Slime: How Algae Created Us, Plague Us, and Just Might Save Us.”