Welcome back: As the United Nations Ocean Conference draws to a close in France this week, we have three oceans-themed stories for you.

On our seas and in our lakes, harmful algal blooms occur when algae grow out of control, often triggered by nutrients entering the water, including fertilizer, sewage and industrial runoff. As the world warms up, some HABs are only going to become more common and the disruptions they cause to ecosystems and economies will become increasingly hard to stop. Now one company believes it may have an answer, not only to the problem of toxic algae but also to locking away carbon for thousands of years.

Meanwhile, a tech company in South Korea is taking starfish that are caught as bycatch by local fishermern, and usually incinerated, and using the carcasses to make everything from de-icing products to fertilizers to skincare products. See below for more on this unusual effort to upscale. 

And finally, water scarcity is projected to become much more acute in the coming decades, owing to more extreme weather patterns, the decimation of the world’s aquifers, saltwater incursion, and growing urban populations. This threatens humanity at a fundamental level—not just because we need water to drink, but because without it there’s no food or manufacturing, and precious little electricity. Now deep-sea desalination is on the cusp of providing a source of clean water from the Caribbean to the Emirates.

Read on for more on these stories and other sustainability news.

An Israeli startup is rethinking how to tackle two of the planet’s most stubborn environmental threats—toxic algae that can lead to dead zones in oceans and rising carbon levels, WSJ Pro Sustainable Business writes.

BlueGreen Water Technologies is developing a novel approach to neutralizing harmful algal blooms, those vivid slicks of toxic plant life that can choke sea and fresh water alike. In the process, it is also offering a potentially scalable new tool in the global effort to fight climate change.

BlueGreen’s solution is a re-engineered formulation of hydrogen peroxide that floats on the water’s surface and releases slowly—just enough to cause stress to the harmful algae but not enough to harm anything else. The aim isn’t to kill the algae directly, but to trick it into activating a natural self-destruct mechanism known as programmed cell death.

BlueGreen says the resulting sinking biomass is an effective way of locking carbon into sea- or lake-bed sediments for thousands of years. This idea forms the basis of its carbon credit offerings. BlueGreen employs satellite imaging, water sampling and sediment coring to quantify how much biomass it removes. It uses this data to sell removal credits.

A South Korean tech company is using starfish carcasses to make deicing products that it markets as being less harmful to the environment and to roads. The products are being rolled out in North America, WSJ Pro Sustainable Business's Clara Hudson writes.

The Northern Pacific seastar is abundant in Korean waters and collected by fishermen as bycatch. The starfish are usually incinerated, but Korean company STAR’s Tech is taking the carcasses before they are burned to turn into its unusual range of products: mainly de-icing powders and liquids that don't damage soil or plants—in contrast to the typical salt or calcium chloride products available on the market.

The company uses starfish bones for the de-icer, and much of the rest is used to make a fertilizer. The business said it processes about 300 to 400 metric tons of starfish carcasses annually, collected mainly from the west coast of Korea.

“We wanted to fully upcycle and leverage everything in the starfish,” said Hando Choi, STAR's Tech’s president.

A radical new kind of desalination technology is on the cusp of helping to slake the world’s thirst: plants on the ocean floor.

First proposed in the early 1960s, this deep-sea process would benefit from both the crushing water pressure and relatively pure seawater more than 1,000 feet down. It has been unworkable until now. Only the recent commercialization of deep-sea robots from the oil-and-gas industry and advanced reverse-osmosis filters now standard in terrestrial desalination—make it viable, the WSJ's Christopher Mims writes.

The principle is easy to grasp: Instead of expending huge amounts of energy to pump seawater onto land, and then pressurize it inside a plant, why not take advantage of the ocean’s extreme natural pressure? At depth, seawater naturally wants to cross a desalination membrane, so long as the fresh water on the other side of it is being pumped to the surface. The result is a net energy savings of up to 40%.

Oslo-based Flocean, Netherlands-based Waterise and Bay Area-based OceanWell are among companies developing the idea.

Tell me what you think: Send me your feedback and suggestions at perry.cleveland-peck@wsj.com or reply to any newsletter. If you were forwarded this newsletter, you can sign up here.

• EPA to repeal air-pollution standards on power plants. (WSJ)
   
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• California, 10 other states sue to block Trump from killing 2035 EV rules. (Reuters)
   
• World’s oceans remain near record temperatures as CO₂ levels rise. (FT)
   
• Submarines are hard to detect. Climate change may make it harder. (NYT)

WSJ Pro Sustainable Business gives you an inside look at how companies are tackling sustainability. Send comments to bureau chief Perry Cleveland-Peck at perry.cleveland-peck@wsj.com and reporters Clara Hudson at clara.hudson@wsj.com and Yusuf Khan at yusuf.khan@wsj.com. Follow us on LinkedIn at wsjperry, clara-hudson and yusuf_khan.