Ocean acidification could weaken diatoms’ glass houses

Ocean acidification doesn’t just erode calcium carbonate shells. It can also slow the rate at which tiny algae called diatoms build their beautiful, intricate silica cell walls. Thinner walls mean lighter diatoms — making the algae less able to transport carbon to the deep ocean, scientists report August 26 in Nature Climate Change.

Vast diatom blooms act as a biological pump in the ocean, adding oxygen to the atmosphere and drawing carbon dioxide out of it. To protect themselves from predators, diatoms also build houses of glass — strong cell walls of silica. When diatoms die, the walls act as ballast, causing the creatures to sink and sequester carbon from the atmosphere.

But as oceans absorb atmospheric carbon dioxide (SN: 6/8/19, p. 24), their waters become more acidic. If greenhouse gas emissions continue on their current track, the average ocean pH will drop from about 8.1 to about 7.8 by 2100, says marine biologist Katherina Petrou of the University of Technology Sydney in Australia.

What that would mean for diatoms isn’t clear. Previous research suggests more CO₂ could increase diatoms’ productivity, helping the algae to grow faster. But Petrou and her colleagues suspected that a lower pH might also affect how well the algae build their glass houses.

The team filled six 650-liter tanks with Antarctic seawater containing about 35 diatom species. Each tank’s seawater was saturated with different amounts of CO₂, resulting in pH values ranging from 8.1 to 7.45.

After 12 days, diatoms in the most acidic water were making 60 percent less new silica compared with those in seawater with a pH of 8.1. And, in that heavily acidic tank, larger, heavier species went from making up about 40 percent of the community to only 3 percent.

But even at pH as high as 7.84, silica production shrank, the team found. That’s “above the pH levels expected by 2100,” Petrou says. “Our study has exposed a new climate change threat to the ecosystem.”