Climatide

Spike Walker, Wellcome Images

Diatom frustules - shells - seen under a polarizing light microscope. Diatoms may be responsible for half of all phytoplankton productivity.

If it’s not one thing, it’s another. Serious questions have been raised about the claim that warming seas have caused a precipitous decline in global phytoplankton abundance. But another study, this one published in Science, says that ocean acidification – the direct chemical result of carbon dioxide dissolving in ocean waters, sometimes called global warming’s evil twin – may threaten phytoplankton populations. Here’s the skinny.

WHAT WE KNOW

As atmospheric carbon dioxide dissolves in the ocean, water and carbon dioxide react to form carbonic acid. One major impact of this reaction is that it limits the amount of calcium bicarbonate available for organisms like oysters, corals, and phytoplankton to build their shells or skeletons (yes, some phytoplankton have shells). Increasing amounts of carbonic acid also shift the pH of the ocean toward greater acidity, and that, according to the new study, can reduce the availability of iron – a nutrient crucial to phytoplankton growth that is already limiting in many parts of the ocean.

HOW WE KNOW IT

The researchers grew four species of phytoplankton – Thalassiosira weissflogii (coastal), Thalassiosira oceanica (open ocean), Phaeodactylum tricornutum, and Emiliana huxleyi – in the lab, varying the pH and the amount and type of iron present. In every case, as the pH dropped (acidity increased) the planktors (that’s what you call individual plankton) took in less and less iron. That result, alone, could be explained one of two ways – a chemical change in the availability of iron, or a physiological change in the planktors’ ability to take in the iron. But the researchers also looked at what happened when pH was held steady and iron levels varied. Again, the result was unanimous – all of the planktors were able to take up more iron if given more iron, even under the most acidic conditions. That suggests that the effect of ocean acidification is to limit the availability of iron, not the physiological ability of plankton to get the iron.

The researchers point out that most iron in the ocean isn’t floating around on its own, it’s bound to organic molecules – long chains of carbon, hydrogen, oxygen, and nitrogen. In the lab, the researchers ran their experiments using a few different organic molecules that bind iron, called chelators. Changing the chelator dramatically changed the end result, completely erasing the effect of acidity on iron availability in some cases.

WHAT WE DON’T KNOW

Whether this will bear out in the real world – where there are a lot of different organic molecules floating around – is an open question. The researchers did try running their experiments with some ocean water samples; the results were undramatic, showing a very slight trend toward decreasing iron uptake that was only statistically significant at pH levels equivalent to carbon dioxide levels two and a half times the current level.

There’s also the fact that other climate change-related processes could counteract the impact of pH on iron availability. For example, most of the iron in the ocean comes from dust and climate change may increase the amount of dust that falls on the ocean.

WHAT IT MEANS

“We’re just at the beginning of research on ocean acidification,” said François Morel of Princeton University, the senior investigator in the team. “This is the first study published of its kind that looks at the uptake of a critical nutrient.”

It’s certainly an important area to consider, says Ken Buesseler of Woods Hole Oceanographic Institution: “The concept of changes to ocean productivity and ecosystems due to acidification is a very important one to consider. If half of the photosynthesis on the planet is in the ocean and if you reduce that because of acidification, that is a big deal.”

But at this point, that’s still a big IF.