Massive bloom or surface aggregation of Trichodesmium near New Caledonia in the tropical Pacific Ocean, as viewed by a satellite from space (long brownish streaks are the bloom; white objects are clouds). Blooms such as these can stretch for hundreds of kilometers across the surface of the tropical oceans. (Photo credit: NASA EarthWatch)

Human activities are causing plenty of obvious changes to the world’s oceans, from mounds of garbage accumulating on the seafloor to bleached coral reefs killed by rising water temperatures. But other equally dramatic changes are happening on a level too small for the human eye to see.

A new study in Nature Communications finds that rising levels of carbon dioxide in the ocean — the result of human greenhouse gas emissions — could have a major effect on microscopic marine organisms known as cyanobacteria, better known as blue-green algae. Cyanobacteria might be most famous for the huge algal blooms they occasionally undergo if there happens to be an excess of the nutrients they feed on in the water. These blooms, which often produce red, blue or green stains on the surface of the water, are known for the harmful toxins they produce, which often drive away or kill off any marine animals in the area. 

But despite these occasional incidents, cyanobacteria generally do much more good than harm. These tiny critters play an important role in the marine ecosystem by pulling nitrogen gas out of the water and converting it into a form that other organisms can use, a process known as “nitrogen fixation.”

Nitrogen is a vital nutrient for many other organisms, so cyanobacteria are critical to the marine food web — in fact, “they’re literally tiny fertilizer factories,” said Tatiana Rynearson, an associate professor of oceanography at the University of Rhode Island.  In many ways, cyanobacteria can be thought of as the cornerstone of the marine food ecosystem, since they supply usable nitrogen that is then consumed by other microorganisms, which are consumed by bigger organisms, and so on throughout the marine food web.

“What supports ultimately everything in the ocean, all the way from plankton up to whales, is that nitrogen coming in from nitrogen fixers,” says David Hutchins, a professor of biological sciences at the University of Southern California Dornsife College and the new study’s lead author. Previous studies have shown that rising levels of carbon dioxide can affect the rate at which cyanobacteria produce usable nitrogen. And since the entire marine food web depends on this nitrogen to survive, it’s important to understand how these bacteria will react as carbon levels in the ocean continue to rise.

Now, the new research shows that a type of cyanobacteria called Trichodesmium fixes nitrogen at much higher rates in a high-carbon environment. This reaction could have big implications for the marine food web because Trichodesmium is such a major player when it comes to nitrogen fixation. Found in tropical and subtropical parts of the ocean, it can be responsible for bringing in up to half the usable nitrogen in the areas where it lives, said Hutchins. 

The paper also shows for the first time that these effects are irreversible — even when the bacteria were restored to lower carbon levels, their nitrogen fixation rates did not return to normal.

Previous research on cyanobacteria in high-carbon environments have included only short-term studies, Hutchins said. This study is the first to examine the effects of elevated carbon dioxide levels over a long period of time, an important step to understanding the long-term effects of climate change on the ocean.

The researchers allowed the bacteria to grow at high levels of carbon — in fact, just the levels that scientists predict we’ll see in the ocean by the end of the century — for 4.5 years. “That’s long enough that we can actually begin to see evolution in action in these organisms and get an idea of if and how they may adapt to this changing chemistry,” Hutchins said.

Sure enough, the bacteria evolved and adapted to their new environment. The researchers observed that, in the high-carbon environment, the bacteria’s nitrogen fixation rates quickly rose and then remained at steady high levels. In fact, at the elevated levels, their nitrogen fixing rates were about 43 percent higher than bacteria kept at present-day carbon dioxide levels.

At first glance, this might sound like a good thing — higher nitrogen fixation rates mean more usable nitrogen for the marine ecosystem, which will allow marine organisms to flourish. But Hutchins cautioned that it might not actually work out this way. In order to fix nitrogen, cyanobacteria rely on other nutrients, such as phosphorus and iron, which tend to exist in much smaller quantities in the ocean. If they fix nitrogen too quickly, the bacteria might start to use up these other nutrients too quickly.

“The consequence could be Trichodesmium is not going to do well, and you might actually have less nitrogen,” Hutchins said. Without more research into the interactions between carbon dioxide levels and other nutrients’ availability on Trichodesmium’s productivity, it’s impossible to say what exactly this scenario would look like. But a marine environment with less usable nitrogen, rather than more, could cause the decline of many marine organisms.

This idea is made more alarming by the fact that the effects seem to be irreversible. After the 4.5 years were up, the researchers returned the bacteria to present-day carbon dioxide levels, assuming that their nitrogen fixation rates would also go back to normal. Instead, the bacteria continued to fix nitrogen at high rates — even two years later.

“No one has ever seen an organism do what this does, which is to get stuck — to evolve into a new space and then not be able to change back,” Hutchins said. The finding suggests that even if humans eventually curb our carbon emissions and carbon dioxide levels in the ocean start to decline, the bacteria may not return to their ordinary activity. For this species, at least, the effects of climate change appear to be permanent.

This finding “really highlights that evolutionary change is an important thing for us to consider when we’re trying to predict how ecosystems are going to respond to climate change,” said Rynearson, the Rhode Island researcher (who also was not affiliated with the study).

But there’s still more work to be done. First of all, just because Trichodesmium behaved this way doesn’t mean all species of cyanobacteria will. And there are plenty of other factors besides carbon dioxide levels that will affect marine ecosystems in the future, said Elena Litchman, a professor of aquatic ecology at Michigan State University, who was not involved with the paper.

“When we think about changing climate, it’s not just going to be high carbon dioxide, Litchman said. “It’s going to be warmer temperatures, it’s going to be perhaps lower nutrient concentrations.” Similar studies in the future should also adjust some of these factors to see how the cyanobacteria react.

This research isn’t just important for marine organisms either, Rynearson said. What happens in the ocean affects humans in a big way, too. “It’s a big deal because many of us rely on the oceans for food and income,” she said. “Regardless of where we live, whether it’s by the ocean or in the country, we’re all subject to the influence that marine microbes have.”