Earth’s oldest and smallest creatures are about to become the winners of climate change – and the repercussions could be huge

The world’s oceans are home to microscopic organisms that are invisible to the naked eye. These tiny creatures, called “prokaryotes,” make up 30 percent of life in the world’s oceans.

These organisms play an important role in maintaining the balance of the oceans. But new research by my colleagues and I shows that this balance is under threat.

We found that prokaryotes are remarkably resilient to climate change and could, as a result, increasingly dominate marine environments.

This could reduce the availability of fish that humans rely on for food and hamper the ocean’s ability to absorb carbon emissions.

A beautiful balance

Prokaryotes include both bacteria and “archaea,” another type of single-celled organism.

These organisms are considered the oldest cellular life forms on Earth. They thrive all over the planet, on land and in water, from the tropics to the poles.

Prokaryotes make up for their lack of size in abundance. Globally, there are about two tons of marine prokaryotes for every human on the planet.

They play a crucial role in global food chains, helping to meet the nutritional needs of the fish that humans catch and eat.

Marine prokaryotes grow extremely fast, a process that releases a lot of carbon. In fact, prokaryotes living at a depth of 200 meters in the ocean produce about 20 billion tons of carbon per year, twice the human output.

This massive carbon production is balanced by phytoplankton, another type of microscopic organism that transforms sunlight and carbon dioxide into energy, through photosynthesis.

Phytoplankton and other ocean processes also absorb up to a third of the carbon humans release into the atmosphere each year, helping to limit the rate of global warming.

How prokaryotes respond to global warming is key to understanding how the delicate balance of the planet’s oceans might change in a warmer world. That’s where our research focuses.

What we found

We wanted to predict the impact of climate change on the “biomass,” or total overall weight, of marine prokaryotes. We also wanted to look at the impact it would have on their carbon production.

To do this, we built computer models that integrated decades of observations from dozens of scientific studies in the world’s oceans.

So what did we find? Prokaryotes are probably the big winners from climate change, compared to other forms of marine life.

For every degree of ocean warming, their biomass will decline by about 1.5%, less than half the 3 to 5% decline we had predicted for plankton, fish and larger mammals.

This means that future marine ecosystems will have lower overall biomass and will be increasingly dominated by prokaryotes. This could divert more of the available nutrients and energy to prokaryotes at the expense of fish, thereby reducing the supply of fish for human consumption.

We found another important change. For every degree of warming, we predict that prokaryotes living in the top 200 meters of the world’s oceans would produce an additional 800 million tons of carbon per year.

This is equivalent to the current emissions of the entire European Union (after converting CO₂ to carbon).

What does all this mean?

Due to human-induced climate change, Earth’s oceans are expected to warm by 1 to 3°C by the end of the century unless humanity changes course.

If the amount of carbon produced by prokaryotes increases as expected, the oceans’ ability to absorb anthropogenic emissions could decrease, making it even more difficult to achieve the goal of net-zero emissions globally.

Furthermore, current projections of global fish stock declines under climate change generally do not account for how warming could restructure marine food webs in favor of prokaryotes. This means that projected declines are likely underestimated.

The decline in fish populations poses a major problem for the global food supply, as the oceans are a major source of protein for about 3 billion people.

What should happen now?

Our analysis represents an important step in uncovering the changing role of marine prokaryotes. But significant uncertainties remain.

Our analysis builds on existing observations. Climate change is already altering conditions in marine ecosystems in ways that our models may not have captured.

We also don’t know how quickly prokaryotes will adapt and evolve to new environments. But existing research shows that within weeks, bacteria can develop new traits that make it easier to survive.

It is clear that scientists need to continue to improve their understanding of prokaryotes and how they may be affected by climate change.