The world's oceans are home to microscopic organisms invisible to the human eye. The tiny creatures, known as "prokaryotes", comprise 30% of life in the world's oceans.
These organisms play an important role in keeping the oceans in balance, recycling nutrients up marine food chains to fish, and regulating the ocean's capacity to absorb human emissions. But new research published in Nature Communications led by Griffith University's Dr Ryan Heneghan showed this balance was at risk.
"We found prokaryotes are remarkably resilient to climate change - and as a result, could increasingly dominate marine environments," said Dr Heneghan, a Research Fellow with the Australian Rivers Institute.
"This could reduce the availability of fish humans rely on for food and hamper the ocean's ability to absorb carbon emissions."
Despite being crucial components of these systems, understanding of how prokaryotes vary across global environments and their response to climate change has previously been limited.
"Prokaryotes are a major part of global ocean life, but we lack a global perspective on how their abundance and activity varies across environmental gradients. We wanted to fix this," Dr Heneghan said.
Using the largest global dataset of prokaryotes ever compiled, Dr Heneghan and the team built statistical models to examine prokaryotic abundance, cell carbon and metabolic activity across the world's oceans now and into the future.
They found that global prokaryote biomass totalled about 15 billion tonnes, or almost 2 tonnes for every person on the planet. Under the influence of climate change, the study projected a decline in global prokaryotic biomass in surface waters by approximately 1.5% per degree Celsius of warming.
"What we found is that prokaryotes are likely to be climate change winners, relative to other marine life. For each degree of ocean warming, their biomass would decline by about 1.5%. But, this is less than half the projected 3-5% decline we predicted for larger plankton, fish and mammals. That means future marine ecosystems will have lower overall biomass and will increasingly be dominated by prokaryotes. This could divert a greater share of available nutrients and energy toward prokaryotes and away from fish, reducing the supply of fish humans eat."
Dr Ryan Heneghan
"We discovered another important change. For every degree of warming, we predict prokaryotes in the top 200 metres of the world's oceans would produce an additional 800 million tonnes of carbon per year. This is equivalent to the present-day emissions of the entire European Union.
"At the moment, the oceans absorb about one-third of the 35 billion tonnes of carbon dioxide humans produce every year. This has a massive impact on slowing the rate of climate change.
"But, if in the future prokaryotes are respiring additional carbon in response to warming, this could reduce the capacity of the oceans to absorb our emissions. This would make the global goal of net zero emissions even more difficult to achieve.
"The oceans are complicated, and there's still a lot we don't know. But, our work can address a critical gap in improving the computer models we use to understand the impacts of climate change.
"We're living in unprecedented times, with climate change already driving marine ecosystems into states not seen before. We don't yet know how prokaryotes will adapt and evolve to these new environments.
"Given there'll likely be 10 billion people to feed in 2050, we're going to need healthy, functioning marine ecosystems more than ever before. Whether for food from fisheries, or to make the global climate habitable. So it's critical that we continue to improve our understanding of prokaryotes, and ocean life more broadly."
The findings 'The global distribution and climate resilience of marine heterotrophic prokaryotes' have been published in Nature Communications.
