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Samples collected during the HMS Challenger expedition 150 years ago are helping scientists figure out how acidification from climate change is impacting the world's oceans today and how it might trigger a feedback loop resulting in even more atmospheric carbon.
The climate crisis is impacting our planet in many ways, from the melting of ice sheets to the increasing frequency of devastating weather events. It is also causing the oceans to acidify.
As the concentration of carbon dioxide in the atmosphere continues to rise due to human activity, more of the gas dissolves into the world's oceans. This lowers the pH of the water, making it more acidic.
How life is coping with these changes in ocean chemistry is not particularly well understood.
To track differences over a 150-year period, researchers have compared modern samples to those collected during the HMS Challenger expedition of 1872-1876. The results have been published in Scientific Reports.
The focus of the research, which has been led by Dr Lyndsey Fox from Kingston University, is on small, single-celled organisms known as foraminifera. These are a significant component of plankton and live in the sediments on ocean floors and in the water column.
Most of these small organisms are known to build a shell from calcium carbonate, often complex and beautiful. Laboratory experiments have shown that the ability to build these shells is dramatically hindered by acidic conditions. What the experiments are unable to show, however, is how the organisms might be impacted over a long period of time.
This is where the Challenger samples come in.
Dr Stephen Stukins, Senior Curator of Micropalaeontology at the Museum, has been working with colleagues to compare the shell thickness of foraminifera collected on the Challenger expedition with those collected during the modern Tara expedition that set sail five years ago.
'We found that the Challenger collections didn't just contain samples of dredges from the ocean floor, but there were also plankton tow residues,' says Stephen. 'The important thing about this is that we realised they contained foraminifera that were alive on the day they were collected.
'We also have all of the papers and notebooks from the expedition, which are remarkable and give detailed accounts of exactly how they collected the samples, which was surprisingly thorough.'
This is significant because it means that the samples collected in the 1870s can be directly compared with those collected in modern times.
HMS Challenger set sail from Portsmouth in December of 1872, with the aim to study the world's oceans. It was the first major global oceanic survey with the express purpose of collecting data and samples.
Over four years, the crew sailed some 130,000 kilometres, measuring everything including temperature, depth, salinity and sediments, while collecting all animals and plants they could find. It resulted in many notable discoveries such as identifying the deepest point on Earth, as well as 4,700 species previously unknown to Western science.
During this expedition, they also conducted what are known as plankton tows. It is this plankton that forms the base of many marine food chains. They are not a single organism, but made up of an entire community of bacteria, archaea, algae, protozoa and tiny drifting animals.
These different types of plankton are found at different depths of the oceans, with foraminifera plankton typically found closer to the surface, associating with photosynthetic algae in a symbiotic relationship.
'The symbionts are at the very bottom of the food chain, and then these foraminifera use the symbionts, and in turn they then get eaten by larger creatures which get eaten by the bigger animals,' explains Stephen.
Despite their small size, plankton such as the foraminifera are also vitally important carbon sinks, as they build their shells out of calcium carbonate. Stephen says, 'Massive amounts of carbon is drawn down to the ocean floor due to these calcareous plankton, and they make up a ridiculous percentage of the biomass of the oceans.'
This biomass is being threatened by ocean acidification.
Stephen and his colleagues were able to use nano-CT scans to look at the thickness of the shells built by foraminifera from the Challenger collection and compare them with those collected recently from similar localities at the same time of year. What they found was startling.
'The samples show that as these organisms find it more difficult to create their shells, they're adapting over multiple generations to build thinner ones because of the changes in pH,' says Stephen. 'It is not about the shells being dissolved, which is a common misconception, but that the foraminifera are struggling to build them.
'This means that not only are their defences weaker, but that less carbon is being drawn down into the carbonate.'
The result is something of a feedback loop. As the concentration of carbon in the atmosphere increases, the oceans become more acidic, thinning the shells of the plankton and resulting in less carbon being locked in the oceans and more remaining in the air, which makes the oceans more acidic.
It will also likely cause some of the species to go extinct, which may well ripple further down the food chain.
'If the plankton are struggling, then that is going to cause a knock-on effect to the larger creatures that consume the plankton and the predators that subsequently feed on them,' explains Stephen. 'It will cause imbalances and you will inevitably get extinctions, because only the most resilient species of foraminifera and other creatures will survive and they will out-compete the other species, causing them to die-out.'
Thanks to the specimens collected by HMS Challenger, this study has provided the first direct evidence of how ocean acidification is impacting carbonate-producing marine life in modern oceans over a long period. The next questions will to look at how these impacts vary across the world's oceans and the species that live in them.
Ocean acidification is caused by the increasing concentration of carbon in the atmosphere due to the continued burning of fossil fuels, to deforestation and to our intensification of agriculture. The only way to slow it - and to help protect the oceans - is to dramatically cut these emissions.