Meanwhile at the foot of the food chain

 

A sneaking scepticism about received plankton theory led a Bristol researcher to uncover a fascinating tale

UNDERSTANDING the evolution of marine creatures smaller than the width of a human hair could teach us about past changes in climate over millions of years, and hold key lessons for the future.

Plankton are single-celled drifting ocean organisms that are the basis of the marine food- chain, explains Dr Daniela Schmidt, who led the research. Their skeletons end up deposited in layers of sediment at the bottom of the world’s oceans, providing a record of their evolution.

“These tiny, but very important creatures produce a quarter of the world’s ocean carbonate,” says Schmidt. Why is that important? Because it is a key component of the carbon cycle – an all-important driver of climate change.

Over the past 65 million years various species of marine plankton have evolved, and changed in size. “Understanding the underlying cause for the evolution of living organisms has been a challenge for centuries. For many groups of plants and animals, past changes in temperature have been suggested as a driving force for body size changes,” explains Schmidt.

North American scientists have previously argued that changes in marine plankton were universally driven by ocean temperature – in other words, the plankton were adapting exclusively to changes in climate.

Schmidt was not wholly convinced, however. “I thought hmm . . . ?” she says. It was this sneaking scepticism, rather than an exclamation of “Eureka!”, that led her to uncover a fascinating scientific tale, details of which were published this summer in the US, in Proceedings of the National Academy of Science.

“If this were true, future changes in ocean temperature might have a profound influence on these organisms which play an important role in the geochemical cycles of the ocean, ” says Schmidt. But she suspected that interactions with other organisms could also be important.

While previous studies had looked at carbonate-based plankton, Schmidt’s study focused on a particular type of plankton called radiolarians, which are based on silica – commonly found in the form of quartz, like the sand on a beach. Silica comes from weathering of rocks, which gets washed into rivers and ends up in the oceans, explains Schmidt.

“Radiolarians compete with other organisms for silica, the material they use to build their intricate skeletons, thereby allowing testing for the two possible explanations – physical changes in the water, or biological interactions with other organisms.”

Unlike some of their carbonate-based cousins, radiolarians have not significantly changed in size over the past 65 million years. How do we know this? Because Benjamin Kotrc, a Master’s student with Schmidt, spent hundreds of hours looking at dozens of samples under a microscope.

“This is absolutely painstaking work,” says Schmidt, a Royal Society Research Fellow in the department of earth sciences at the University of Bristol. But this hard work paid off.

What the team found is that while the size of radiolarians didn’t change, their structure did. “The results show radiolarians were able to decrease their thickness and increase the pore area – both ways to construct skeletons with less and less silica,” explains Schmidt.

This evolutionary adaption is consistent with a decrease in the availability of silica in the oceans as well as competition from other silica-using organisms. Most significantly, it has nothing to do with temperature, as was previously supposed.

“Our study suggests that both, the biological competition for the silica and the reduced silica in the water were the underlying change in silica use. Hence not all marine plankton have, or will react in the same way to temperature changes.”

The lesson is that no single factor determines the evolution of plankton, but rather a complex combination of biological and physical factors are at play. Life really is a complicated business – even for single-celled organisms.

Beyond the obvious scientific understanding that this research has generated, it has important implications for climate models. Schmidt strongly believes that we need better models that include greater detail.

“This warns us not to over-generalise. Lots of assumptions are based on climate models that include just one species, which could have a totally different driver. We have to make biology in climate- change models much more realistic.”

Jeremy O’Brien is based at the University of Bristol and is on placement at The Irish Times as a British Science Association Media Fellow