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December 4th, 2017

The hard science of soft substances: Julio Sepúlveda uses lipids to decode ancient ecosystems

Graduate students analyze samples in the Organic Geochemistry Lab in fall 2017. Photo by Julio Sepúlveda.

Julio Sepúlveda is a unique fossil-hunter. Rather than examining the hard exoskeletons of ancient organisms, Sepúlveda analyzes a softer biological component, fats.

Sepúlveda, an INSTAAR fellow and assistant professor in the department of Geological Sciences, looks at the distribution of organic molecules in nature from soils and sediments to lakes and the ocean, with a focus mostly on lipids, the molecular constituent of fat. He uses this data to not only reconstruct ancient environments but also to contribute to predictions for how our current climate might respond to similar environmental conditions.

“We try to match lipids with specific biological sources,” says Sepúlveda. “We are trying to see who produces what, and why.”

Compared to other biological macromolecules like DNA and proteins, lipids have a high preservation potential. Although they degrade over time, their hydrogen and carbon backbones can be preserved for millions and even billions of years.

Currently, among many other projects, Sepúlveda is working on extracting lipids from bacteria found in ancient soil samples as old as 70 million years. These bacteria can change the types of lipids they use to make their cell membranes depending on the temperature in which they’re growing. The types of lipids Sepúlveda finds in these samples will give him an idea for what the temperatures were like at that time in Earth’s history.

“They are a proxy for temperature in the past,” Sepúlveda says.

To determine what lipids are actually in a given sample, Sepúlveda uses a number of mass spectrometers and chromatography techniques in the newly opened Organic Geochemistry lab (OG Lab) in the Sustainability, Energy and Environment Complex (SEEC) on east campus.

“This is exciting because some of these instruments with high-mass resolution are relatively new, especially for the application to lipidomics,” Sebastian Kopf, co-PI of the OG Lab and assistant professor in Geological Sciences.

Using these instruments, Kopf and Sepúlveda can determine very specifically what kind of organism made which compound because, like the soil bacteria, different microbes have distinct signatures.

It helps that the instruments are optimized for low sensitivity.

“We are able to detect minute concentrations of organic compounds in the environment, which is terrific,” says Sepúlveda.

Because it can discover such low levels of a desired compound, the food industry uses the same kind of instrumentation and methodology to test the levels of pesticides in food as does the sporting industry to test for doping in athletes.

But even this is not enough.

“Our hope is that we can push down the sensitivity even lower so we can look both in environments that are not as well preserved and further back in time,” says Kopf.

Instruments in the Organic Geochemistry lab provide a well-rounded view of a sample by allowing for both a broad characterization and a targeted analysis of molecules in a given sample.

“The goal with analyzing [these compounds] is, once we know who is producing what and why and in what kind of environment, then when those lipids get preserved we can try to use that information to reconstruct the paleoecology and the kind of environmental conditions of an ecosystem for a particular time period,” says Sepúlveda.

The Organic Geochemistry team in summer 2016, including lab manager Nadia Dildar, undergraduate student Michael "Avi" Gross, postdoc Ari Meilijson, Julio Sepúlveda, and PhD students Garrett Boudinot and Lina Pérez-Ángel.

The temperature data Sepúlveda generates can contribute, for example, to climate change models that try to estimate climate sensitivity.

“We can use lipidomics to study how biology responds to abrupt changes in our climate system by, for example, reconstructing how entire ecosystems might have been able to respond to periods of time when there was more CO2 in the atmosphere than today and our planet became very warm, and all of that information is relevant to our understanding of what might happen in the future.”

The same goes for studies in the ocean.

Sepúlveda works in regions of the ocean off the coasts of Chile and Peru known as oxygen minimum zones. In these areas water from the deep ocean rises to the surface, bringing an abundance of nutrients for plankton and fish, but also water very low in oxygen. Changes in temperature can reduce the amount of ocean mixing and its oxygen content, which has direct implications for global economies, food supplies and ocean geochemistry.

Sepúlveda analyzes the lipid composition of microorganisms inhabiting these areas to determine how marine ecosystems can respond to changes in water chemistry, including acidification and the loss of oxygen. Understanding the microbial processes taking place in these oxygen minimum zones has implications for the chemistry of the ocean as well as the chemistry of the atmosphere because some of the by-products of their metabolism, like nitrous oxide, are powerful greenhouse gases.

Sepúlveda has a number of projects on the docket for the coming year. His collaborators and the Organic Geochemistry lab will help him fulfill them.

“We can share the techniques and expertise in our new lab. It’s like a very fun playground to solve scientific questions,” says Sepúlveda.

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