Graduate Students

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Emma Bertran

Fifth year Ph.D student

The intimate interaction between microbial life and earth leaves specific chemical signatures that record information on microbial activity, and paleo-environmental conditions. As a geobiologist, my goal is to interpret these signatures and reconstruct an accurate picture of paleo-environments and microbial communities, and the nature of their co-evolution through time. To date, I have focused on the sedimentary biogeochemical sulfur cycle and its main metabolic pathways: microbial sulfate reduction and microbial sulfur disproportionation. These run the reductive and oxidative branches of the sedimentary sulfur biogeochemical cycle, respectively. As such, they respond to and track the evolution of Earth’s surface redox conditions. Deconstructing the net preserved isotopic signature into its individual metabolic components requires a thorough understanding of the biochemistry of each, and of the environmental information enclosed in their specific sulfur isotopic signatures. For this, I apply a wide range of research tools. These include stable isotope geochemistry, environmental observations, pure culture microbial experiments (using both wild-type and deletion mutant strains), modeling of intracellular dynamics, and construction of three-dimensional models of crucial proteins in these key metabolic pathways.

For more details on current projects and a list of publications, please visit my website!

Ana Gonzalez Valdes

Ana Gonzalez Valdes

Fourth Year Ph.D Student

Ana Gonzalez Valdes received her B.A. in Earth Sciences from Columbia University in 2016. At Columbia and Caltech she worked to understand how microbes in unique environments cycle nutrients like phosphorus and sulfur. At Harvard Ana will be spanning the Johnston and Pearson labs and hopes to dig deeper into microbial nutrient cycling and how it can be measured isotopically.

Ella

Ella Hughes

First Year Ph.D. Student
Ella is coming to Harvard from the UK, where she completed a BA degree in Natural Sciences and MSc degree in Earth Science at the University of Cambridge. Her... Read more about Ella Hughes
K Keller

Katherine Keller

Second Year Ph.D Student
She/Her/Hers
Katherine received her BA in Environmental Chemistry from Columbia University in 2014. Since then, she has worked at the US Geological Survey on Quaternary paleoceanogprahic and hydroclimate reconstruction in the Arctic. At Harvard, she will be working in the Johnston and Schrag labs.
Frasier Liljestrand

Frasier Liljestrand

Fifth year Ph.D student

Frasier got his BS in Earth Science from Rice University in 2013. While he was at Rice, he did research in biogeochemistry focusing broadly on trying to quantify global carbon fluxes and budgets. Here in the Johnston group Frasier hopes to expand his knowledge of global sulfur cycling and how the global cycles are expressed locally.

Haley

Haley Olson

First Year Ph.D. Student

She/Her/Hers

Haley received a BA in Geology from Carleton College in 2017. After graduating, she worked as a TA for a geology field camp in New Zealand and as a lab assistant in the Johnston lab. As a graduate student she plans to merge her interests in field geology, sedimentology, and stable isotope geochemistry. She will focus on using minor oxygen isotopes as a proxy to reconstruct past climates.

Anna Waldeck

Anna Waldeck

Fourth Year Graduate Student

She/Her/Hers
Anna completed a BSc in Chemistry and in Geophysical Sciences at the University of Chicago in 2014. Since 2013 she has worked in a stable isotope ratio laboratory there, measuring oxygen isotope ratios in phosphates contained in small shelly fossils and tooth enamel. Anna’s interests pertain to reconstructing chemical cycling in ancient environments through a combination of field work and laboratory techniques.