Zehr Lab Personnel


Jonathan P. Zehr

Distinguished Professor of Ocean Sciences

Rachel headshot.jpg

Rachel Clark (Harbeitner)

Postdoctoral Scholar

email Rachel:​

rharbeit AT ucsc DOT edu


​Link to Rachel's Google Scholar Citations


Francisco Miguel Cornejo-Castillo

Postdoctoral Scholar

My main research interest is to bring single-celled symbioses (i.e., symbiosis between unicellular organisms) to a higher level of understanding by providing knowledge on their ecological and evolutionary patterns. Symbiotic interactions are key drivers of ecological diversification and evolutionary innovation on Earth. For example, one of the major innovations in nature was postulated in the Theory of Endosymbiosis by Linn Margulis, which posits that plastids and mitochondria in eukaryotes originated from bacterial endosymbionts. However, the underlying processes remain difficult to address because they occur over geological time scales. Thus, the study of present-day symbiotic associations between unicellular eukaryotes and prokaryotes might be an alternative way to understand how symbioses are established. Therefore, the objective of my research is to gain knowledge into the ecology and the evolution of the UCYN-A nitrogen-fixing symbiosis to, eventually, get a better understanding of the mechanisms conducting to the plastid formation.


email Fran:

​frcornej AT ucsc DOT edu

​Link to Fran's Research Gate Profile


Mary Rose (Rosie) Gradoville

Postdoctoral Scholar

I am interested in the diversity, activity, and ecology of marine microorganisms, especially nitrogen-fixing bacteria. My research in the Zehr lab is based on the “Gradients” research project, in which a team of scientists are mapping the gradients in diversity and productivity between the North Pacific Subtropical Gyre and the North Pacific Subarctic Gyre. For this project, we departed Honolulu and sailed over 1000 miles north, collecting physical, chemical, and biological samples. I am exploring how the diversity and activity of marine nitrogen-fixers changed along this journey from the warm, clear Hawaiian waters—a classical habitat for nitrogen fixation—to the colder, more nutrient-rich waters of the subarctic North Pacific. My research uses DNA and RNA sequence analyses, fluorescence-based cell sorting, and single-cell nitrogen fixation rate measurements. 


email Rosie: 

mgradoville AT ucsc DOT edu

Link to Rosie’s Google Scholar Citations

Link to Rosie’s Research Gate Profile


Katie Harding

Graduate Student

I am studying Nitrogen fixation in the Arctic. I would like to answer such questions as; if and where nitrogen fixing bacteria (diazotrophs) are present, what diazotrophs are present and are they fixing nitrogen. I will accomplish this through a combination of research cruises and laboratory work. Another interest of mine is non-cyanobacterial diazotrophs. Most well studied diazotrophs are cyanobacteria and use sunlight for energy. But as more sequencing applications become easily available, non-cyanobacterial, presumably heterotrophic, diazotroph sequences are now commonly found. I am interested in the lifestyle of a specific, widespread, non-cyanobacterial sequence known as GammaA.

e-mail Katie:

kharding AT ucsc DOT edu


Esther Mak

Graduate Student

e-mail Esther:

wimak AT ucsc DOT edu


Britt Henke

Associate Specialist

I am interested in researching and communicating the important work of marine nitrogen (N2) fixing bacteria, especially the unique and ubiquitous Unicellular Cyanobacteria group-A (UCYN-A). N2-fixing bacteria support primary production by supplying nitrogen (N) to the vast oligotrophic areas of the world’s oceans. In this way N2-fixing bacteria effect global nutrient cycles and support phytoplankton oxygen production, which accounts for half of all the oxygen we breathe! By communicating our interdependence with marine microbes, I hope to inspire scientific understanding and inquiry including broad acceptance of and honest discussion about human-induced climate change. In my capacity as a Lab Specialist I do science, support the research of graduate students and postdocs, mentor undergraduates, and generally keep the lab running, including ensuring that our lab is a safe and friendly working environment for all.

e-mail Britt:

bhenke AT ucsc DOT edu


Jonathan Magasin 


My goal is to improve our understanding marine microbial communities in their natural environments.  To this end, I enjoy creating software tools to help analyze large data sets derived from marine environmental samples (metagenomic and metatranscriptomic) produced by next-generation sequencing and our MicroTOOLs microarray.  I am also interested in how the staggering number of public marine environmental data sets archived in "omics" repositories can be used to discover marine microbes that are widespread but have been overlooked due to biases toward studying cultivable types.

email Jonathan:  

jmagasin AT ucsc DOT edu


Kendra Turk-Kubo

Project Scientist

My current research focuses on understanding the environmental factors driving the diversity, biogeography and activity of marine nitrogen-fixing microbes. Vast areas of the sunlit surface ocean have no detectable nitrogen, yet are teeming with microbial life. Life in these “ocean deserts” is made possible, in part, by microbes that are able to convert dinitrogen (N2) gas into biomass through the process of N2 fixation. This process has been known to be important in the marine environment for several decades now, yet we are still in the beginning stages of understanding the organisms responsible. Many N2-fixers have yet to be isolated in culture, so my research relies on applying cultivation independent techniques to detect and study these cryptic microbes. I am currently involved in three collaborative research projects, funded by the National Science Foundation (NSF) and The Simons Foundation (P.I. Jonathan Zehr). Broadly, these research projects are focused on: 1) Determining whether N2 fixation is occurring in the Alaskan Arctic Ocean, identifying which N2-fixers may be responsible, and determining the environmental drivers of their activity and distributions in these cold, high latitude waters; 2) Identifying the quantitative significance of a unique N2-fixer that lives in symbiosis with another single-celled algae (UCYN-A/haptophyte association) in coastally-influenced marine waters; and 3) Measuring N2-fixer growth rates and microzooplankton grazing rates on N2-fixers in oligotrophic marine waters, to gain insight into their distribution patterns in the North Pacific Subtropical Gyre.


e-mail Kendra:

kturk AT ucsc DOT edu

Link to Kendra's Google Scholar Citations

Link to Kendra's Research Gate Profile