Diazotrophs in the Arctic

Kendra Turk-Kubo’s research focuses on diazotrophs. Diazotrophs are microscopic organisms that have the special ability to fix nitrogen. They come in all shapes and sizes, as bacteria or archaea. Turk-Kubo talks about their individual characteristics with a fond familiarity – Trichodesmium, Nostoc, UCYN-A, Richiela, Nodularia, Crocosphaera – all diazotrophs she’s spent the last decade of her career getting to know.  

Bundles of Trichodesmium trichomes forming a tuft (Photo from B. Bergman/Stockholm University). Scientists previously thought Trichodesmium was the only diazotroph in the ocean.
Trichodesmium blooms sometimes get called “sea sawdust” for their appearance in the water. (Credit: ohnemusatsea – Flickr)

Diazotrophs fascinate her because they continue to challenge our assumptions about what organisms are capable of doing. Nitrogen fixation is a crucial process that was previously thought to only happen in warmer waters. So scientists haven’t thought to look for diazotrophs in the Arctic. But now, after preliminary studies from the VIMS team and others that show nitrogen fixation is happening the Arctic, Turk-Kubo is looking for them up here. 

Turk-Kubo is a research specialist in the Dr. Jon Zehr’s laboratory at the University of California Santa Cruz (UCSC). The VIMS team and UCSC have worked closely together on this project for about two years now. Her goal for this cruise is to characterize the diazotrophs in the water here.

She wants to understand the diazotrophs’ cell specific nitrogen fixation rates – that is, how quickly one cell can take up nitrogen gas. She’s also looking at what the environmental drivers are to their biogeography – for example, how influenced they are by the temperature, salinity, and exposure to light. She’s interested in the activity of these diazotrophs, more so than last year, since last year the team was looking to see if these organisms are here at all. Now that it’s known the diazotrophs are here, more hypothesis driven questions can be asked about how their activities may change in response to longer day length.

Kendra Turk-Kubo in front of her lab setup in the main lab

Some of her work is done onboard but most of the work happens back in the lab in California. Onboard, she does molecular analyses for direct sample and manipulation experiments. She uses a qPCR to analyze some of the diazotroph DNA, looking specifically for the nifH gene. The nifH gene is what diazotrophs need to fix nitrogen. It doesn’t necessarily mean the organisms are fixing nitrogen, but they could be. (Learn more about nifH on the Zehr lab website.) 

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Turk-Kubo’s lab station in the wet lab, where she filters samples before taking a look at them under the microscope.

Peristaltic pumps bring water samples through filters that she’ll take back to the lab. In the UCSC lab, Turk-Kubo extracts the DNA from the diazotrophs and sequences for the nifH gene. 

nifH diveristy in oligotrophic marine waters from different ocean basins from Turk-Kubo et al., 2014

Turk-Kubo has been on close to 20 research cruises, mostly in the North Pacific Sub-Tropical Gyre and the Eastern Tropical South Pacific. She completed her undergraduate degree in marine biology and chemistry, with a focus on environmental chemistry and toxins at UCSC. Afterwards she worked as a research associate at the NASA Ames research center in Mountain View, California, running a stable isotope facility. She went to graduate school at Cal Tech for geological and planetary sciences. She then spent a few years back at NASA Ames, where she shifted her work to microbial ecology using molecular methods. From there she went to Jon Zehr’s laboratory, and has been there ever since. 


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