Explorations of Oxygen-Free Seas on the Oceanus (Part IV: Methane)


Guest post by Abbie Johnson

This is the fourth and final blog post of a series.
Here are links to the first, second and third posts.

Abbie Johnson signing on… Hi everyone! I’m a future PhD student of Jen Glass and have been onboard the R/V Oceanus learning the ropes (or perhaps “learning the lines” when you’re at sea) and helping collect and process samples.

We are now towards the end of this expedition, packing away our experiments, and reflecting on our hard work during the cruise. It’s important, while we’re all in the same place, to discuss our findings and the new and exciting questions to be answered.

Before we sign off, we’d like to share with you some of our thoughts on the importance of methane in the work we’re doing out here in the Oxygen Minimum Zone (OMZ) of the Eastern Tropical North Pacific (ETNP), the largest open ocean reservoir of methane in the world.

Methane is a potent greenhouse gas and an important energy and carbon source for microorganisms. In the ETNP OMZ, methane is transported laterally from sediments where the OMZ meets the continental shelf (Chronopoulou et al., 2017, ISME J). 

After organic carbon raining down from surface waters is consumed, methane-oxidizing microbes known as “methanotrophs” can take advantage of an ecological niche using oxygen from overlying waters, and methane from below. When oxygen is fully depleted, anaerobic methanotrophs may “eat” methane using oxidized nitrogen, such as nitrate and nitrite, instead of oxygen.

Georgia Tech PhD candidate Cory Padilla, from the lab of chief scientist Frank Stewart, has linked methane oxidation to the nitrogen cycle in the ETNP and other OMZs (Padilla et al., 2016, ISME J; Padilla et al., 2017, Frontiers in Microbiology). This research showed that OMZ microbes express genes for denitrification coupled to methane oxidation at depths in the water column where both methane and oxidized nitrogen compounds are available.

These studies sparked further questions about rates and mechanisms of methane cycling in OMZs, supported by grants to PI Stewart and Co-I Glass from the National Science Foundation and to PI Thamdrup from the European Research Council.

On this cruise, I helped my PhD advisor, Jennifer Glass, collect water samples from 130 meters depth, where nitrite concentrations are highest in the OMZ. We set up anoxic enrichment cultures with methane, nitrite and other nitrogen compounds to select for anaerobic methanotrophic microbes. Back in the lab in Atlanta, our group members will carefully track the chemical constituents and microbial diversity over many months to study these mysterious microbes in the methane cycle.

Bo Thamdrup, a Professor at the University of Southern Denmark and Herdís Steinsdóttir (now a Master’s student at Danish Technical University), are trying to understand how fast OMZ microbes are turning over methane. Thamdrup is always in search of the state-of-the-art techniques for measuring rate processes. On this trip, Thamdrup and Steinsdóttir used tritium-labeled methane, a more sensitive method than they’ve used in the past to detect methane turnover in OMZs.

This is Steinsdóttir’s first research cruise, and she has been motivated by the fast-paced experience of measuring samples immediately after they are collected in situ.

This cruise was especially special because we’ve had beautiful sunsets nearly every night. On one particularly clear night, some of us even saw the green flash! Thamdrup said this was the best green flash he’d ever seen, which is saying something, given his vast experience on research cruises.

Photo courtesy of Abbie Johnson 

Before we sign off, we’d like to give a big shout out to the ship’s crew for their generous hospitality and never-ending support of all of our scientific needs. Our crew members were Marine Technician Brandon D’Andrea, Cook Theresa Wisner, Asst Cook Marc Madula, Captain Bryon Wilson, Second Mate Jim Howard, Chief Engineer Mark Booher, ABs Rob Weaver and Marc Simpson, QMED Jay Bart, C/M Jim Moore, Bosun Doug Beck and 1 A/E Chip Millard.

We hope you enjoyed following along on our scientific voyage!
Please leave comments if you have any questions we can answer.

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