Explorations of Oxygen-Free Seas on the Oceanus (Part I)

Greetings from the Pacific Ocean! I am currently at sea for three weeks with an awesome team of astrobiologists, geochemists and molecular biologists. We are floating off the coast of western Mexico, about 100 miles west of Manzanillo, Mexico, on the Oregon State University research vessel Oceanus.

Ocean waves stretch to the horizon on all sides of us. Boats passing by would never expect this area of the ocean is different from any other. Yet, hiding beneath the surface is the largest anoxic water body on modern Earth.


Only about 100 feet below the surface, oxygen begins to disappear. By 300 feet, it is completely gone. These oxygen minimum zones, abbreviated “OMZ”, often underlie highly productive parts of the ocean surface. They are stable over tens of thousands of years. Fish swimming through OMZ depths will suffocate for lack of oxygen unless they ascend to surface or dive a thousand feet to the bottom waters that carry oxygen around the globe.


Due to circulation patterns and higher temperatures, the largest OMZs occur in tropical regions of the north and south Pacific Ocean. Unlike marine “dead zones” produced by nutrient pollution from agricultural runoff, these tropical OMZs occur naturally.


OMZs are produced by algal blooms in nutrient-rich upwelled waters. These algae need sunlight to photosynthesize, confining them to the upper water column. Eventually the algae sink and are eaten by bacteria, the most prominent of which are members of the genus Pelagibacter. These marine bacteria suck down oxygen to concentrations below detection to all but the very most sensitive optic sensors.


Led by Chief Scientist Frank Stewart of the Georgia Institute of Technology with funding from the National Science Foundation, our international research team has members from eight countries (USA, Canada, Mexico, Iceland, Denmark, Austria, Spain, and Germany).

While we stop work to snap photos when whales are spotted in the distance, our true passion is for the invisible creatures that populate every drop of seawater that splashes onto the ship’s deck. These microbes are extraordinary chemists that have evolved to take advantage of a plethora of anaerobic metabolisms that animals cannot perform under low oxygen conditions.


With samples from this cruise, Mike Henson, a PhD candidate in Cameron Thrash’s lab at Louisiana State University, Baton Rouge, aims to cultivate the first Pelagibacter bacterium from anoxic waters. He designed a new type of “open source” culture medium that mimics natural ocean water (Henson et al., M-sphere, 2016). Henson’s medium is completely modular; he adds and subtracts different substrates to customize the medium for particular strains of Pelagibacter bacterium. This also avoids having to filter hundreds of liters of seawater on each cruise. Henson has had success cultivating Pelagibacter bacteria from the Gulf of Mexico, and wants lots of people to go out and use his recipe: “The more cultures we have, the more questions we can test.”

Professor Sean Crowe of the University of British Columbia is interested in chemical cycles that control nutrients, and in turn, global productivity. Some of these cycles are hard to detect, essentially invisible to standard methods of seawater analysis, but provide critical links between anaerobic life and the strength of nutrient cycles.


To see these cryptic cycles, Crowe is combining measurements of microbial activity rates using the radioactive isotope sulfur-35 with DNA sequencing. In order to avoid contamination inside the ship, he must do his experiments outside in the “rad van”, a special shipping container modified to contain radioactive material.


Crowe collaborates with fellow research team member, Professor Bo Thamdrup of Southern Denmark University, to carefully measure nanomolar oxygen in his experiments with ultra-sensitive optodes.


Some of the invisible processes also leave signals in the rock record that can be revealed through heavy metal geochemistry to reconstruct past ocean processes. Crowe will analyze stable isotopes of the heavy metal chromium in these samples as a tracer of the past.


Stay tuned for more blog posts over the next ten days of our expedition. In each, I will feature scientists from the expedition and the exciting research that inspires them to go to sea.

P.S. If you are interested in more about our previous research in this area, here is a MicrobeNet blog post about the last time I visited this region, and a video I made about the cruise is here:

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