Exploring the deep ocean
Photo Illustration by Tammy Beeson | photos courtesy of George Luther, NSF/HOV Alvin December 20, 2021
UD alumni recall their undergraduate research trip to the floor of the Pacific Ocean
Nicole Coffey and Richard Rosas were undergraduate students at the University of Delaware in 2017 when they joined UD professor George Luther on a research cruise to the East Pacific Rise — a mid-oceanic ridge located on the floor of the Pacific Ocean. The region is known for its hydrothermal vent activity and is located more than 500 miles off the coast of Acapulco, Mexico.
While that experience took place more than four years ago, enough data were collected that Coffey, who is now a doctoral student at Oregon State University, and Rosas, a doctoral student at Texas A&M University, were recently co-authors with Luther on a paper published in the Proceedings of the National Academy of Sciences.
Timothy Shaw, a professor from the University of South Carolina, served as the lead author on the paper and was also a member of the research cruise.
The research paper showed that hydrothermal vents act as a previously unmeasured source of reactive oxygen species (ROS) in the ocean. ROS are highly reactive chemicals formed from oxygen, such as hydrogen peroxide and superoxide.
This process has long been recognized in surface waters and attributed to photochemical and biochemical reactions, but this paper showed that large amounts of iron coming from hydrothermal vents react with the oxygen in ambient bottom waters to form hydrogen peroxide as a metastable ROS species.
Iron can exist in different forms or oxidation states in nature. The researchers found a lot of iron(II) — which is typically thought of as dissolved or ‘free’ iron and is not stable in oxygenated waters — but not a lot of iron(III) — which is usually present in waters with a lot of oxygen, such as the bottom waters that were being sampled.
They discovered that any iron(III) that was formed would react with the hydrogen sulfide to form elemental sulfur and regenerate the iron(II) as a catalyst.
This ROS as hydrogen peroxide was detected at concentrations 20 to 100 times higher than the average for photo produced ROS in surface waters. Additionally, the hydrogen peroxide was measured at a concentration up to six micromolar, which is 6% of the original dissolved oxygen concentration of the cold bottom waters that mix with the vent waters.
The concentration of hydrogen peroxide would be expected to form at the same rate as the elemental sulfur produced, which was 20 micromolar. The imbalance showed that hydrogen peroxide further reacts with other components in seawater.
“The intensity is really spectacular compared to what you see in surface waters and cloud waters,” said Luther, the Maxwell P. and Mildred H. Harrington Professor of Marine Studies in the School of Marine Science and Policy. “Rainwater and cloud water usually get the highest amount of reactive oxygen species that you can measure, but our data rivals it and surpasses it.”
In addition, the finding has implications for the ocean’s global carbon cycle. The hydrogen peroxide can react further with reduced iron to form hydroxyl radical in a process known as the Fenton reaction. This hydroxyl radical can react with dissolved organic carbon (DOC) to mediate the mineralization of DOC to carbon dioxide and form hydroxylated benzene compounds, thus impacting the carbon cycle in the ocean.
Both Coffey and Rosas were integral in the process of collecting the samples and processing the data.
Rosas, who graduated in the spring of 2018 from UD’s College of Earth, Ocean and Environment, worked closely with Shaw on the physical instrumentation to collect the samples from the vent sites.
In order to do this, Rosas descended to the seafloor twice in Alvin, a submersible, and helped collect samples from the hydrothermal vents using syringe samplers that were held by an arm known as a manipulator on the Alvin submersible. The syringe samplers were filled with horseradish peroxidase and a reagent that would permit hydrogen peroxide to react quickly with a sample to form a colored agent for its detection.
“I had no idea that I would have the opportunity to actually be inside Alvin and go down for a dive, let alone two,” said Rosas. “It was a highlight of my life so far, being able to go down in the ocean and see the vents in person was astounding. It’s so hard to express what that meant to me as someone who was aspiring to do marine science and oceanography.”
Coffey also had the opportunity to travel in Alvin to look at the seafloor vents as she was on the first science dive of the cruise to canvas the seafloor to see how many vent sites would be good for sampling.
“Vents are so ephemeral. They might be on one month and off the next,” said Coffey. “Our job was to go out, see what the situation was down there, and report back to say ‘P vent will be great for this’ or ‘Q vent will be good for that.’ ”
Coffey conducted the iron measurements for the paper and said the presence of iron(II) and absence of iron(III) was at first confusing because there was plenty of oxygen available at the vent sites.
“We found plenty of reactive oxygen species down there, as well as reduced sulfur, and one of the ideas we had is that the iron is acting as a catalyst,” said Coffey. “When you generate superoxide or peroxide, those are reactive and can facilitate a lot more reactions within marine chemistry. The idea behind this was that the iron and sulfur are playing a role in generating these reactive oxygen species that are then transported away from the vents and then take part in other chemistry.”
Both Coffey and Rosas stressed that this was a formative experience for them as undergraduates, and they were happy to get to help out on the research cruise all the while taking classes onboard the ship as undergraduate students.
“It’s hard to express how grateful I am about that opportunity,” said Rosas. “The experience was invaluable, especially as someone who wants to study the ocean. You don’t really grasp how massive the ocean is until you’re out far enough that you can’t see land in any direction.”
Coffey said that for any undergraduates out there, one of the most important things they can do if they are interested in research is to reach out and tell someone. She said she never would have gotten to go on the cruise if she hadn’t told her adviser about her interest in ocean chemistry.
“Looking back, it really set me up because I got to do my thesis with George, and he helped me figure out that I wanted to stay at UD for my masters,” said Coffey. “Without that conversation with my adviser, I don’t know if I would have gotten here. I’m sure I would have done well and gotten somewhere I was happy, but those doors might not have been opened.”