|Manganese Nodules on the bottom of the Pacific Ocean|
What’s your cell phone made of? Over forty different elements are needed for the modern phone with its touch screen and battery. Many of those elements are ones that most people have never heard of like dysprosium and terbium. Where do we find rare elements like these? Today, most of the rare earth elements, as they are called, come from third world countries that still tolerate having their citizens work in dangerous mines where rock strata known as ore deposits have accumulated over Earth’s 4 billion year history.
Economic geologists are the scientists who study how ore deposits form. They find rocks that have higher than normal concentrations of desirable elements in them (like gold or silver), bring them into a laboratory for chemical analysis, and finally determine if the ore deposit is economically feasible for mining. In the early 1980s, it was finally being recognized that microbes living in oceanic sediments were involved in precipitating minerals—some of them with economic importance--although very little was known about how bacteria did this.
I was encouraged to study the role of microorganisms in the formation of ore deposits by Carnegie Director Hatten Yoder. In 1983, I organized my first international conference on “Organic Matter in Ore Deposits”, inviting an interdisciplinary group of scientists from around the world. Ken Nealson, then at Scripps Institution of Oceanography, was working on manganese oxidizing bacteria, which related presumably to the formation of deep sea manganese nodules that carpet the bottom of certain places in the Pacific Ocean. The biogeochemistry field at this time was small enough that you bumped into people at conferences as disparate as the American Society of Microbiology (ASM) and the Geological Society of America (GSA). Ken attended my first graduate student talk at ASM. He was intrigued by my work on Rubisco and stable isotopes. When we met a few years later, we struck up a collegial friendship that has included sharing students and postdocs. Ken is a magnetic person—always optimistic, positive, innovative, and funny. I was fascinated by the possibility that stable isotopes might prove useful in studying the mechanism by which microbes turned soluble manganese into mineral precipitates. We began to discuss in greater detail a potential collaborative project.
|(L-R): Wes Huntress, Doug Rumble, Marilyn, Ken Nealson, circa 2002|
It was unknown if manganese oxides (MnO2—meaning one manganese atom with two oxygen atoms) were formed with molecular oxygen (O2) or with oxygen from water (H2O). Ken and his students had made manganese minerals in the laboratory catalyzed by bacteria or bacterial spores. Conversely, manganese ore deposits could also be formed at very high temperatures or spontaneously by different non-biological mechanisms. How to figure out where the oxygen atoms came from? Stable isotopes to the rescue!
The stable oxygen isotope composition of atmospheric oxygen is very different from oxygen isotopes in water. Air has significantly more of the heavy isotope (18O), while water has significantly less. Because of these basic differences in oxygen isotopes, we had a naturally occurring tracer.
I ended up working on this problem with Ken’s Ph.D. student Brad Tebo, who later replaced Ken at Scripps when he moved to the University of Wisconsin. We also worked with Brad’s first graduate student, Kevin Mandernack, who grew the microbial cells, isolated the manganese oxides, then came to Washington DC to analyze the oxygen isotope compositions of the oxides using a special metal and glass vacuum line called a fluorination line.
The work was physically demanding in several ways. You weighed out a few milligrams of sample into a nickel “bomb”--a cylinder of nickel metal about 40 cm long--and attached it to the vacuum line where a highly reactive compound—bromine pentafluoride (BrF5)--was frozen into the bomb using liquid nitrogen. We then attached furnaces to the bombs and heated them to 600°C for 20 hours. Bromine pentafluoride is explosive. We used it with extreme caution. The laboratory required a specialized ventilation system. In the old Geophysical Lab, this was a huge fan that was connected to a window built into the side of the building. When the fan was turned on, the window flipped open automatically. Often, the fittings attaching the bombs to the vacuum line leaked. The lab often had a smell like an old, indoor YMCA swimming pool.
|Fluorination line with Doug Rumble and Craig Shiffries, circa 1995|
This was my first and last project using bromine pentafluoride. Manganese minerals had never been analyzed before using this technique, so I had to develop methods to remove adhering water before reacting the manganese oxides. As a student, Kevin Mandernack was a good looking, blonde fellow well above 6-feet tall. Coming from UC San Diego’s Scripps Institution, he carried himself like a Southern Californian, dressed in tank tops in summer and surf clothes the rest of the year. He also sported a healthy tan and often had a wide smile and mischievous chuckle. Compared with many of the Harvard-trained postdocs, Kevin exuded self confidence to the young people of the lab. I suspect some of them were jealous.
It may be controversial to discuss appearance and demeanor of colleagues, but there is no question in my mind that these traits are important in affecting a person’s perceived standing. In the very conservative environment of the Geophysical Laboratory, people viewed as “different” sent small ripples throughout the small enclave. Speaking for myself, I feel that I was judged as being a bit of a light-weight during my career because I enjoyed and choose to work with “different” people. At times people would say to me: “You always work with tall guys” or “You always work with women”—neither of which was strictly true. Thirty years ago, it was commonplace to treat women this way. Good looking, magnetic people evoke strong opinions from others: either they become presidents and CEOs or people mumble behind their backs. I saw this repeatedly in working with the many people that I’ve mentored over my career.
I served on Kevin’s PhD committee and made several trips out to Tebo’s lab during that time to discuss the findings we’d discovered, as well as participate in Kevin’s examinations. We collaborated also with Alan Stone at Johns Hopkins University, who prepared manganese minerals of various flavors using purely chemical methods. We compared these minerals with ones that were biologically precipitated by active live, growing microbes or passive spores (Mandernack et al., 1995).
Oxygen isotope analysis of the manganese minerals revealed that a significant proportion (up to 50%) of the oxygen in the minerals came from molecular O2 with both purely chemical as well as biological mechanisms. Oxygen from water formed the remaining portion. This finding means that if a manganese mineral is found in an ore deposit or sediment, there probably was some atmospheric oxygen involved. In some of the Earth’s very old rocks, manganese minerals provided evidence that oxygen was present in the atmosphere at that time.
Brad Tebo, now at Oregon Health and Science University, continues to investigate bacterially-mediated manganese cycling and is a leader in that field. He has isolated many strains of bacteria, as well as studied their genomes to determine which enzymes are involved in the catalysis. Tebo’s recent work challenges the paradigm that the manganese cycle operates between soluble manganese and solid manganese phases. He's found an important intermediate phase formed by microbes in many environments that's at the heart of the manganese cycle. Tebo and his colleague George Luther (University of Delaware) have made significant strides in unraveling the complexity of manganese cycling to a much greater degree than our earlier experiments in the 1990s.
|Map of location of manganese nodules|
Today, Brad Tebo visited us in Mariposa with his wife Margo Haygood. We “skyped” with Kevin reminiscing about this project. Brad and I are planning retirement in a few months, while Kevin is now the Dean of Letters and Science at California State University Maritime, a small campus providing degrees in maritime transportation, business, and engineering.
Kevin’s PhD defense was a real learning experience for all of us. He had decided to invite Scripps’s most influential professors to be on his examining committee. The PhD student presents his/her findings to his/her committee typically in a one hour lecture followed by one to two hours of questioning. After this, there is a deliberation period when the student leaves the room, and the committee discusses the merits of the defense.
It didn’t take long for a fight to break out. Not a fist fight, mind you, but a battle of egos.
Those with the biggest, showiest egos trumpeted the loudest and longest. I was appalled at that behavior. Although Carnegie does not grant academic degrees, I had served on several PhD committees worldwide. I’d never seen an exam go this way. This was Brad’s first student. Tebo, a thoughtful, reflective microbiologist, is on the quiet side. When he speaks, he knows what he’s talking about. At that time, he didn’t have the stature to shut down the egos that were gobbling up the oxygen in the room.
I finally spoke up. I recognized that some of the committee members had not thoroughly read Kevin’s dissertation. It may be—and this is just a guess—that they had pre-judged him. And I think, they even pre-judged me as I heard a lot of man- and even woman-splaining about isotope fractionation and things I was an expert in. I had worked with Kevin and Brad from the get-go on this project. The work was sound and has stood the test of time. I defended Kevin’s work, his writing, and his conclusions. It felt good to stand up for both men. I was well on my way from being the shy, retiring postdoc to the outspoken scientist supporting others who need the help.
Finally, the egos subsided and the PhD was approved!