Morphing manganese
An often-overlooked form of manganese, an element critical to many life processes, is far more prevalent in ocean environments than previously known, according to a study by U.S. and Canadian researchers published this week in Science.
The discovery alters scientists’ understanding of the chemistry that moves manganese and other elements, like oxygen and carbon, through the natural world. Manganese is an essential nutrient for most organisms and helps plants produce oxygen during photosynthesis.
“You wouldn’t think manganese is that important, but without manganese, we wouldn’t have the molecular oxygen that we breathe,” said study co-author George Luther, a professor of oceanography in the School of Marine Science and Policy within the University of Delaware’s College of Earth, Ocean, and Environment.
Manganese is present in the environment in three forms — manganese(II), manganese(III) and manganese(IV) — the difference related to the oxidation state, or number of electrons present. When elements lose or gain an electron, the oxidation state changes in a “redox reaction,” as when iron turns into rust by losing electrons to oxygen in air.
The second-most common metal in the earth’s crust, manganese rapidly changes between oxidation states while reacting with other elements in the environment.
Traditionally, dissolved manganese(II) and solid manganese(III, IV) were believed to be the dominant forms in aquatic environments, but, in the mid-2000s, Luther found that manganese(III) was also present in dissolved form in a Black Sea “transition zone,” an area where oxygen levels are relatively high near the surface but gradually diminish deeper down in the water.
Suspecting that this intermediary form wasn’t confined to the unusual conditions of the Black Sea, he and his Canadian colleagues Alfonso Mucci of McGill University and Bjørn Sundby, a professor at the University of Québec at Rimouski and adjunct professor at McGill, set out for the St. Lawrence maritime estuary, east of Tadoussac and the Saguenay Fjord. Previous research led by Mucci, a professor of geochemistry and oceanography in McGill’s Department of Earth and Planetary Sciences, had drawn attention to the oxygen-depleted bottom waters of the St. Lawrence Estuary.
“The sediments in the estuary are particularly rich in iron and manganese oxides,” Mucci notes. “… and we have about 30 years of manganese and ancillary geochemical data in the St. Lawrence,” making the area particularly well-suited to the research.
The researchers pulled up samples of mud from the seafloor, where in the top few centimeters of sediment, there is also a transition zone of diminishing oxygen levels. Andrew Madison, lead author on the Science paper and Luther’s former graduate student, used a new technique to differentiate between manganese forms.
His results showed that manganese(III) comprised up to 90 percent of the total dissolved manganese present in the Canadian study sites. The implication is that this dissolved form of the metal is found in other marine environments where there is a gradation of oxygen concentrations, whether in the water column of the Black Sea, sediment in the St. Lawrence Estuary or a Delaware salt marsh.
“We saw it all through the St. Lawrence Estuary where we studied,” Luther said. “We did some work in a local Delaware salt marsh and found it. Wherever we’ve been able to look for it, we’ve found it. By implication, it should be found in all ocean sediments.”
The findings help explain anomalies in manganese models that have puzzled scientists. Other researchers studying manganese did not make specific measurements for manganese(II) versus manganese(III), Luther said. Rather, they measured total dissolved manganese and assumed it was the former.
This missing link in the manganese cycle may shed light on the complex connections between the biology, geology and chemistry — called biogeochemistry — in ocean environments.
The biogeochemistry of marine sediments revolves around organic matter, such as bits of dead algae, that falls through the water to the bottom of the ocean. Bacteria consume that debris, setting off a chain of redox reactions.
In their paper, the researchers call for the conceptual model of the sedimentary redox cycle to be revised to include dissolved manganese(III).
“Manganese is helpful to produce organic matter in the surface waters through photosynthesis,” Luther said. “But in the sediments, the higher oxidation state manganese is used to decompose organic matter. So it’s a really interesting cycle.”
The study, titled “Abundant porewater Mn(III) is a major component of the sedimentary redox system,” appears in the Aug. 23 issue of Science.
The research was funded by the National Science Foundation and the Natural Sciences and Engineering Research Council of Canada. The research was conducted in 2009 and 2010 on the research vessel Coriolis II, with Mucci as chief-scientist. The vessel is owned by a consortium of Quebec universities, including McGill, and operated by Reformar, a non-profit organization based at l’Institut Maritime du Québec in Rimouski.
IMAGE CREDIT: Courtesy of George Luther, University of Delaware