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Manganese Can Keep Toxic Hydrogen Sulfide Zones In Check Aquatic Systems, Researchers Report In Science

Manganese, in tiny amounts, helps build strong bones in the human body and is thought to enable photosynthesis in plants.

Now a research team from Oregon Health & Science University, University of Delaware, Scripps Institution of Oceanography and the University of Hawaii has discovered there is much more of the dissolved form of the essential element in aquatic systems than previously thought.

Their research indicates that dissolved manganese helps keep zones of a naturally occurring but toxic gas in check. It also has implications for understanding the ways in which metals are cycled through the ocean - perhaps affecting the ability of phytoplankton to photosynthesize and the availability of other essential elements like iron.

The results are reported in today's issue of the journal Science.

The study's lead author, doctoral student Robert Trouwborst, and senior author, professor George Luther, Ph.D., are both from the University of Delaware; co-authors include Bradley Tebo, Ph.D., professor at Oregon Health & Science University; Brian Glazer, Ph.D., assistant professor at the University of Hawaii; and Brian Clement, formerly a doctoral student at Scripps Institution of Oceanography and now a researcher at OHSU.

The study is based on research that was conducted in 2003 during a month-long expedition, funded by the National Science Foundation, to explore the chemistry of the Black Sea. Nearly 90 percent of that mile-deep system is an anoxic, or no-oxygen, zone (popularly known as a "dead zone") containing large amounts of naturally produced hydrogen sulfide lethal to most marine life. Only a few bacteria can survive in the underwater region.

Above this large, underwater anoxic zone in the Black Sea is another layer known as the suboxic zone, containing both tiny amounts of oxygen and neglible amounts of hydrogen sulfide. This layer can have a depth thickness up to 130 feet.

The scientists found that a chemical form of dissolved manganese - Mn(III) - can compose accumulate in the water or come from sediments originating from the continental slope and other sources. It can maintain the existence of the suboxic zone by reacting with both oxygen and hydrogen sulfide, preventing the deadly hydrogen sulfide from reaching the surface layer of water where most fish and algae live. The finding is surprising, Luther says, because dissolved manganese as Mn(III) was assumed not to form in the environment and thus was largely ignored by scientists.

"Geochemists have typically thought only of manganese in its dissolved, reduced form, Mn(II), or in its solid, oxidized form, Mn(IV)," Tebo said. "This research shows that there can be a substantial amount of oxidized manganese that is soluble - in some cases almost all the dissolved manganese in suboxic water columns - and that it is quite reactive."

Luther goes on to say that "The role of dissolved manganese is particularly critical to the Black Sea - the largest body of water in the world containing poisonous hydrogen sulfide - but also is helpful in other waterways such as the Chesapeake Bay, where another element, iron, also plays a minor role in preventing hydrogen sulfide zones from reaching the surface."

During the research mission in the Black Sea, the scientists used a specialized device called an electrochemical analyzer to locate and map the chemistry of the suboxic zone in real time under changing salinity, temperature, and depth. The sensor portion of the analyzer was developed by Luther's group, and enabled measurement of a host of chemicals simultaneously - a huge advance over previous sensors that could measure only one chemical at a time. Samples collected at the same time as the rReadings taken with the analyzer were then measured for particulate and dissolved manganese and used in microbiological analyses by Tebo's group., Band oth groups collaborated in working through the chemistry of the samples.

The research may ultimately be helpful in understanding the so-called "dead zone" off the coast of Oregon. "The Black Sea happens to be a stable anoxic body of water with predictable chemical gradients and a basin that never mixes," Tebo noted, "so it's an excellent environmental laboratory that allows repeat experiments with similar samples. But the system is similar to anoxic and suboxic zones elsewhere."

"In addition," Tebo continues, "the presence of more reactive manganese in the Black Sea system suggests that there is more manganese cycling going on in aquatic systems than had previously been assumed." Since Mn(III) tends to bind to many of the same organic compounds that iron does, competition between the two elements may prove useful in understanding areas of the ocean that seem to be iron-limited, or in understanding their respective roles in providing nutrition for phytoplankton.

"Our research shows that the impact of dissolved manganese (III) is significant in any aquatic environment, including lakes, plus sediments on the seafloor and soils on land," Luther notes. "And for the public who live near the water, dissolved manganese(III) actually helps prevent naturally occurring hydrogen sulfide from getting to the surface, so it prevents both fish kills and the foul odors from this compound's telltale 'rotten egg' smell."

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