Newswise — Methane as a waste of phosphorus capture
Methane, a powerful greenhouse gas, is constantly escaping from the sea into the atmosphere and contributes significantly to global warming. Methane is mainly produced by microorganisms and mainly in places where there is no oxygen. However, a few years ago, researchers demonstrated that bacteria were capable of what is called aerobic methane production. These bacteria produce methane as a waste product when acquiring phosphorus, a nutrient crucial for survival and extremely rare in the sea. Using special enzymes, the bacteria can release phosphorus from organic compounds, such as methylphosphonate. It is important to note that these enzymes also work in the presence of oxygen, for example in surface waters.
Study in the tropical Atlantic: propagation well below the water surface
The distribution and importance of these bacteria as well as their capacities are still little studied and understood. Researchers from the Max Planck Institute for Marine Microbiology in Bremen now present a study in which they investigate bacterial methane production in surface waters off the Caribbean island of Barbados. There is a lot of oxygen and little phosphorus in the water. “So far, this process has only been studied in a few regions, mainly in the Pacific,” explains first author Jan von Arx. “We have now examined it for the first time in the western tropical North Atlantic.” Researchers show that methane production is higher near the water surface. “But we were also able to detect methane at depths of up to 200 meters, even though at these depths there is actually enough phosphate and the bacteria would not need to use methylphosphonate,” explains lead author Jana Milucka, head of the Greenhouse Gas Department. Research group of the Max Planck Institute in Bremen. Additionally, the types of bacteria that produce methane despite the presence of oxygen also change with depth: while cyanobacteria Trichodesmiuma well-known and widespread marine primary producer, dominated surface methane production, called Alphaproteobacteria were predominant at the greatest depths.
Atmospheric carbon: give and take
By using methylphosphonate, an otherwise unavailable source of phosphorus, it is possible for bacteria to fix more carbon in surface waters than if they relied solely on phosphate. “According to our calculations, bacteria can cover around a tenth of their phosphorus needs with methylphosphonate,” explains von Arx. “This allows them to remove significant amounts of carbon dioxide from the atmosphere in this region. This clearly highlights the ecological importance of phosphonates in the carbon cycle of nutrient-poor ocean regions.
On the one hand, microorganisms capture carbon dioxide, but on the other hand, they release methane, a much more potent greenhouse gas. “Our study shows high methane production in the oxygen-saturated water column – something that has long been considered impossible, but is now increasingly observed,” explains Milucka. “As the bacteria involved are present in all the world’s oceans, the methane produced from methylphosphonate likely contributes significantly to the release of this greenhouse gas from the sea, particularly in phosphate-poor environments.”
Increased methane emissions due to climate change?
The amount of methane released into the environment depends on the ratio between its production and its oxidation. “However, we still do not have a clear idea of where methane in the ocean comes from and how it disappears. We also don’t know how these so-called sources and sinks of methane in the ocean will respond to ongoing climate change,” says Milucka. “We believe that aerobic methane production will increase in the future as phosphate becomes even more scarce due to ocean warming and the resulting stronger stratification of the water column. This is problematic because this process takes place in surface water and the methane thus produced can therefore escape immediately into the atmosphere,” adds von Arx.
In order to be able to predict future changes in climate-impacting gas emissions, the processes involved and the driving factors need to be further researched. “If we understand how a process works, we are more likely to predict and/or counteract its negative effects,” concludes von Arx.
General information: The marine methane paradox
In 2008, American researchers made a remarkable discovery: they showed how methane can be formed in the presence of oxygen – so-called aerobic methane production. It may sound trivial, but this study solved one of the world’s oldest mysteries of methane biogeochemistry: the so-called marine methane paradox. The methane paradox refers to the supersaturation of methane in oxygen-rich surface waters – a place where methane production should not take place because oxygen is traditionally harmful to methane-producing microorganisms (archaea) . The recently discovered process of methane production in oxic waters is carried out by bacteria that use an enzymatic pathway insensitive to oxygen. Unlike classical methane-producing archaea, bacteria do not produce methane to obtain energy from this process. In their case, methane is a by-product of a reaction which is mainly used to obtain phosphorus. Because inorganic forms of phosphorus (such as phosphate) are only available in small quantities, many marine bacteria are forced to rely on organic phosphorus (such as phosphate esters and phosphonates). The latter group of compounds, specifically methylphosphonate (MPn), has been shown to serve as a precursor to aerobic methane production in the sea.