Sulfur Switch
In addition to the well-known greenhouse gas CO2 and the ozone killers CFC, there are other climate-relevant substances: One of them is a waste product of sea plankton, promotes cloud formation and goes by the abbreviation DMS. But some microorganisms extract it from the cycle.
A highly volatile, colorless, flammable, toxic liquid found in coffee and cocoa and used as a pheromone on some insects. The properties of dimethyl sulfide - DMS for short - don't sound very exciting. Only a few botanists and entomologists were interested in the simple molecule, which consists of two methyl groups linked by a sulfur bridge.
That changed abruptly in the 1970s when the chemist James Lovelock - the father of the "Gaia hypothesis" - drew the world's attention to the climate-relevant role of DMS: dimethyl sulfide is oxidized in the atmosphere to sulfuric acid; the resulting droplets promote cloud formation and thus cool the earth.
So where does the cloud factor come from? From the sea. Here, plant and bacterial plankton organisms produce a substance called DMSP (dimethylsulfonium propionate) from sulfates that are abundant in seawater, which they use as a versatile protective agent against herbivores, frost and free oxygen radicals or simply to dispose of sulphurous waste. When the cells die or are nibbled on by predatory zooplankton, the enzyme DMSP-lyase breaks down the substance into acrylic acid – and DMS – in the final agony. An estimated twenty million tons of sulfur enter the atmosphere from the sea every year; the DMS release from the oceans is considered the largest natural sulfur source.
No wonder atmospheric chemists are keenly interested in the small molecule. However, calculations of the DMS turnover, which was estimated using the plankton density, have so far failed. The fate of the precursor substance DMSP is obviously more complicated than initially assumed.
Meanwhile it has been shown that released DMSP is not completely converted into DMS. Rather, some plankton, such as the cyanobacterium Synechococcus, reabsorb it. The researchers led by Maria Vila-Costa from the Institute of Marine Sciences in Barcelona now wanted to know whether there are other DMSP feeders [1].
Indeed, the marine researchers found what they were looking for. In water samples from the Gulf of Mexico, the northwestern Mediterranean Sea, the waters around Gran Canaria and from the Sargasso Sea in the Atlantic, a group of organisms emerged whose role in the sulfur cycle had apparently been completely underestimated: diatoms. The unicellular algae, which occur in huge numbers of individuals, are considered an important carbon pump, removing CO2 from the atmosphere through photosynthesis and transferring it to the depths of the sea as it sinks. However, since their cells contain little DMSP, their role in the sulfur cycle was thought to be insignificant.
A mistake. As experiments with radioactively labeled DMSP have shown, the diatoms literally suck up the substance as soon as they feel bad. Since plankton density varies greatly with the seasons, the ocean's DMS release also fluctuates accordingly.
The research group led by Mary Ann Moran of the University of Georgia delved even further into the depths of strain gage production [2]. "These are necessary steps to understand how our planet works "
(William Whitman) The precursor substance DMSP does not necessarily have to be broken down into dimethyl sulfide. It has long been known that another enzyme, DMSP demethylase, occasionally snatches a methyl group from DMSP, converting it to methyl mercaptopropionate. This can then be broken down further into mercaptopropionate or methanethiol - in any case, it is not affected by climate change.
Certain photosynthetic bacteria, such as Silicibacter pomeroyi, possess this DMSP killer, and the researchers were now looking for the relevant pathway gene. In an S. pomeroyi mutant that did not produce methanethiol, they discovered an alteration in a specific section of the bacterial chromosome. This genetic factor codes for the first step: the demethylation of DMSP. The gene was found and named dmdA.
The researchers then searched for traces of dmdA in water samples from the Sargasso Sea - with success: it is estimated that it contains a third of the bacterioplankton on the sea surface and can therefore prevent the release of DMS into the atmosphere. Both bacteria in coastal waters and on the high seas seem to have a significant impact on the sulfur cycle and thus on the climate.
"This breakthrough in the microbial physiology of DSMP metabolism opens a door for us to finally understand the biology and ecology of this globally important process," said William Whitman, who helped discover the sulfur gene switch. "These are necessary steps to understand how our planet works."
