Functional and molecular biodiversity of the bacterial production of the climate-changing gas dimethyl sulphide.

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We've all been to the seaside and we've all been told by a knowing parent to 'breathe in that ozone', because it's 'good for you'. Well, firstly, it's not ozone and second, it's not terribly good for you. That distinctive aroma is, in fact another gas, called dimethyl sulphide (DMS) and it has been known since 1971 that it is hugely important, with some 30 million tons of it being liberated into the air, world wide, every year. And once in the atmosphere it has other major effects, being the 'seed' that sets off cloud formation over the oceans. Indeed, the production of this molecule is on such a scale that it has major effects on the world's climate, thanks to its effect on the cloud cover over the oceans. Yet, despite all this, we have only very recently begun to understand, at a molecular level, how this process occurs. This is all the more surprising since we have known for some time that many marine bacteria, some of which are easy to grow on the lab, can liberate DMS if supplied with the key precursor molecule, called Dimethylsulphiopropionate - DMSP for short. Not a compound one reads about every day, yet there are over two billion tonnes of it in the world's oceans, seas and seashores. That's the weight, give or take, of another seaside symbol, the Blackpool Tower - 70,000 times over. Amazing. The DMSP is used by the great masses of marine plant life - seaweeds and microscopic plankton - as a buffer, or osmo-protectant, against the saltiness of the sea, and against other stresses. When these plants die, some of the DMSP that escapes from them is used as food by some marine bacteria and, when they do so, they convert some of it to that DMS gas in the process. We recently isolated one such DMSP-consuming bacterium, called Marinomonas, from the Norfolk coast. We used various molecular techniques to get our hands on some of the genes that are involved. By looking at their sequences, we could guess what the genes might be doing and, so far, it looks as if the mechanisms are very different from those hypothetical ones that had been proposed before. We also saw that very similar genes exist in some other, very unexpected, types of bacteria, such as those that live, symbiotically, on the roots of land plants. So the extent of DMS production by bacteria may be far wider and varied than we had thought. It was also very striking that other bacteria that are known to make the DMS gas from DMSP do not contain the gene that we discovered in 'our' strain of Marinomonas. So, there must be some fundamentally different ways in which different bacteria can break down DMSP. We now plan to sample all sorts of environments that are known to be very rich in DMSP and to isolate DMSP-degrading bacteria in a search for these 'novel' forms of DMS emission. These environments will range from the mouths of giant clams, to the roots of some plants in Hawaii, to the open seas (especially when they have just had a massive 'bloom' of tiny plankton cells that liberate huge amounts of DMSP in their death throes) and also the root surfaces of some of the land plants that exude DMSP. We will also look for the genes in some of the bacterial species that are already known to be DMS producers, but which do not have the genes that we had identified in Marinomonas. All this will let us amass a genetic inventory of the different ways in which this climate-changing process occurs in different bacteria. So, in the not-too-distant future, we, and others, can use this information to make molecular tools that will allow us to investigate, even more thoroughly, the biodiversity that underpins the smell of the seaside.