The weather gene

Credit: Right: © Ron Bergeron"> Credit: Right: © Ron Bergeron Andrew Johnston, of the University of East Anglia in Norwich, United Kingdom, is usually found studying the molecular genetics of the nitrogen-fixing bacteria that live in the roots of leguminous plants such as peas and beans. One day, he found himself in a chance lunchtime conversation about dimethyl sulfide (DMS). The DMS story started in 1972 with the publication of an influential paper by James Lovelock in Nature, which showed that DMS produced by marine plankton was the major natural source of atmospheric sulphur. With colleagues, Lovelock went on to develop a model for a climate feedback mechanism involving DMS, which was known to seed the formation of clouds. He suggested that on cloudless days, plankton produce more DMS, which seeds cloud cover, causing DMS production - and, therefore, cloud cover - to fall again. Johnston learned that tens of millions of tons of DMS per year enter the atmosphere. The genes that enable marine bacteria to produce DMS, it stood to reason, could be considered genes for weather. While research dollars were being plowed into testing Lovelock's ideas (by the National Oceanic and Atmospheric Administration (NOAA), for example) basic research on the biology of DMS hadn't moved far. "It intrigued me that no one had done anything - anything! - on its molecular aspects," says Johnston. He and his postdoc, Jonathan Todd, set about plugging the gap by enlisting undergraduate students and the remnant of an unrelated research grant. Rather than heading for the open ocean, their starting point was an English coastal salt marsh, where DMS output is "about 10,000 times higher per square meter than it is in the middle of the Pacific," says Johnston. Within a month they had the gene and set about sequencing it. "The whole thing took three months start to finish," says Johnston. The result, published in Science (315:666-9, 2007) was the dddD gene, whose product produces DMS by cleavage of a precursor molecule called dimethylsulfoniopropionate (DMSP), which marine algae produce. At least one atmospheric scientist thinks it was a significant advance. "I see this as one more piece in a very complicated puzzle," says Timothy Bates of the Pacific Marine Environmental Laboratory at the NOAA in Seattle. "We are a long way from incorporating this type of genetic work in climate models, but in the long run it will be essential to predict future climate." Johnston now has the luxury of a proper research grant from the UK Biotechnology and Biological Sciences Research Council to explore further the long-neglected molecular aspects of DMS production. There is the question of why bacteria should produce DMS in the first place, for example. After all, some marine bacteria metabolize DMSP by demethylation rather than cleavage, which does not leave DMS as a waste product. As Johnston puts it, producing DMS is like "a hungry man leaving half his dinner on the side of his plate." Is the dddD gene common to all DMS producers? Molecular fishing of genomic databases is already proving fruitful in that regard, says Johnston. "We've had it crop up in the Sargasso Sea, and just last week, we found it in a bacterium that lives in association with a plant in mangrove swamps in West Africa." Johnston even considers it "more than likely" that bacteria are not the sole producers of DMS. It might yet turn out that the algae that produce the DMSP also manufacture DMS, perhaps with the help of bacterial symbionts. He says researchers still have a lot of catching up to do. The molecular biology of DMS production is at a stage equivalent to a farmer looking at his field of peas and wondering if it is the peas themselves that fix the nitrogen or the bacteria that live in their roots. "It's 2007, and we don't know the answer," he says. "That's embarrassing."

The weather gene

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