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microbe of the month: Clod

A virulent pathogen is bringing speedy destruction to the world's fragile and slow-growing coral reefs. Bernard Dixon reports
With plenty of other things to worry about, scientists would seem perverse to fret over an infection that spreads at a rate of only three millimetres a day. Yet there are good reasons to worry about one particular microbe which travels at this lowly rate. It causes coralline lethal orange disease, known as Clod. Although it does not attack humans, Clod is a real menace. Its victims are the coral reefs of the South Pacific. As the corals and algae which make up the reefs grow at no more than a few millimetres every year, they are in danger of being overtaken and destroyed by their much speedier enemy.

Clod first came to light two years ago, and has already been reported on reefs as far as 6,000 kilometres apart. It is provoking considerable concern among marine ecologists, who have no ideawhere it came from. Pacific reefs have been studied intensively over recent decades, yet no one had ever reported the bright orange bacterium before. Either it came from some other, obscure location, or it is a previously unremarkable organism that has become newly virulent as a result of changes in its genes.

While corals, which are relatives of sea-anemones, are the best-known constituents of coral reefs, so-called coralline algae are also very important in the construction and maintenance of reefs. The corals produce a hard calyx, made from calcium carbonate, which protects them from predators and from which their much softer, finger-like polyps extend for feeding. The algae form additional calcium carbonate as well as cementing together sand and dead coral to form an exceedingly tough, durable structure. Algae are especially important in safeguarding the outer rim of the reef, preventing it from being eroded away by the pounding of waves.

It is these coralline algae which Clod has targeted. In June 1993, researchers found it destroying reefs surrounding Aitutaki Island, one of the group of Cook Islands. Although microbes are, by definition, invisible to the naked eye, aggregations of millions of them can be seen, and the investigators noticed bright orange dots on the reefs' coralline algae.

Despite Clod's deceptively slow rate of advancement on any one reef, it has spread throughout the Pacific, including the Solomon Islands and Papua New Guinea. Last year it was found at every one of 50 different sites examined in Fiji, covering twice as much coral as it had the previous year.

Most of these observations were made by Mark Littler, Diane Littler and others at the Smithsonian Institution in Washington DC. The researchers have now gone one stage further, describing in Science their discovery of the rod-shaped bacterium responsible for Clod. Examining material from diseased algae under the scanning electron microscope, they found that the leading edge of the infection consisted of a "conglomeration of mostly parallel gliding rods that inundated the surface of the host cells". Behind the advancing band of disease, "only stark, white, skeletal material remained". Clearly Clod is the work of a virulent microbe.

Another reason why biologists are interested in what is happening in the South Pacific is that the interaction between coralline algae and what is provisionally termed "Clod pathogen" provides a real-time model for the wider question of how disease-causing microbes interact with their hosts.

Biologists used to argue that extremely lethal microbes were exceptional, and unlikely to endure for very long in evolutionary terms. There was simply no future for a bacterium or virus that killed 100 per cent of its victims: their death would necessitate its demise. Instead, biologists argued, natural selection would facilitate the emergence of organisms causing non-fatal disease in at least a substantial proportion of those infected. Such microbes would never eliminate a greater percentage of their hosts than were necessary to ensure their own further transmission.

While this scenario is valid, greater attention to time and numbers indicates that natural selection can actually increase the virulence of a virus or bacterium. This is likely to happen whenever a disease-causing microbe encounters an abundance of new hosts. It can then cause almost limitless destruction without running out of victims.

Mark and Diane Littler believe that Clod may be facing such a future. Having developed its virulence relatively recently, the bacterium may now become increasingly dangerous as it is transmitted across the oceans to coralline algae in the tropics and elsewhere throughout the world.