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
Tuesday 20 June 1995
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.
- 1 JK Rowling horrified by Harry Potter actor Matthew Lewis's raunchy photoshoot
- 2 As a white man, I'm surprised more women aren't tweeting the hashtag #KillAllWhiteMen
- 3 Saudi Arabia 'seeking to head United Nations Human Rights Council'
- 4 Georgia Army veteran arrested for breaking window to save dog has charges dropped
- 5 New Zealand 'the best country to work as a prostitute', says sex worker advocacy group
JK Rowling horrified by Harry Potter actor Matthew Lewis's raunchy photoshoot
Evany José Metzker dead: Decapitated body of Brazilian journalist and blogger investigating child prostitution ring found
Georgia Army veteran arrested for breaking window to save dog has charges dropped
Saudi Arabia 'seeking to head United Nations Human Rights Council'
The ten most unequal developed countries in the world
Scotland may have to leave the EU even if it votes to stay in, David Cameron confirms
Report finds that Britain's wages are the most unequal in Europe
As a white man, I'm surprised more women aren't tweeting the hashtag #KillAllWhiteMen
Almost a third of school pupils believe 'Muslims are taking over our country', study claims
Labour leadership: Battle lines are drawn as members battle over party's ideology at first hustings of the contest
Gay marriage 'Bert and Ernie' cake bakery found guilty of discrimination in Northern Ireland
£35 - 40k + Benefits: Guru Careers: We are seeking a Software Developer (JavaS...
£18000 - £23000 per annum + Commission: SThree: As a Trainee Recruitment Consu...
£25000 - £30000 per annum: Ashdown Group: Graduate UI Application Developer - ...