THERE are more microbes on Earth than there are stars in the universe. They occupy every ecological niche, from deep-sea vents to the human gut. So you鈥檇 expect them to be staggeringly diverse. But the latest studies suggest there may be far less variation than thought.
of Liverpool John Moores University in the UK says the diversity of microbes depends on the ease with which they are transported across geographical boundaries. 鈥淥ne of the reasons why there are so many species of big things is because of geographical isolation,鈥� he says. 鈥淏ut if you have widespread dispersal, one would guess that there would be fewer species.鈥�
To see how widely microbes could be dispersed, Wilkinson鈥檚 team used computer models designed for studying the dispersal of dust particles. They modified the particles to be more like microbes 鈥� which tend to be bigger but less dense 鈥� and looked at what would happen if such particles were released from the southern tip of South America.
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To their surprise, they found that airborne microbes smaller than 20 micrometres in diameter could travel thousands of kilometres. Microbes less than 9 micrometres across went as far as Australia. These would include the majority of fungal spores and bacteria, and some amoebae. 鈥淥nce airborne, microbes of 20 micrometres and below can easily get everywhere,鈥� says Wilkinson, who presented the findings at a recent meeting of the in Hatfield, UK.
鈥淭his study has important implications for estimates of the total biodiversity of free-living micro-organisms,鈥� says of the Swiss Federal Research Institute in Lausanne, Switzerland. The fact that microbes can travel huge distances makes it less likely that there are large numbers of endemic species, Heger says.
鈥淢icrobes can travel huge distances, making it less likely that there are a lot of endemic species鈥�
Heger鈥檚 recent work supports this idea. He has been studying the biodiversity of amoebae on the remote Amsterdam Island in the Indian Ocean, which might be expected to harbour a host of unique microbe species, just as the Galapagos Islands support unique species of animals and birds. 鈥淎lthough Amsterdam Island is among the most remote islands in the world, there is no clear evidence for endemism [amongst amoebae],鈥� he says (Journal of Biogeography, ).
Wilkinson鈥檚 work may help us to understand how diseases spread. Most microbes carried by wind are likely to be harmless, but outbreaks of certain diseases, such as meningitis in the Sahel region of north Africa, and coral fan disease, have been linked to dust storms. Another recent study suggests that many more pathogens can be blown long distances with dust, which could protect them against UV radiation, desiccation and extreme heat.
Paraskevi Polymenakou of the Hellenic Centre for Marine Research-Crete in Heraklion, Greece, and her colleagues analysed the DNA content of air on Crete, in the eastern Mediterranean, during a Saharan dust storm. They found DNA from a broad range of bacteria associated with small dust particles that could easily be inhaled. These included relatives of human pathogens linked to pneumonia, meningitis, septic shock and inflammation of the heart (Environmental Health Perspectives, ).
鈥淒ust storms may be associated with more diseases than we once thought,鈥� says Polymenakou. They will also become more frequent, she adds, as climate change increases desertification.
