Mass movements of microbes

14 September 2017
Posted by Chris Smith.

Human population, agriculture, tourism and trade are moving microbes around the world at unprecedented levels, scientists are warning.

Human population now tops 7.5 billion people, and, combined with the animals we rear agriculturally, we outweigh - thirty-five fold - all other mammals in the wild. Human activities move more sand and rock than all natural processes combined, and current estimates suggest that natural erosion displaces only about 20 billion tonnes of material per year, considerably less than the 75 billion tonnes of soil lost owing to agriculture.

Tourism and travel are also booming: more than 1.2 billion international tourist movements happen each year. Meanwhile, boats are dumping millions of tonnes of ballast water - replete with cargoes of exotic microbes - in remote geographies, and hundreds of thousands of square kilometres of agricultural land are  now irrigated by wastewater supercharged with bacteria, antibiotics, heavy metals and disinfectants.

Together, these factors are dramatically and rapidly reshaping the microbial ecosystem, a team of researchers, including Macquarie University's Michael Gillings, argue in a perspective published this week in the journal Science.

This, they say, is driving antibiotic resistance and a loss of microbial diversity. It also threatens to jeopardise the natural biogeochemical relationships that underpin processes like nutrient cycling, the carbon cycle and nitrogen fixing. And because these changes are occurring literally at the microscopic scale, the authors point out, they are easy to overlook.

"Human activity," say the researchers, "is having the same effects on the otherwise invisible microbial world as it is on the world of larger organisms: increasing homogeneity, extinctions of endemic organisms and instability in ecosystem processes."

Gillings and his colleagues point to the wide-scale spread of a genetic component called the class 1 integron as an objective measure of the extent of the problem. This DNA element plays a key role in helping bacteria to acquire and deploy new tricks and traits, like acquiring the genes for antibiotic resistance from their environment.

Winding back the genetic clock, the class 1 integron appears to have originated by chance in a single cell in a single place earlier in the last Century. Now it's in every continent, in a diverse range of bacteria and a range of different hosts, including in the bugs we carry.

"Realisation of the global extent of pollution with these xenogenetic [foreign] elements, and the organisms that carry them, should stimulate questions at a much more global scale," says the researchers, concluding with the poignant counsel that "microorganisms usually perform their essential ecosystem services invisibly, but we ignore them at our peril..."

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