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Polizones de la calima: miles de millones de microbios viajan en las tormentas de polvo

Calima sobre Las Palmas de Gran Canaria en febrero de 2020. / Bengt Nyman


The Saharan particles that redden the Canarian sky carry biologically active bacteria, viruses, fungi and archaea. Researchers around the world, including in Tenerife, are studying this great air belt that fertilizes the planet and can affect human health. His discovery has been a surprise.

Monica G. Salomone 

For two years now, researcher  Cristina González Martín , from the University of La Laguna, has been taking dust samples near the Teide volcano, 2,400 meters above sea level in Tenerife, to study which microorganisms travel like stowaways attached to the particles. She also does the same in the city of La Laguna, at about 600 meters above sea level. Her objective is to find out if part of these microbes that fly over the island’s airspace make a stop on the island.

It is the starting point for analyzing its possible effect on human health, for example, dispersing antibiotic resistance genes. There is evidence of the airborne transport of these genes, as well as of pathogens —especially fungi that attack plants— in other parts of the planet. In the Sierra Nevada and Pyrenees, specifically, airborne microorganisms potentially harmful to plants, animals and humans have been found.

González recalls that in the Canary Islands the relationship between the arrival of Saharan dust and a higher incidence of asthma and allergies is well documented. But this has traditionally been attributed to the physical effect of the inhaled particles, never until now has it been studied whether pathogens arrive with them.

In recent years, a microbiome of the troposphere has been discovered that is more than 99% unknown

Actually, awareness of the biological importance of atmospheric dust is relatively recent. In recent years it has been discovered that there are millions and millions of bacteria, viruses, fungi and archaea in the atmosphere. An entire microbial community, a microbiome of the troposphere, that is more than 99% unknown. Some describe it as a global genome that engulfs the Earth.

One of the pioneers of his study is  Dale Griffin , a microbiologist at the US Geological Survey and Cristina González’s thesis director. «When we began to study the aerial dispersal of microorganisms, about twenty years ago, there was a lot of skepticism,» Griffin tells SINC. The troposphere was considered an environment practically sterilized by ultraviolet radiation.

«We now know that there are large clouds of desert dust that harbor very diverse microorganisms, which survive and travel very long distances,» adds Griffin. «But we still have a lot to learn.»

Polizones de la calima: miles de millones de microbios viajan en las tormentas de polvo

Image of the Canary Islands on February 23, 2020 on the web and shared on Twitter by journalist Javier Salas.


10,000 bacteria in one gram of dust

The deserts emit every year 5,000 million tons of dust, tiny grains —of between a half and several thousandths of a millimeter— that form authentic air highways of particles around the planet. Satellite images show them coming mainly from the Sahara, but not only. One million tons  come to Japan every year  from the Gobi and Taklamakan.

Microorganisms, an enormously abundant and diverse ‘aeroplankton’, adhere to the grains. In one gram of airborne dust there are about  10,000 bacteria .

Dust and its occupants travel thousands of miles and then fall down. From the Sahel to the Pyrenees it takes three days; to cross the Atlantic, one week

The vast majority are beings that we do not know how to cultivate in the laboratory, so their diversity has only been able to come to light through genetic markers.

«The markers tell us that we have thousands of different entities, but in reality we don’t know who they are or what they do,»  Emilio Casamayor , director of the CSIC’s Center for Advanced Studies in Blanes, explained to SINC.

Dust and its occupants travel thousands of miles and then fall down, dry or in rain or snow. From the Sahel to the Pyrenees it can take three days; to cross the Atlantic, one week. The heaviest fall first; the viruses, in the smallest grains, travel further. What happens in the place where they land is just beginning to be investigated now.


They fertilize the Amazon and disperse pollutants

Part of its impact is due to the chemistry of the particles. For example, the Amazon jungle would not be without the constant contribution of Saharan dust, with phosphorus and other minerals that fertilize  a poor soil , constantly washed by tropical rains.

But in the dust there are also contaminants such as  flame retardants  or  endocrine disruptors . It is possible to detect them at the LTER high-mountain observatory of Global Change in the Aigüestortes National Park, in the Pyrenees, «a potentially pristine place where these compounds can only come from the sky,» says Casamayor.

Sampling station of the Spanish Long-Term Ecological Research Network (LTER).

At this observatory, Casamayor has carried out  the  longest study on aeroplankton in time. Twice a month for seven years his group took samples of rain and snow.

Their genetic analysis has shown not only that most of the sequences found are very similar to those described in other parts of the planet, but also that the microorganisms that arrive change depending on the seasons. This data suggests an organized, cyclical system with a specific ecological function.

A year ago it was revealed that more than 800 million viruses are deposited every day in one square meter of Sierra Nevada

Another study point for aeroplankton is in the Sierra Nevada, at about 2,500 meters of altitude – at that altitude the dusty highways of the troposphere intersect, and dust is known to arrive from other continents. Isabel Reche , from the University of Granada,  revealed  a little over a year ago that «more than 800 million viruses» are deposited every day in one square meter of Sierra Nevada.

This is the first work «that quantifies the magnitude of the dispersion of viruses through the troposphere,» says Reche, and also explains «the enigmatic discovery» 20 years ago of genetically similar viruses «in very different and distant environments.»

So much resemblance does not add up because the genetic diversity of viruses is high even within the same population. It now seems clear that viruses can travel very far, very fast, and not necessarily by plane.


human pathogens

Casamayor assures that «it is not an alarming phenomenon» from the point of view of human health but admits that «it is a wake-up call to raise our heads and also look up.» Among the potential pathogens found are those related to allergies and respiratory diseases.

“It is common to find pathogens, what determines the outbreaks is how many they are [low, in this case], and if the environmental conditions and the host’s immune system favor their growth. Furthermore, we have not verified whether the variant is virulent, what we show is the potentiality of the phenomenon”.

Griffin, for his part, does not doubt that «diseases caused by bacteria, fungi, viruses or protozoa can be transmitted by dust storms,» ​​he told SINC. “We have identified numerous plant or animal pathogens. In Taiwan, the influenza virus has been identified   in the atmosphere when dust from China arrived in the region.


antibiotic resistance

Another possibility is that microorganisms carrying potentially harmful genes arrive. It is important, because the inhabitants of the microbial world pass genes to each other easily. This gene trafficking would allow, for example, antibiotic resistance genes «to be incorporated into local pathogens,» writes  Yinon Mazar  of the Weizmann Institute in Israel in a 2016 paper.

Air traffic of harmful genes would create superbugs immune to antibiotic treatments

This scenario is «particularly worrying» for Mazar, because it would create  superbugs  that are immune to the treatments. This researcher is one of the few who have found antibiotic resistance genes in airborne dust.

Casamayor, Griffin and González also look for them in their samples.

Among the questions that remain to be answered is how microorganisms manage to survive in the atmosphere. It is known that part of them use the dust grains as a shield against UV radiation and make the journey latent, without reproducing in the atmosphere but capable of reproducing massively if they land in the right place.

Others, the least, maintain their metabolic activity during the trip and reproduce in the air; they surely participate in chemical reactions that occur naturally in the atmosphere. Understanding all this may also be relevant when looking for  life on other planets .

The truth is that investigating the microbiome of the troposphere has become a booming area, fueled by satellite data and genomics. Also, due to its relationship with the  global pattern of rainfall . Because everything indicates that climate change is altering a key ecological process for the Earth, even before its operation is understood.


A global genome that strengthens ecosystems

The microbiome of the troposphere could fulfill an ecological function related to the maintenance of ecosystems: it would function as a global network of genetic resources that come into play depending on the environment in each region. “A kind of interconnected functional global genome”, explains Emilio Casamayor, “that allows microbial life to respond quickly to environmental changes”. If a sudden change kills the microorganisms in a certain place, other fallen from the sky that could take advantage of the new conditions.

Isabel Reche uses the ‘gene bank’ concept in  a recent work  : “The dispersal of viruses and bacteria over long distances can contribute to increasing their distribution in a dormant or inactive state but with the capacity to activate quickly.(…) An influence An important release of viruses and bacteria from the atmosphere can have effects on the receiving ecosystems (…), providing biological functions that should allow ecosystems to quickly adapt to environmental changes”.


From Darwin

It is not an entirely new phenomenon to science. In the mid-19th century,  Darwin became interested in African dust  with microorganisms collected on ships far away from the continent. A century later, the versatile aviator Charles A. Lindberg took samples of arctic air in which microalgae and spores were found, and warned in  The Scientific Monthly  of the need for more research on the matter.

But it is only in the last decade that people have begun to appreciate how abundant and diverse the microbial community in the troposphere, the ‘aeroplankton’, is .


Source:  SINC
Rights:  Creative Commons
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