Review
Aerobiology and the global transport of desert dust

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Desert winds aerosolize several billion tons of soil-derived dust each year, including concentrated seasonal pulses from Africa and Asia. These transoceanic and transcontinental dust events inject a large pulse of microorganisms and pollen into the atmosphere and could therefore have a role in transporting pathogens or expanding the biogeographical range of some organisms by facilitating long-distance dispersal events. As we discuss here, whether such dispersal events are occurring is only now beginning to be investigated. Huge dust events create an atmospheric bridge over land and sea, and the microbiota contained within them could impact downwind ecosystems. Such dispersal is of interest because of the possible health effects of allergens and pathogens that might be carried with the dust.

Introduction

Dust clouds generated by storm activity over arid lands can result in soil particulates being transported to altitudes >5 km [1]. The Sahara–Sahel region of Africa is the largest source of aerosolized soil dust on Earth, contributing as much as one billion metric tons of dust yr−1 to the global atmosphere [2]. Deserts continuously discharge dust, but it is the large-scale dust events, visible from space and capable of crossing oceans (Figure 1), that could have the biggest impact on the biology and ecology of downwind ecosystems. The intercontinental transport of African desert dust has been studied for decades [3], but, as we discuss here, research on the biological particles traveling between continents with the dust has only recently been initiated (Table 1). The topic has generated interest because of concerns about health effects of allergens carried in the dust 4, 5, 6 and the possible transport of pathogens [7].

Section snippets

Dust (and biology) in the wind

Several mechanisms contribute to the microbial load of African desert dust, in addition to the ∼106 bacteria g−1 of soil estimated to occur naturally [8]: as they move across the continent, local winds lift large quantities of arid soil. Garbage disposal in many parts of Africa is accomplished by burning [9], which can contribute bacterial and fungal spores in the rising smoke [10]. Finally, the trade winds blow the dust out over the Atlantic Ocean, where additional marine microorganisms

Evidence for LDD of microbiota via dust events

There is no doubt that microbes and pollen are contained within these large desert dust events; however, only recently have data been presented that implicate these dust events as mechanisms for transporting aerosolized microbiota around the globe. These data come from satellites and classical microbiology and molecular biology studies.

Consequences of the LDD of desert dust

Interest in the LDD of desert dust has been increasing as questions have arisen about the potential effects of associated chemical pollutants and pathogenic microbes on human health and ecosystems 9, 33, 34. Recent work in this area raises issues of pathogen transport and the biogeography of microbes and pollen.

Future directions

While more work is clearly required on both Asian and African dust systems, it would be of interest to have microbiological and pollen data from the large Australian dust events that impact New Zealand and the southern Pacific. Each dust system is likely to have unique microflora owing to regional geographical influences; once standard methodologies have been defined (Box 2), it will be of interest to compare the three systems.

The community composition and frequency of occurrence data generated

Summary

Huge dust events create an atmospheric bridge over land and sea. Although satellite images leave no doubt that desert dust particles have an intercontinental distribution, we are only now beginning to address the questions relating to the dust-associated biological particles that also make this trip (how many, how often, and even which types). Data suggest that dust events can transport pathogens and allergens with the potential to impact the health of downwind populations and ecosystems. The

Acknowledgement

We thank Betsy Boynton for creating the original illustration used in Figure 2.

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