27/10/2024
African Dust Carries Microbes Across the Ocean :
Are The y Affecting Human and Ecosystem Health?
Atmospheric transport of dust from
northwest Africa to the western Atlantic
Ocean region may be responsible for
a number of environmental hazards,
including the demise of Caribbean
corals; red tides; amphibian diseases;
increased occurrence of asthma in
humans; and oxygen depletion (eutro-
phication) in estuaries. Studies of
satellite images suggest that hundreds
of millions of tons of dust are trans-
ported annually at relatively low alti-
tudes across the Atlantic Ocean to the
Caribbean Sea and southeastern United
States. The dust emanates from the
expanding Sahara/Sahel desert region
in Africa and carries a wide variety of
bacteria and fungi .
The U.S. Geological Survey, in
collaboration with the NA SA /Goddard
Spaceflight Center, is conducting a study
Figure 1. The satellite image, acquired by NASNGoddard Spaceflight Center's SeaWiFS Project and ORB IMAGE
on February 26, 2000, shows one of the largest Saharan dust storms ever observed by SeaWiFS as it moves out
over the eastern Atlantic Ocean . Spain and Portugal are at the upper right Morocco is at the lower right.
Figure 2. This half of an air filter represents 40 liters of air (roughly the amount it would take to fill a 10 gallon
aquarium) sampled during a dust storm in Mali, Africa . The fil t er is placed on nutrient media for 48 hours so
the viable microbes can grow . The shiny, colorful circles (ind i cated by the black arrows) are bacterial colo-
nies . The fuzzy patches (indicated by the red arrows) are fun gi. These colonies range in size from about 1 to
5 millimeters .
U.S. De partmen t of the Interi or
U.S. Ge ol og ic al Survey
to identify microbes -bacteria , fungi ,
viruses-transported across the Atlantic
in African soil dust. Each year, mil -
lions of tons of desert dust blow off the
west Aftican coast and ride the trade
winds across the ocean, affecting the
entire Caribbean basin, as well as the
southeastern United States. Of the dust
reaching the U.S., Rorida receives about
50 percent, while tl1e rest may range
as far nortl1 as Maine or as far west
as Colorado. The dust stonns can be
tracked by satellite and take about one
week to cross the Atlantic.
How many microbes can a dust event
carry? There are not enough data at this
point to answer that question. However, a
conservative estimate of 10,000 microbes
per gram of soil, suggests that in 1 mil-
lion tons of dust (airborne soil) there
would be 10 quadrillion (1016
) microbes!
Conventional wisdom says ultraviolet
radiation from the sun would kill most
microbes during the 5- to 7-day trip
US GS Open -File Repor t 03-028
Jan uary 20 03
Figure 3. The eight brightly colored bacteria streaked
on this petri dish of agar were isolated from a dust
event in Africa. While many bacteria are colorless
or off-w hite in appearance, the intense colors and
opaque appearance of these streaks are examples
of how deeply pigmented many of the dust bacteria
are . Having lots of pigment is thought to help shield
the microbes from harmful solar radiation, sort of like
wearing sunscreen .
across the Atlantic. Our studies thus far
indicate that of the microbes that become
airbome, hundreds are surviving the aeri -
al joume y in each gram of dust, apparent-
ly sheltered within the particle s, shrouded
by protective pigments, or shielded by
overlying dust layers . Air samples taken
in the Virgin Islands show an increase of
3 to 10 times as many microbes during
dust events than during clear conditions.
Air samples taken in the countTy of Mali,
west Africa, can contain as many as 15
viable bacteria per liter of air (approxi-
mately one breath).
What types of microbes are in the
dust? Bacteria, fungi, and viruses - some
of tl1em patl10 genic (capable of causin g
diseases) and some of them common to
many environments. Roughly 30 percent
of tl1e bacteria isolated from airbome soil
dust are known patl10gens, able to affect
plants, animals, or humans.
What arc the po ss ible impacts of
these microbial travelers on downwind
ecosystems? USGS scientists have
monitored coral reef vi tality for nearly
40 years and have observed that the
Caribbean and Borida coral reefs have
been in a state of decline since tl1e late
1970s. Moreover, a number of other
mmine species, including the grazing
sea urchin , Diadema , and sea fm1s , have
experienced a widespread and sudden
demi se, roughly coincident with the
deaths of the stony coral populations
and increased pulses of African dus t.
Recent studies at the University of South
Carolina Aiken have identified several
species of soil fungi , Aspergillus, in
Africm1 dust samples collected from the
Caribbean atmosphere. One of these ,
Aspergillus sydowii, has been shown to
be tl1e causative agent of a disease in sea
fm1 s (now known as aspergillosis). Many
coral diseases are not well characterized
and could possibly be caused by microbes
that may have rained down from passing
dust clouds.
It is also po ss ible that microbes in
the A fJi can du st could be linked to some
of tl1c well -known outbreaks of infectious
di seases in endangered marine spec ie s,
such as manatees, dolphins , a nd tmtlcs.
Loggerhead sea turtles in th e Canary
fslands (off the west coast of Africa) have
been dying from a bacterial infection
caused by Slaphylococcus xylosus, which
has been found in a dust sa1.11 ple taken in
Mali.
The soil dust contains a number of
plant and animal pathogens tJwt could
affect agticulture and livestock in down-
wind areas. Fungal di seases, affecting
commercial crops like sugarcane and
bananas, have appeared in the Cm ibbc an
witl1in a few days after aJ.l outbreak in
Africa, suggesting the spores could have
traveled in a dust storm . Our research
has identified bacterial pathogens of ri ce
and beans in the Caribbean air smnplcs,
as well as those that cause disease in
fruit and a variety of trees, from Afri cm1
air samples. It has been speculated
that African dust may catTy tl1e virus
responsible for Foot and Mouth Disease
(which is endemic to sub -S al1 aran A fri ca)
because tentative links have been made
between dust storms that pas se d over
Great Brit.:'lin m1d subsequent outbreaks
of the disease at multiple points. We wiU
be testing future air smnples for evidence
of tlli s and other viruses. The dust h as
Figur e 4. This image , acquired by NASA's Earth Probe TOMS (Total Ozone Mapping Spectrometer) satellite, shows pulse s of Afri can dust moving across the Atlantic
Ocean . Our study sites, the Virgin Islands and Mali, have been outlined for clarity.
been found to contain bactetia that cause infec-
tions in birds, pigs, and cattle.
The African dust events also have a direct
effect on human health. During a du st event,
airborne particulate concentrations in Mali
exceed international health standards tenfold.
African dust clouds arriving in the Caribbean
arc still thick enough to obscure visibility and
coat windshields. Several investigators are
currently studying a possible link between
high rates of asthma in tlte Caribbean and
African dust events. For example, tltere has
been a 17 -fold increase in the incidence of
asthma on the island of Barbados since 1973.
Whether tlli s is due to tiny mineral particles
initating the ltmgs, bacterial and fungal spores,
pollen, or some combination of these and other
factors remains to be determined. In addi -
tion to allergic or astl1matic responses, there is
the separate issue of microbes that are able to
cause inf cctions. Although we have detected
bacteria and fungi in ilie dust that arc capable
of causing infection in people with weakened
immune systems, tltere have been no cases of
infectious illness in tlte Caribbean or U. S. that
have been directly linked to intercontinental
dust events.
In addition to hosting microbes, the
A frican dust is can·y ing otl1cr tmplcasant pa s-
se ng ers. Dust from tl1e extraordinary dust
event of February 26 , 2000 (shown on tlte
fir st page), was collected by researchers at tlte
Figure 5. The hallmark brick red color of the African dust can be seen on the street as well as in the air in this
photo taken in the city of Bamako , Mali . Dust concentrations in Mali (west Africa) range from 26 · 13,735
~~g/m 3 , sometimes greatly exceeding international health standards . Easily inhaled , the small particles become
trapped in human lungs and are difficult to expel.
Figure 6. The dark purple coloring surrounding dead
tissue on this sea fan is indicative of the disease
aspergillosis. The fungus (Aspergillus sydowir), now
known to cause this disease in the tropical western
Atlantic, was identified by Garriet Smith 's laboratory
at the University of South Carolina Aiken . The fun ·
gus was isolated in its active pathogenic form from
air samples taken during dust events in the Virgin
Islands and in Mali.
University of the Azores. The smallest
particles (one micrometer) were found to
contain 2 parts per million of the element
mercury, which is many times greater
than tlte amounts normally fotmd in air.
The mercury may have originated from
open-pit mercury mines in Algeria and
from the rock fonnations from which the
mercury is mined.
