A new study has found that Saharan dust travelling across the Atlantic Ocean carries a distinct community of bacteria to the Amazon rainforest in Brazil. The arrival of the dust also coincided with a nearly threefold increase in airborne endotoxins, substances that can trigger inflammation in humans.
The research led by Jens Weber and Bettina Weber of the University of Graz in Austria, involved scientists from the Max Planck Institute for Chemistry and Brazilian research institutions. Their findings were published in the journal Communications Earth & Environment.
The team collected air samples at the Amazon Tall Tower Observatory (ATTO) a 325 metre research tower surrounded by extensive areas of undisturbed rainforest in Brazil’s Uatumã Sustainable Development Reserve.
Africa’s drylands are the world’s largest source of atmospheric dust, releasing an estimated 11–15 teragrams of mineral particles every year. These dust clouds regularly travel across the Atlantic, carrying nutrients such as phosphorus and iron that help fertilise the Amazon basin.
However, scientists knew relatively little about the microorganisms transported along with these particles.
To investigate researchers installed high volume air samplers at two heights on the ATTO tower: 42 metres within the forest canopy and 323 metres above it. Samples were collected daily between February 10 and March 2, 2020.
A major Saharan dust event occurred between February 18 and 22. Atmospheric modelling confirmed that the air masses had travelled from North Africa.
During this period average airborne particle concentrations increased nearly eightfold, rising from approximately 2.7 micrograms per cubic metre under normal conditions to more than 21 micrograms. The concentration peaked at 45.78 micrograms per cubic metre.
The particles also contained calcium a characteristic component of Saharan mineral dust.
Genetic analysis revealed a significant change in the bacterial communities present in the Amazonian air during the dust event.
Under normal conditions, bacteria at lower heights mainly originated from local sources such as tree bark, leaves and lichens. However, during the Saharan dust intrusion, bacterial communities at both sampling heights became more similar, reflecting organisms associated with dryland soils, biological soil crusts and marine environments.
Spore forming bacteria belonging to the group Bacillota became particularly prominent. Researchers also identified soil-associated bacterial genera such as Clostridium, Brevibacillus and Tumebacillus.
Cyanobacteria commonly found in biological soil crusts, including Schizothrix and Trichocoleus, were also detected.
Surprisingly, marine bacteria such as Synechococcus appeared in the samples despite the observatory being approximately 1,000 kilometres from the nearest coastline. This suggests that the dust plume collected microorganisms from the ocean during its journey across the Atlantic.
While bacterial populations changed considerably, fungal communities showed little response to the dust intrusion.
Wood decomposing fungi belonging to the group Basidiomycota remained dominant before during and after the event. At the lower sampling height, they accounted for 93.8% of the fungal genetic sequences.
Researchers suggest that the large number of locally produced fungal spores may have masked the influence of fungi arriving from Africa.
According to Bettina Weber, the genetic characteristics of the detected bacteria matched microorganisms found in Saharan biological soil crusts, supporting the conclusion that they had travelled from African drylands to Brazil.
Endotoxin Levels Nearly Tripled
One of the study’s most significant findings was the sharp increase in airborne endotoxins during the dust event.
Endotoxins are components of the outer membranes of Gram negative bacteria. They can be released when bacterial cells die and are known to trigger inflammatory responses in humans. Their chemical stability allows them to remain active even after the bacteria themselves are no longer alive.
At 323 metres above the forest endotoxin concentrations increased from 0.45 to 1.27 endotoxin units per cubic metre, nearly tripling during the dust intrusion. Levels reached a peak of 2.10 units per cubic metre.
The researchers linked the increase to bacterial material from African dryland soils and marine microorganisms picked up over the Atlantic Ocean.
Jens Weber said he was surprised by the high concentration of endotoxins. Previous research in the Caribbean has associated Saharan dust arrivals with increases in asthma attacks, although whether a similar relationship exists in the Amazon remains unknown.
The findings could have implications for the health of people living in the region including approximately 3.9 million residents of Brazil’s Amazonas state and communities along the dust plume’s route.
Despite the findings several important questions remain unanswered. The genetic sequencing method used in the study can detect microbial DNA whether organisms are alive or dead. Therefore researchers cannot yet confirm whether the bacteria survived the approximately 10 day journey across the Atlantic.
Microorganisms travelling through the atmosphere face intense ultraviolet radiation, dehydration and changing temperatures. The research team plans further investigations using fluorescence microscopy and flow cytometry to determine whether the transported bacteria remain viable.
The study also examined whether potentially harmful pathogens were carried in the dust. Researchers did not find an increase in obligate pathogens although some opportunistic bacterial genera, including Staphylococcus and Clostridium, increased at the higher sampling level.
However, identifying bacteria only at the genus level does not establish whether disease causing species were present.
The researchers therefore suggest that Saharan dust primarily transports soil bacteria and inflammatory microbial material rather than necessarily carrying large numbers of infectious organisms.
Further research will be needed to understand the effects of these biological particles on rainforest ecosystems and human health.