Limited dispersal of giant bioaerosol particles

Very recently, a study from ATTO revealed what kind of plant and fungi species are present in the Amazonian atmosphere across all particle sizes. Another study now focuses on so-called “giant bioaerosol particles” larger than 5 µm. The main focus of this new study was to identify how the distribution of different types of particles in this large size fraction changes with height. The scientists also wanted to find out if changes in weather and climate affect particle distribution and composition.

Giant bioaerosol particles

Particles in this size range typically include pollen grains and fungi spores. The team around Cybelli Barbosa was interested in their distribution patterns because they carry genetic information for pollination, colonization and infection. Scientists also believe that pollen and large spores might undergo long-distance transport and influence local precipitation processes. However, we know little about their abundance in the air above the canopy. Most prior studies have looked at distribution patterns only at ground level and focused on temperate regions.

Labor-intensive work

But now the team set out to look at distribution patterns of such giant bioaerosol particles in the near-pristine atmosphere at the ATTO site.

Over a time period of five years, Cybelli Barbosa and her colleagues performed eight intensive sampling campaigns, during both the wet and dry seasons. To collect the air samples, they climbed the 80m tower and tall tower every day, no matter how hot or wet it was.

They then individually identified the incredible number of 500 000 individual particles with a light microscope. This tremendous effort took several months and included inspecting each induvial particle and measuring its size. This allowed the team to categorize the particles into the main groups: pollen, spores (fungi, fern, bryophyte), canopy residues (leaf/wood parts, glands, insect parts, amorphous plant and fungal debris), and non-organic particles.

The effort was worth it though because the scientists made some exciting and a few surprising discoveries.

Microscopic images of giant aerosol particles found at ATTO. © Cybelli Barbosa

Exciting discoveries

Thanks to their manual microscopic analysis, the team found lots of giant bioaerosol particles beyond pollen and spores. The samples contained lots of canopy debris that consisted of fragments of plant and fungal tissue. This debris made up around two-thirds of all particles in many of the samples. Some of this debris contains DNA and therefore could readily be mistaken for pollen or spores in genetic analyses.

Second, the amounts of coarse and ‘giant’ bioaerosols high above the Amazon rainforest decreased substantially compared to right above the canopy. This suggests that many of these particles stay close to where they are created and dispersed. Long-range transport and cloud interaction due not seem to be a large factor. For that, the particles would be to be transported higher up.

However, times, when there were peaks in pollen abundance above the canopy, coincided with increased wind speeds associated with convective thunderstorms. On the other hand, precipitation and increased relative humidity lead to decreasing numbers of giant particles, as rain washes them out of the air.

Crucially, with this ‘limited’ transport range, the large fungi spores and pollen particles have a strong influence on rainfall periods locally for this tropical area. This might guide future studies on the role of giant bioaerosol particles in precipitation cycles.

Thirdly, the team assembled the first-ever illustrated inventory of all their analyzed bioaerosols. This atlas of “Bioaerosols from the Brazilian Amazon Rainforest: Pollens and Spores” is freely available online, in bilingual form (English and Portuguese), in addition to the publication of the study.

Barbosa et al. published the study “Identification and quantification of giant bioaerosol particles over the Amazon rainforest” Open Access in npj Climate and Atmospheric Science. doi: 10.1038/s41612-022-00294-y

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