Tiny but mighty: bioaerosols
Learning more about aerosols is one of our major assignments at ATTO. This includes inorganic aerosols like soot and other particles associated with biomass burning, for example. But it also includes bioaerosols emitted by the forest itself. We care about them so much because they are important for a whole range of processes. In the atmosphere, all aerosols scatter and absorb light or act as cloud condensation nuclei. In addition, bioaerosols such as bacteria, fungi spores and pollen disperse biological material and species far and wide.
But we need to learn more about the composition, quantity, dispersal, as well as sources and sinks of bioaerosols in remote areas to better understand the pre-industrial background state of the atmosphere. Knowing their role in the climate system will then help us to estimate future climate change.
The Amazon is one of those remote regions that, in parts, still has a near-pristine atmosphere. Furthermore, it is such a large source of bioaerosols that it might be relevant on a global scale.
Therefore, Luciano Huergo and this team, including the Ph.D. students Felipe Souza and Price Mathai took a close look at the diversity of bioaerosols, and specifically bacteria, in the Amazon rainforest. They collected air samples during distinct seasons and at different heights above the ground at ATTO. Then they filtered out all bioaerosols and sequenced a piece of DNA of bacteria. Specifically, they analyzed the sequences encoding the 16S rRNA, a gene marker used for bacterial classification.
Seasons over height
Their data reveal that the bacteria of the Amazon Rainforest are diverse. Felipe Souza, Price Mathai and their co-authors found up to 1000 species in a single sample. Out of those, 128 species were present in all samples. Those appear to make up the core Bacteria species of the region. The composition of the other species varied mainly with seasonal changes in temperature, relative humidity, and precipitation. These meteorological parameters may affect the bacterial communities directly or indirectly because they influence the seasonal cycle of plant communities.
On the other hand, they did not detect significant differences between the ground and canopy levels. The collected data suggest that the soil is not a major contributor to airborne bacteria in the Amazon rainforest. Instead, the identity of some bacteria implies that the leaf surface of plants may act as a source of airborne bacteria. Because of the immense number of plants in the Amazon rainforest, the surface of their plant leaves outnumbers the soil surface area by some orders of magnitude. So it may not come as a surprise that this habitat promotes such a large population of bacteria.
They also identified in the aerosol samples of bacterial species that participate in the nitrogen cycle. Specifically, they identified species that can convert the inert atmospheric nitrogen gas into other nitrogen forms which plants can use to build biomass. These bacterial species may act as biofertilizers within the Amazon forest.
Souza et al. published the study “Influence of seasonality on the aerosol microbiome of the Amazon rainforest” in the journal Science of the Total Environment. DOI: 10.1016/j.scitotenv.2020.144092
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