Organisms across the tree of life, from plankton to humans, follow day-night cycles. New research shows that the airborne microbiome cycles through day-night cycles as well.
Researchers in Brazil and Singapore captured the microbes present in the air of a tropical environment using air samplers located at Nanyang Technical University. Considering the popularity of microbiome studies in both humans and ecosystems, the microbiomes in the atmosphere are incredibly understudied, especialy compared to human-, water- or soil-based microbiomes.
The study sampled every two hours each day, which is high resolution for experiments collecting air samples. The researchers took almost 800 samples that spanned over a year, which covered Singapore’s wet and dry seasons. This dataset includes two monsoon seasons and microbes that dispersed from different directions (generally the northeast and southwest).
The atmospheric microbiome is challenging to study. Often, a low amount of biological material is typically collected, which can hinder next-generation sequencing methods identifying microbial species. The researchers developed a method to collect enough biological material that allowed them to look at which species and how many of each across fungi, bacteria, virus and plants.
The air sampling site in Singapore was ideal for this type of study because it’s so close to the equator that there is little variation in daylight hours throughout the year, as well as minimal variability in temperature and relative humidity, all of which remove sources of noise in the data.
The air microbiome followed a clear day-night cycle that was regulated by environmental conditions, particularly temperature. This pattern was stronger than variation over days, weeks or months.
The airborne microbiome had diversity levels similar to other ecosystems. The researchers consistently saw over 1,000 microbial species during any five-day period.
Various microbial groups would increase up to ten-fold during midday or during rain events while others became more common during nighttime. The nighttime air microbiome was much more consistent than the daytime air microbiomes.
The amount of DNA the researchers were able to get from samples, a loose proxy of biomass, could increase 100 times between 5:00 am and solar noon at 1:00 pm during the wet season. Bacteria were three times more abundant during daytime versus nighttime.
Temperature, followed by carbon dioxide levels (which increase during day) were the most important of the meteorological factors tested driving the changes in the air microbiome. Hotter temperatures reduced airborne fungi. In contrast, bacteria went up.
Generally, bacteria made up about two-thirds of the cells with fungi comprising the last third. The researchers tracked hourly concentrations of environmental pollutants, including nitric oxides and sulfur oxides, but found no relationships with the day-night cycle of the microbiome.
There was also a large core of microbial groups that did not change over the entire 13 month sampling period. The researchers found specific fungal and bacterial species associated with certain environmental conditions. These indicator species could be used as biomarkers for future research on the bioaerosol dynamics.
Moving forward, future studies on airborne microbiomes using similar methods could uncover changes in the concentration of disease-causing or pathogenic microbes.