Aerosols as potential pathogen carrier?

Bacteria, viruses and other small particles can be transported in the atmosphere by aerosols which also occur in marine habitats. The particles have been found on the surface of bubbles which are produced by ship’s wake, rain or wave breaking. Rising up through the water column bubbles burst at the water surface and therefore release the small particles attached to their surface to the atmosphere and the surface microlayer (SML). The SML is a layer between the hydrosphere and the atmosphere where bacterial activity is enhanced when compared to subsurface waters. In marine aerosols the concentration of bacteria and viruses is even higher.

Aerosols in general have an important impact on the distribution of particles across the land over long distances. Marine aerosols can transport microbes over hundreds of kilometers across the sea and can remain liquid. Thus, the microbes in the aerosols stay viable even after a longer time under stress conditions as increased UV-radiation. Marine aerosols containing pathogenic microbes can be ingested or inhaled by humans and are consequently suggested as risk factor for human health especially in coastal regions.

Aller et al. (2004) hypothesized that ‘the number of viable bacteria in aerosols was greatly underestimated’ and they conducted a study to examine the enrichment of marine aerosols compared to subsurface waters and the SML.

The researchers sampled marine aerosols, subsurface waters and probes of the SML at locations next to Long Island, New York using a small catamaran. Aerosols were sampled 3 – 4 m above the sea surface by collecting the bubbles produced by the ship’s wake. To count the bacteria and viruses the installed an artificial bubble source and sampled the aerosols at 10 cm above the surface. An epifluorescence microscope was used to determine the number of contained microbes. Additionally, Aller et al. used different dyes to make the bacteria, virus like particles (VLP), TEP and damaged cell membranes visible (DAPI, SYBR Gold, Coomassie and Alcian Blue).

As expected, the aerosols were enriched by bacteria (6-fold) and VLP (10-fold) when compared to subsurface waters. TEP was colonized by microorganisms and enriched 20 – 40 fold in the aerosols. These results confirm the researchers’ suggestion that aerosols can enhance the risk of infections by human pathogens. Additionally Aller et al. found that while bacteria and VLP in the subsurface waters are normally free-living, they are associated to microorganisms and TEP in the SML (bacteria: 23 %, VLP: 15 %) and in the aerosols (bacteria: 59%, VLP: 35 %). Concluding this association can serve as protection for bacteria and VLP against changes in salinity, temperature, UV-radiation, etc while being transported in aerosol droplets.

In the last part of their study Aller et al. showed that a higher percentage of damaged cells occurred in the SML than in subsurface waters. Active cells declined from more than 60 % in subsurface waters to about 8 % in the SML which can be due to scavenging of damaged cells or the cells could stop growth when delivered to the SML, but this has to be examined in further research. It is possible that the percentage of damaged cells is even higher in aerosols because the exposure to UV-radiation and the stress in general is higher than in the SML. Furthermore an enrichment of VLP from the subsurface waters to the SML was detected, suggesting that these VLP could be produced in the SML.

All in all it has been shown that the viral abundance is enhanced in the SML, which creates a high potential for infection. Furthermore, marine aerosols exhibit much higher concentrations of bacteria and VLP than the SML and subsurface waters which provides a high risk of infections by human pathogens especially in coastal regions.

This paper gives an important insight in the enrichment of potential human pathogens in aerosols and shows that aerosol production is an important mechanism in the distribution of particles and has been cited more than 140 times since. It would be interesting to investigate the composition of particles in the SML and the marine aerosols to see whether only specific bacteria and VLP are enriched.

Reviewed paper:

Aller, J. Y., Kuznetsova, M. R., Jahns, C. J., & Kemp, P. F. (2005). The sea surface microlayer as a source of viral and bacterial enrichment in marine aerosols. Journal of aerosol science, 36(5), 801-812.