The importance of the sea surface microlayer: influences on the atmosphere

Aerosol formation is the main vector for transport of bacteria and viruses across the air–sea interface that can transport microbes for long distances, sometimes even surviving overland. Several studies have shown that marine aerosols are large enough to contain microbes and organic particles such as viruses, which can be transported for hundreds of kilometers from their source. This process involves materials being transported to and through the SML to the atmosphere and results in enrichment by the SML with microbes and other organisms. Although published almost ten years ago, I feel this study by Aller et al, has made an important contribution to the understanding and study of the SML and this can be confirmed by the amount of times it has been cited in other literature (120+).

Over a four-month period subsurface water, SML and aerosol samples were collected from locations adjacent to Long Island, New York. It is extremely important how you sample the SML (M.Cunliffe pers. comms. [first seminar]); SML samples in this study were obtained using a polyester 1-mm mesh screen, and I feel the poor size was too large,  I am sure if this study was carried out today a smaller nuclear pore membrane would be used.

It is clear from Fig.1. that the SML is enriching these aerosols, promoting microbial communities to be dispersed into the atmosphere. There was considerable enrichment of virus- like particles (VLPs) from subsurface water to the SML; but there was no evidence to determine whether these were produced in the SML, or scavenged and delivered to the SML. A very important finding in this study was that the majority of microorganisms in aerosols (59±3% of bacteria and 35±11% of VLPs) and the SML (23±1% of bacteria and 15±1% of VLPs) were found embedded in the organic particles, while in subsurface waters most of them were free-living; these were suggested to be transparent exo-polymer particles (TEP). It was most interesting in the study that these authors were one of the first to mention TEP; they even suggested the importance of it. TEP was suggested to provide physical protection as well as a nutrient source allowing some bacteria and VLPs to remain viable for extended periods in the atmosphere. This is important as microorganisms are subject to substantial temperature and salinity changes and higher exposure to UV light when in aerosol droplets.

Fig.1. The average concentrations of bacteria and VLPs on 6 sampling dates from mid-June to the end of September 2003 were substantially elevated in aerosols compared to subsurface waters

They also showed that a larger percentage of damaged and less active bacterial cells occur in the SML than in subsurface waters, and preliminary data suggest that an even greater percentage occurs in aerosols. This could be due to inactive cells being scavenged or that cells stop growing when delivered to the SML- this has interesting as now the SML is thought to be it’s own ecosystem, with cells being produced in it.

This paper demonstrates how SML studies have developed since 2005. I found it unfortunate that bacterial and VPLs (hopefully viruses!) were not identified, as it is extremely important to look at this, as it is unknown in this study that species were even the same between the different types of samples (subsurface/ SML/ aerosol). Cunliffe & Murrell (2010) later discovered there was a great diversity in microorganisms found in SML, even protists! Aller et al mentioned this work on aerosols to be of an importance because the microbes could be a public health risk, but how can they determine this by not identifying the species. Most marine microorganisms are not harmful to mammals and other animals, unless the microbes were a result of sewage inputs that contain a spectrum of human-borne pathogens. However again, due to the lack of identification, this cannot be confirmed, so would not be useful in any risk assessments or protocols. An positive aspect to take away also is that this study does highlight another mechanism (e.g. one mechanism is copepods) as to how viral and bacterial infections/ pathogens can be transported from one water body to another. 

References: 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.

Cunliffe, M., & Murrell, J. C. (2010). Eukarya 18S rRNA gene diversity in the sea surface microlayer: implications for the structure of the neustonic microbial loop. The ISME journal, 4(3), 455-458.

Publisher: Science Direct