The oceans are covered in a thin layer of slime!

Sea-surface microlayers comprise a thin layer on the surface of the water roughly about 1mm thick. Also known as the neuston, these are unique microbial ecosystems which form an interface between atmosphere and water column. Sea-surface microlayers have an essential role in global biogeochemical cycles and impact air-sea gas exchange. The microlayer allows reactive gases and particles such as aerosols and CO₂ to move between sea and atmosphere as bursting bubbles. These biofilm environments are also important for marine food webs, in particular the microbial loop, in which larger organisms feed on bacterial and archaeal cells.

These layers have been known about for nearly 100 years but until recently, their significance was not appreciated. The pioneering work of Sieburth in 1983 on sea-surface microlayers helped form the basis of current knowledge. Sea surface microlayers were previously thought to be stratified but are now considered to be a heterogenous aggregation of molecules. The current view is more complex system, of a gelatinous mass of carbohydrate particles and microbes including grazing protists.

Sea-surface microlayers display different chemical properties to the water below as compounds can concentrate in these upper layers of the ocean. The study found that total carbohydrates, sticky gel-like particles known as TEP (transparent exopolymer particles) and DOM were significantly enriched in the microlayer. The dynamic microbial communities within microlayers exhibited higher levels of extracellular enzyme activity and respiration compared to subsurface waters. In addition, the study showed that microlayers are enriched with virus-like particles (VLP) which may be transported via sprays and aerosols. FISH analysis showed that bacterial cells were more aggregated in the microlayer than in the water below and the reason for this may be that the particles in the microlayer provide a substrate for bacterial colonization.

This paper reviews several microlayer studies and explains that it is difficult to compare current studies as firstly, there are so few that have been conducted and secondly, there is no consistent way to sample aquatic surface microlayers. Many techniques exist but the study recommends using a membrane to sample microbial communities. The authors argue that membranes reduce the risk of contamination as membranes are less likely to make contact with sub-surface water and the associated micro-organisms compared to other techniques.

I feel that this paper highlights an important development in marine microbiology. The updated view of aquatic microlayers has revolutionized our understanding of the air-water interface and its role in aquatic food webs. Viruses are a crucial part of the microbial population within oceans, yet these are not referred to in this paper in any great detail. Relatively little is known on how viruses and VLPs function in sea-surface microlayers and further research should focus on these areas. I feel that this is particularly important as microlayers have such a central role in biogeochemical cycles and the function of an ecosystem as a whole. Viruses may prove to be yet another crucial link linking the sea with the atmosphere, enabling us to further understand the role of these complex systems within global climate change.

Reference: Cunliffe, M., Upstill-Goddard, R.C. and Murrell, J.C. (2011) Microbiology of aquatic surface microlayers, FEMS Microbiological Review, 35, 233-246.

 

http://onlinelibrary.wiley.com/doi/10.1111/j.1574-6976.2010.00246.x/full