Aquatic surface microlayers: A review of our current knowledge

A review of Cunliffe M, Upstill-Goddard RC, Murrell JC. 2010. Microbiology of aquatic surface microlayers. FEMS Microbiology Reviews. 35:133-146.

This article reviews the current knowledge on surface microlayers, amalgamating the findings of recent published work with an emphasis on the molecular and microbial ecology.

Surface microlayers are only a few tens of micrometres deep but yet may prove to be instrumental in influencing global biogeochemistry as they are the main barrier between air and water for the exchange of reactive gasses and particles. They are comprised of a ‘loose gel of tangles macromolecules and colloids’ which is produced from dissolved organic matter and neuston (organisms found exclusively in the layer) a significant portion of this DOM being carbohydrates. Recently, it has been discovered that transparent exopolymer particles (TEP), which is a sticky gel like substance formed by coagulated DOM, are also a large component. This is important as they readily form aggregates with other particles such as detritus, and then contributing to marine snow, transporting nutrients downwards. They are a unique habitat, interacting with both hydrosphere and atmosphere simultaneously.

One characteristic of surface microlayers is that they have higher concentrations of many components when compared to subsurface layers below them. This results in highly active microbial communities, as there are more nutrients available, with increased extracellular enzyme activity and enhanced conversion of POM into DOM. A further characteristic is the high levels of UV radiation microbes are exposed to. Whilst there are conflicting arguments as to the extent to which UV radiation is harmful, it has been demonstrated that certain species appear to have adaptations to allow survival in this environment such as very effective photorepair systems.

The sampling of surface microlayers is widely debated with many different methods and protocols. What this paper concludes is that much further research is needed to establish which methods to apply in different situations.

As one of my previous reviews on microbial communities and climate change demonstrated, the uptake of trace gasses such as methane and carbon dioxide via microbial activity is a very important process and a large sink for increasing anthropogenic atmospheric carbon dioxide. It is the microbial communities that influence the water to air interactions due to differing partial gas pressures at the surface microlayer, caused by bacterioneuston metabolism. This paper provides examples of bacterioneuston actively influencing rates of gas transfer between water and air. It is likely that bacterioneuston are very closely linked to gas cycling as well as proving a small gas sink.

Previous studies have demonstrated many cases where the bacteria present significantly differ, not only in density but in the species between the surface microlayer and the adjacent surface water. This alone may not have the ability to affect gas transfer as this process relies on specific species and/or metabolic processes. Further studies on the surface layers of estuarine systems have found methane producing bacteria which may account for the high levels of methane release in these systems.

Despite the physical environmental pressures exerted on neuston, there are also many biological factors, chief amongst which is that of grazing by organisms such as copepods and amoeba. At this micro scale, organisms like amoeba are able to treat the microlayer as a hard substrate as it moves across it grazing. Another main predator are protists, of which both sessile and motile forms have been discovered in the microlayers. It has also been suggested that the ciliates have the potential to produce large amounts of the surface-active organic compounds within these layers. The effect of this is very important in determining the composition of the microbial community. It is know that grazing pressure, due to things like size selectiveness by the predator has the potential to dramatically dictate the composition of bacterial communities.

Experiments on freshwater surface layers have identified that within the surface layers, there is a much larger aggregation on bacteria onto aggregates. These attached bacteria are responsible for the extracellular enzyme production that lead to elevated levels of DOM being released into the environment. It is the significance of these attached cells to buoyant aggregated which is one of the main distinguishing ecological characteristics of the neuston.

This paper provides a well synthesised comprehensive review of our current knowledge of what surface microlayers are and of the microbial community interactions that occur. It suggests that far more research is needed using molecular techniques as to date there have been relatively few. Despite surface microlayers being one of the most abundant habitats and playing an important role in gas exchange, there have still been relatively few papers that examine them or that link them to ecosystem functioning.