One of John McN Sieburth's many contributions to biological oceanography was his proposal of a hydrated gelatinous surface microlayer film at the air–sea interface (Sieburth, 1983). The purpose of this commentary is to highlight John Sieburth's contribution to surface ocean science and the recent evidence that supports his proposal of a gelatinous microlayer film. His hypothesis was based on many observations made over several years, including those during a Trichodesmium bloom in the Sargasso Sea, when a distinct slick was reported directly above the bloom (Sieburth and Conover, 1965). At the air–sea interface, the sea-surface microlayer is the physical boundary between the ocean and the atmosphere. Roughly considered to be the upper-most 1 mm of the ocean, the sea-surface microlayer is physico-chemically distinct compared with subsurface water and is characteristically enriched with biogenic organic compounds, such as lipids, proteins and polysaccharides (Liss and Duce, 2005). The presence of the surface film and surface tension properties means the sea-surface microlayer is a unique habitat that is often referred to as the neuston. Bacterial communities that are present in the surface microlayer are known as the bacterioneuston (Liss and Duce, 2005). Surface films occur on all water bodies, marine, estuarine and freshwater, sometimes as visible slicks, and are rapidly reformed after mixing by wind or waves (RC Upstill-Goddard, personal communication). The widely known effect of pouring oil on troubled waters occurs naturally, with organic films changing the physical properties of surface water, reducing roughness and affecting air–sea gas transfer (Liss and Duce, 2005). The structure of the sea-surface microlayer film, potentially 70% of the Earth's surface, is of great importance in the exchange of chemicals between the oceans and the atmosphere. During summer blooms of filamentous cyanobacteria, the net oxygen flux is significantly highly between the atmosphere and the surface microlayer compared with the ocean and the surface microlayer, which highlights the importance of autotrophic and heterotrophic organisms in this environment (Ploug, 2008). Also, bacterial activity within the microlayer can mediate the air–sea flux of other gases, such as methane (Upstill-Goddard et al., 2003).

The molecular microbial ecology of the sea-surface microlayer has only recently been studied. Franklin et al. (2005) showed that the bacterioneuston at a site in the North Sea was distinctly different compared with subsurface water just 0.4 m below the surface. The bacterioneuston was dominated by two genera, Vibrio and Pseudoalteromonas. Both organisms have physiological characteristics that are indicative of adaptations for biofilm survival (Franklin et al., 2005). If the sea-surface microlayer is a gelatinous film, then organisms that have biofilm capabilities will have a selective advantage.

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