The Ocean's Thin Slice: Drivers of the Community Composition of the Sea Surface Microlayer

The sea surface microlayer (SML) is defined as the top 1 mm of the ocean’s surface and is biologically and chemically distinct from the underlying water (UW) beneath. The SML is primarily composed of dissolved organic carbon and transparent exopolymer particles (TEP), a gelatinous mix of polysaccharides derived from phytoplankton. This concoction of carbon supports high amounts of biomass in the SML, with the inhabitants of this thin slice of the sea (known as the neuston) often found associated with TEP. However, the communities of bacterioneuston in the SML can be altered depending on abiotic factors for example temperature and wind speed, or biotic factors such as nutrient availability. The study presented here aimed to better understand the bacterioneuston communities and the mechanisms that controlled these communities off the Peruvian coast.

SML samples and UW samples were collected from 11 stations during the SO243 cruise to a Peruvian upwelling. Various abiotic and biotic parameters were measured during the cruise, such as wind speed, temperature, salinity, the concentration of TEP and available nutrients (phosphate, nitrate, silicate, and carbohydrates). The bacteria from both depths (SML and UL) at each station were identified by Illumina high-throughput sequencing of 16S rRNA sequences and total bacterial abundances were determined by flow cytometry. Finally, the most abundant families were tested for enrichment or depletion in the SML.

Twenty-four families that made up over 1% of the communities were found across the 11 stations, none of which were found to be significantly depleted in the SML. Of the 24 families, 4 (unknown Flavobacteriales, Flavobacteriaceae, Crymorphaceae, and unknown Bacteroidetes) were significantly enriched in the SML but their abundances were similar at both depths. Overall, the community compositions and bacterial abundances were similar between both depths within a station but more different between stations. Negative correlations of the enriched families were found for temperature and wind speed as this can cause disruption of the SML and prevent enrichment. The enriched families did show positive correlations with nutrient concentrations and TEP, suggesting that the increases in abundance were likely found at upwelling stations. This is not surprising as the upwelling will bring nutrients up from deeper waters that the bacteria can utilise and aid with the transport of TEP and TEP associated bacteria, like the Flavobacteriales, to the surface.

However, the authors suggest that TEP has a limited influence on the community structure of the SML, given that TEP concentrations and community structures were similar at the SML and UL. This was quite surprising to me, as it was my understanding that TEP was the major player in determining SML communities. However, from this study, it is difficult to determine if TEP enhances the growth of bacterioneuston or, is simply a vehicle for some bacteria to float to the surface. It may be that the availability of nutrients (nitrate, phosphate, silicate) coupled with a favourable external environment with low wind speed and suitable temperature could be more important to the enrichment of SML communities.

Paper reviewed:

Zäncker, B., Cunliffe, M., & Engel, A. (2018). Bacterial community composition in the sea surface microlayer off the Peruvian coast. Frontiers in Microbiology, 9, 1-11.