Marine Biogeochemistry

Microbial dynamics at the air-sea interface and their impact on cycling of climate-active gases

- The Surface Microlayer as an active biofilm

The sea surface microlayer (SML) is a thin organic biofilm, located at the very surface of the ocean, linking the hydrosphere with the atmosphere, and central to a range of global biogeochemical and climate-related processes. Organic compounds in the SML may modify air-sea gas exchange rates in two principal ways: as a physical barrier and by changing sea surface hydrodynamics. Organics within the SML are also a source of primary aerosols that are emitted to the atmosphere by sea spray and may serve as cloud condensation nuclei.  

Our studies investigate the relationship between SML formation in the ocean, its thickness and composition, and biological activity. Among the organic compounds that accumulate in the SML and are primarily are of planktonic origin are dissolved exopolymers, specifically polysaccharides and polypeptides, and gel particles, such as transparent exopolymer particles (TEP) and coomassie stainable particles (CSP). Our studies aim at a better understanding of how accumulation and composition of organic matter at the SML influence gas exchange rates, or POA emission and composition.

Figure: Gel particle colonized by living bacteria, stained with Syto 9. Sample collected from the SML in the Eastern Tropical Pacific during the SONNE (SO243) cruise in 2015. Picture: B. Zäncker.


Results from the METEOR cruise (M91) to the upwelling region off Peru showed that SML formation, composition and stability are related to biological productivity (Engel and Galgani 2016). Increasing wind speed disrupts the SML biofilm as indicated by lower accumulation of gel particles such as transparent exopolymer particles (TEP). At high wind speed a depletion of TEP in the SML was often observed, indicating TEP export from the SML, either by sea spray or by aggregation with sinking particles.

- The surface ocean under high CO2 conditions

The results of a large-scale mesocosm study indicate that ocean acidification can affect the abundance and activity of microorganisms during phytoplankton blooms, resulting in changes in the composition and dynamics of organic matter in the SML (Galgani et al. 2014). Phytoneuston and bacterial abundances in the SML were positively affected by CO2. Proteinaceous gels in the SML seemed to respond differently to acidification compared to polysaccharidic gels.

- Bacterial cycling of climate-active trace gases

Oceanic bromocarbons are highly reactive volatile organic compounds and may contribute up to 40% of stratospheric ozone depletion in mid latitudes. High sea-air fluxes of bromocarbons in the tropical ocean have been related to microbial cycling in the surface ocean, mainly due to phytoplankton and bacteria, but the underlying processes and magnitude of the biogenic sources in the diverse marine environments are poorly known.


Besides that, it is also unclear how environmental parameters such as light, temperature and pH may influence bromocarbon cycling rates in the ocean. In order to understand seasonal and spatial fluctuations of oceanic bromocarbon emissions and to project their future development, we study microbial production and removal processes in the surface ocean during cruises to low and high latitudes and combines them with observational data of bromocarbon and organic matter concentrations in the water and atmosphere.

See also Halocarbon-Group (