The composition of SML... what effect does it have on future climate change scenarios?

As we know in recent lectures; composition of the sea-surface microlayer (SML) has been recently characterised and is vitally important for microbial processes and carbon cycling in the ocean. Physical accumulation of gels particles in the SML can result from gels rising in the water column due to their low density, and by adsorption of gels onto rising bubbles. In the ocean, and also within the SML, organic gel particles represent substrates for marine phytoplankton and bacterioplankton to attach and grow upon, facilitating the formation of an active biofilm.

These particles might provide a new source for submicron marine primary organic aerosols (POA) during the emission of sea spray to the lower atmosphere. As the bubbles burst; they release a polysaccharidic composition and are suggested to act as cloud condensation nuclei in regions such as the high Arctic, where low-level clouds play a climate-regulating role by reflecting incoming solar radiation. Marine POA-cloud feedback processes are beginning to change the way we look at present day and future scenarios of surface ocean-lower atmosphere interactions. However, little is known about the factors controlling the accumulation, size distribution, and composition of gels in the SML, which is a suggested source for POA. Rising uptake of anthropogenic CO₂ may enhance autotrophic carbon fixation and increase the extracellular release of organic polymers from phytoplankton. Thus, a high production of extracellular polymers supports the accumulation of gel particles that may contribute to surface biofilm formation. A counteraction to enhance organic matter production may be met by higher heterotrophic activity under future ocean conditions and may change the balance of autotrophy vs heterotrophy which may provide positive feedbacks on rising atmospheric CO₂.

By experimenting with six the Kiel Off-Shore Mesocosms for future Ocean Simulation (KOSMOS) in Raunefjord, Norway, results were shown from a large-scale pH perturbation experiment in the aftermath of a spring bloom. The aim of this study was to examine the coupling between phytoplankton bloom development and the accumulation, composition and microbial dynamics of organic matter in the SML in response to CO₂ enrichment as expected for future ocean acidification scenarios. Two phytoplankton blooms were observed in the water column of the mesocosms, as derived from increases in chlorophyll a concentrations. In all mesocosms, the first bloom occurred around day 3 and was mainly dominated by chlorophyta and diatoms. The second bloom was induced by the addition of nutrients, with the main species being diatoms, cryptophythes, chlorophytes, and haptophytes.

For both classes of marine gels that were investigated, their dynamics in the SML and in future ocean scenarios were closely related to autotrophic and heterotrophic metabolism. Proteinaceous gels contributed even more to the SML gel-like composition, and may explain the enrichment of proteinaceous material in natural SML samples and in sea-spray aerosols. High surface-active properties of amino acids in addition to the active bacterial release of dissolved organic matter as extracellular enzymes or by cell lysis contribute to a biofilm matrix and may support proteinaceous gel accumulation at present CO₂ levels (control treatment of this study). These results show that acidification of seawater has a high potential to affect the amount and composition of organic matter, in particular with respect to gel particles in the SML. It reflects the sensitivity of marine microorganisms and organic matter within the SML to environmental change and altered characteristics of the SML could be relevant for air-sea gas exchange with detrimental knock-on effects becoming apparent. An improved understanding of its structure and dynamics will help to better estimate ocean-atmosphere interactions in a future high CO₂ world.

This study was a very snapshot insight to the composition of the SML due to the experiment running for one month. Also the fact that nutrient addition was under lab conditions may suggest that these finding could be slightly exaggerated. In the natural world, factors such as food web dynamics need to be taken into consideration as they would have an negative impact (as in cost of net growth - not that it would be a bad thing) on the net accumulation of all composition. However, personally, this study reveals new implications and that now is a great time for anyone looking into ocean processes and climate change as this looks like a real game changer, especially when it comes to implementing climate scenario models for future use.

Apologies for the obvious pun, but we are only now scratching the surface!

Galgani, L., Stolle, C., Endres, S., Schulz, K.G., Engel, A., 2014. Effects of ocean acidification on the biogenic composition of the sea-surface microlayer: Results from a mesocosm study. J. Geophys. Res-Oceans. 119(11), 7911-7924.