Effects of ocean acidification on the biogenic composition of the sea-surface microlayer: Results from a mesocosm study

Abstract

The sea-surface microlayer (SML) is the ocean's uppermost boundary to the atmosphere and in control of climate relevant processes like gas exchange and emission of marine primary organic aerosols (POA). The SML represents a complex surface film including organic components like polysaccharides, proteins, and marine gel particles, and harbors diverse microbial communities. Despite the potential relevance of the SML in ocean-atmosphere interactions, still little is known about its structural characteristics and sensitivity to a changing environment such as increased oceanic uptake of anthropogenic CO 2 . Here we report results of a large-scale mesocosm study, indicating that ocean acidification can affect the abundance and activity of microorganisms during phytoplankton blooms, resulting in changes in composition and dynamics of organic matter in the SML. Our results reveal a potential coupling between anthropogenic CO 2 emissions and the biogenic properties of the SML, pointing to a hitherto disregarded feedback process between ocean and atmosphere under climate change.

The organic sea-surface microlayer in the upwelling region off the coast of Peru and potential implications for air–sea exchange processes

“…During this study, the SML was sampled with the glass plate at ∼ 20 cm s −1 , yielding a thickness between 45 and 60 µm, with an overall mean value of 49 ± 8.89 µm (n = 39). This value is in good accordance with the proposed apparent sampling thickness of 50 ± 10 µm (Zhang et al, 1998) and fits well to previous observations for the SML sampled with a glass plate at the same withdrawal rate (Cunliffe et al, 2011; Galgani and Engel, 2013;Galgani et al, 2014;Zhang et al, 1998;Zhang, 2003). Using direct pH microelectrode measurements, Zhang (2003) later confirmed an in situ thickness of ∼ 60 µm for the SML, which they defined as the layer of sudden change of physico-chemical properties.…”

“…Significant correlations between organic matter concentration in the SML and in the ULW were determined and showed that the SML basically reflects the underlying seawater system. The close connectivity between SML organic properties and biological development was also shown during a recent meso- cosm study, indicating that ecosystem changes impact SML organic matter composition and concentration (Galgani et al, 2014). Despite this finding that relates to a more general characteristic of the SML, clear differences in the accumulation behavior of different organic matter components were determined during this study and are in good accordance with previous observations.…”

“…Accumulation of organic matter in the SML may be tightly coupled to phytoplankton abundance in the water column (Bigg et al, 2004;Galgani et al, 2014;Gao et al, 2012;Matrai et al, 2008). Thus, organic matter accumulation and composition in the SML may also reflect the sensitivity of marine microorganisms in the surface ocean to environmental changes, which was shown during previous mesocosms studies Riebesell et al, 2009;Schulz et al, 2013).…”

Primary marine aerosol emissions from the Mediterranean Sea during pre-bloom and oligotrophic conditions: correlations to seawater chlorophyll <i>a</i> from a mesocosm study

“…chla concentrations, total prokaryotic cells and virus-like particles abundances), there were no strong discernible differences between the control, C3, or the acidified mesocosms, P3 and P6, along the course of the experiments; however, there were often large differences between the two campaigns due to the pre-bloom and non-bloom conditions. While several studies have shown the effects of ocean acidification on biogeochemical parameters in eutrophic waters (Galgani et al, 2014;Schulz et al, 2013), observations from the MedSea experiment showed no effect of ocean acidification on most of the biogeochemical parameters in these oligo-to mesotrophic areas. These results are discussed more fully in Gazeau et al (2015).…”

Plastic Accumulation in the Sea Surface Microlayer: An Experiment-Based Perspective for Future Studies

“…This modified organic "skin" has the potential to influence the flux of gases across the air-water interface [51]. As the biological response to ocean change affects the SML compounds and processes [10,13,[52][53][54], we expect that plastic pollution will have tangible effects on SML composition and reactivity as well.…”