Organic Matter in the Surface Microlayer: Insights From a Wind Wave Channel Experiment

Abstract

The surface microlayer (SML) is the uppermost thin layer of the ocean and influencing interactions between the air and sea, such as gas exchange, atmospheric deposition and aerosol emission. Organic matter (OM) plays a key role in air-sea exchange processes, but studying how the accumulation of organic compounds in the SML relates to biological processes is impeded in the field by a changing physical environment, in particular wind speed and wave breaking. Here, we studied OM dynamics in the SML under controlled physical conditions in a large annular wind wave channel, filled with natural seawater, over a period of 26 days. Biology in both SML and bulk water was dominated by bacterioneuston and-plankton, respectively, while autotrophic biomass in the two compartments was very low. In general, SML thickness was related to the concentration of dissolved organic carbon (DOC) but not to enrichment of DOC or of specific OM components in the SML. Pronounced changes in OM enrichment and molecular composition were observed in the course of the study and correlated significantly to bacterial abundance. Thereby, hydrolysable amino acids, in particular arginine, were more enriched in the SML than combined carbohydrates. Amino acid composition indicated that less degraded OM accumulated preferentially in the SML. A strong correlation was established between the amount of surfactants coverage and γ-aminobutric acid, suggesting that microbial cycling of amino acids can control physiochemical traits of the SML. Our study shows that accumulation and cycling of OM in the SML can occur independently of recent autotrophic production, indicating a widespread biogenic control of process across the air-sea exchange.

Air-Sea CO2-Exchange in a Large Annular Wind-Wave Tank and the Effects of Surfactants

“…In our study, decrease on surfactants over the time of the study indicates that primary and bacterial production, as the source of surfactants (Ẑutić et al, 1981;Satpute et al, 2010;Engel et al, 2018), was very low (see section The Non-zero Intercept of Wind-based Parametrization of CO 2 Transfer Velocity). Also the nutrients and chlorophyll concentrations were very low (Rahlff et al, 2017).…”

“…However, during the entire campaign, no increase was observed in chlorophyll-a (Rahlff et al, 2017). Bacterial abundance and community composition have been reported elsewhere (Rahlff et al, 2017;Engel et al, 2018).…”

“…A limitation of our study was that algal biomass was of low abundance (Rahlff et al, 2017;Engel et al, 2018), probably due to the required storage and transport of the seawater from the North Atlantic to Heidelberg. On the other site, bacteria were well and active (Rahlff et al, 2017;Engel et al, 2018).…”

“…A limitation of our study was that algal biomass was of low abundance (Rahlff et al, 2017;Engel et al, 2018), probably due to the required storage and transport of the seawater from the North Atlantic to Heidelberg. On the other site, bacteria were well and active (Rahlff et al, 2017;Engel et al, 2018). Therefore, the measured surfactant concentrations were in a typical range (300-1000 µgL −1 ) for natural seawater (Frka et al, 2009;Wurl et al, 2011), and probably also in its composition.…”

The Ocean’s Elevator: Evolution of the Air-Seawater Interface During a Small-Scale Algal Bloom

“…Biogenic enrichment at the air-seawater interface occurs throughout the course of a marine algal bloom. [1][2][3][4] Marine algae produce proteins, lipids, and carbohydrates, among other surfaceactive organic molecules that partition to the air-seawater interface and contribute to the composition of the thin layer at the ocean's surface, known as the sea surface microlayer. 5,6 The sea surface microlayer serves as the boundary between the ocean and atmosphere and is therefore crucial to the exchange of gases 7,8 and a wide range of chemical and physical processes in the ocean and atmosphere.…”