On the chemical dynamics of extracellular polysaccharides in the high Arctic surface microlayer

Abstract

Abstract. The surface microlayer (SML) represents a unique system of which the physicochemical characteristics may differ from those of the underlying subsurface seawater (SSW). Within the Arctic pack ice area, the SML has been characterized as enriched in small colloids of biological origin, resulting from extracellular polymeric secretions (EPS). During the Arctic Summer Cloud Ocean Study (ASCOS) in August 2008, particulate organic matter (POM, with size range > 0.22 µm) and dissolved organic matter (DOM, <0.22 µm, obtained after filtration) samples were collected and chemically characterized from the SML and the corresponding SSW at an open lead centered at 87.5 • N and 5 • E. Total organic carbon was persistently enriched in the SML with a mean enrichment factor (EF) of 1.45 ± 0.41, whereas sporadic depletions of dissolved carbohydrates and amino acids were observed. Monosaccharide compositional analysis reveals that EPS in the Arctic lead was formed mainly of distinctive heteropolysaccharides, enriched in xylose, fucose and glucose. The mean concentrations of total hydrolysable neutral sugars in SSW were 94.9±37.5 nM in high molecular weight (HMW) DOM (> 5 kDa) and 64.4±14.5 nM in POM. The enrichment of polysaccharides in the SML appeared to be a common feature, with EFs ranging from 1.7 to 7.0 for particulate polysaccharides and 3.5 to 12.1 for polysaccharides in the HMW DOM fraction. A calculated monosaccharide yield suggests that polymers in the HMW DOM fraction were scavenged, without substantial degradation, into the SML. Bubble scavenging experiments showed that newly aggregated particles could be formed abiotically by coagulation of low molecular weight nanometer-sized gels. Aerosol particles, artificially generated by bubbling experiments, were enriched in polysaccharides by factors of 22-70, relative to the source seawater. We propose that bubble scavenging of surface-active polysaccharides could be one of the possible mechanisms for the enrichment of polysaccharides in the high Arctic open lead SML.

Size-resolved atmospheric particulate polysaccharides in the high summer Arctic

“…6). The above results are not only consistent with the relatively more biologically active waters of the Greenland Sea-Fram Strait area (Leck and Persson, 1996a, b;Gao et al, 2012) but also consistent with observations of the aerosol source strength based on the eddy-covariance flux measurements during AS-COS (Held et al, 2011a) and during a similar expedition to the same area and season during 1996 , which showed an order of magnitude stronger flux of bubblebursting aerosol particles over the open sea than from open leads.…”

“…Moreover, the relatively high abundance of extracellular deoxysugars in the Aitken mode in air being advected over the pack ice is indicative of the presence of highly surfaceactive constituents (Gao et al, 2012) promoting cloud droplet activation of polymer gels (Ovadnevaite, 2011). The polysaccharide subunit monomer mass of the Aitken mode particles, observed in this study, associated with the gels found in the open lead surface microlayer (Gao et al, 2012) could thus be important for cloud droplet formation over the inner summer Arctic.…”

“…In a parallel study during ASCOS (Gao et al, 2012), it was proposed that bubble scavenging of surfaceactive polysaccharides was one of the possible mechanisms for the observed enrichment of polysaccharides in the SML located at the air-sea interface. Gao et al (2012) also observed that newly released gel polymers from sea ice algae were more favorably scavenged into the SML, which is consistent with the idea that the porous nature of sea ice not only provides a habitat for ice algae but also opens a pathway for exchanges of organic matter with the seawater below. The authors concluded that the melting sea ice containing high standing stocks of microalgae and bacteria elevated the input of polymer gels into seawater and subsequently enhanced the enrichment of polysaccharides in the SML at the air-sea interface.…”

“…The authors concluded that the melting sea ice containing high standing stocks of microalgae and bacteria elevated the input of polymer gels into seawater and subsequently enhanced the enrichment of polysaccharides in the SML at the air-sea interface. According to Gao et al (2012) the same processes promoting the formation of SML polymer gels and their saccharides would also apply in the MIZ. Table 2 gives basic descriptive statistics of all monosaccharides (hydrolyzed polysaccharides) measured during the expedition.…”

“…Chang et al (2011) reported a sub-micrometer organic mass of about 0.055 µg m −3 . Gao et al (2012) estimated the fraction of THNS to be about 1.5-3 % (2.7 % on average) of the organic matter dissolved in surface seawater during ASCOS. By assuming the same fraction of THNS in atmospheric organic particulate matters, the THNS in the organic aerosols studied by Chang et al (2011) would correspond to the order of 10 pmol m −3 THNS.…”