Impact of phytoplankton‐generated surfactants on air‐sea gas exchange

Abstract

The effect of surface-active organic nmtter generated by seven common species of rnarine phytoplankton on gas ex•e rates under turbulent conditions at the sir-water interface was determined. Reductions in oxygen evasion rates ranging from 5 to 50% were observed relative to clean seawater controls. Relative oxygen exchange coefficients (expressed as R = Kw[sarnple]/Kw[control]) were shown to be sensitive to small changes in total dissolved c•rbohydrate at concentrations <1 mg C (carbon) L -• and to asymptotically decrease to & lower limit (R '-55-70%) at concentrations between 2 and 6 rag C L -1. A corresponding rel&tionship was observed in which R decreased with increasing relative surfactant •mounts derived from surface pressure-area measurements. However, gas exchange reductions were significant for plankton exudate samples displaying surface pressures •<1 mN rn -1. It thus seems that condensed monol•yer films axe not & prerequisite for reduced gas ex•e and that re•atlvely soluble surfactants derived from phytoplankton can strongly •ect the dissipation of near-surface turbulence and lead to changes in the Schmidt number dependency of K•. Based on detailed analyses of carbohydrate-containing surface-active exu•t• isolated by solid phase extraction from one of the species, Phaeodaci•l•n tricor•turn, it appears tkat small g!ucar•s and heteropolysaccharides associated with proteins and possibly lipids were responsible for the observed reductions in R. mN m -• (milliNewtons per meter) even when R was •50%. Since the bulk of r values measured in visible slicks using spreading oils at coastal and open ocean sites are of the order of 1 mN m -• or greater [Hunter and Liss, 1981], we can surmise that the development of coherent surface films is not a prerequisite for the reduction of interfacial gas exchange. Possibly, soluble surfactants, which are known to be effective in reducing gas exchange through rapid readsorption at the surface [Springer and Pigford, 1970], but which do not display concentration-dependent surface pressures, may have played a major role in retarding gas exchange in our experiments. Phytoplankton exudates and degradation products generally are believed to be major sources of marine surfactants [Zutic et al., 1981]. Typical excretion products of many marine phytoplankton species are complex heteropolysaccharides of high molecular weight that often are acidic due to the presence of varying combinations of uronic acids and carboxyl or sulfate ester groups [Allan et al., 1972; Ramus, 1972; Sinestad et al., 1974, 1975; Percival et al., 1980]. Highly soluble fi-glucan-type polysaccharides often are major components of cellular carbohydrate pools in diatoms [M•kelstad, 1974; M•kelstad et al., 1982] and frequently are found in abundance in natural marine waters during phytoplankton blooms [Sakugawa and Hands, 1985a]. While many of these compounds are quite soluble, they are likely to contain sufficient hydrophobic groups to be at least weakly surface-active [oeeenheer, 1985] and thus influence interfacial...

The effect of protozoan grazers on the cycling of polychlorinated biphenyls (PCBs) in marine systems

“…Cognizance of the effects of microbial cycling on different components of DOC is important because sub-pools of DOC play different roles in ocean processes. For example, surface-active material 95 influences air-sea gas exchange (Frew et al, 1990) and lipid-rich material affects organic contaminant speciation. This chapter focuses on the effect of protozoan grazing processes on the production of surface-active material, lipid-rich DOM and bulk DOC within a simple microbial food web (predator + prey).…”

Seasonal signatures in SFG vibrational spectra of the sea surface nanolayer at Boknis Eck Time Series Station (SW Baltic Sea)

“…Hence, it is further hypothesized that a high concentration of chlorophyll in the water column can be taken as a direct indicator for pronounced, surfactantenriched microlayers (Asher, 1997) and nanolayers, leading to a decrease in gas exchange (see e.g. Duarte et al, 2009;Frew et al, 1990;Salter et al, 2011). A review of the current understanding of physicochemical aspects of the microlayer has been given recently by Cunliffe et al (2013).…”

The relationship between near‐surface turbulence and gas transfer velocity in freshwater systems and its implications for floating chamber measurements of gas exchange

“…However, the existence of a unique and universal wind-k relationship for all systems is highly questionable given that for any wind speed, its effect on gas exchange is unlikely to be the same in the ocean and, for example, in a small kettle lake. Moreover, many studies have shown that other factors will affect k, such as wind fetch (Frost and Upstill-Goddard 2002;Borges et al 2004;Guérin et al 2007), tidal currents (Borges et al 2004;Zappa et al 2007), rainfall (Ho et al 1997(Ho et al , 2007, microscale breaking waves (Zappa et al 2004), thermal convection (Schladow et al 2002;Eugster et al 2003), organic matter or suspended matter (Abril et al 2009;Calleja et al 2009), and surfactants (Frew et al 1990;McKenna and McGillis 2004).…”