Running title: Sea-surface microlayers 2 Chemical composition and physico-chemical characteristics of natural and ex-situ reconstructed sea surface microlayer samples were studied using a complex methodological approach. Surface microlayer samples and the underlying seawater were collected in different seasons and different weather conditions in the northern most part of the Adriatic sea. The techniques used were thin layer chromatography with flame ionization detection for lipid classes analysis, electrochemical characterization of adsorbable organic substances using the ONP probe and monolayer techniques for surface pressure () and surface potential ( V) measurements, as well as Brewster angle microscopy.Our results indicated higher enrichment of organic matter for the microlayer sample collected in the warm period of the year (summer). This was reflected in the high surface activity of the present organic substances, and formation of a more condensed layer than for the other two samples collected in the spring. Of the two spring samples, the one sampled on a sunny day was reacher in surface active organic material. We concluded that the most important source of surface active substances is the insitu production of organic susbtances by the present plankton and bacteria, promoted by sunlight, while antrophogenic input comprises a smaller part of the present organic matter, around 10 % for all samples.Key Words: sea-surface microlayer; lipid analysis; o-nitrophenol; surface active organic substances; voltammetry; monolayer studies; Brewster angle microscopy; northern Adriatic. 3 INTRODUCTIONSea surface microlayer is an important boundary, as an area of exchange of matter and energy, that either affects or is affected by global change (1, 2). Sea surface layer is generally enriched in organic substances, particularly those which are surface active, metal ions, bacteria and other microorganisms, relative to subsurface water (3-5). Organic substances found in the microlayer originate from the in-situ production or from different sources and have been brought there by different transport mechanisms from the seawater bulk, atmosphere or land. Adsorbed organic substances change the physical and optical properties, depending on the nature of organics, i.e. the nature of polar groups, architecture of the hydrophobic chain, and ionic strength, pH and temperature.Although the physical and chemical properties of the sea-surface microlayer have been studied extensively, there is still lack of knowledge about the physico-chemical processes governing the formation and properties of the surface microlayer.Electrochemical methods have been widely used for the investigation of surface active substances (SAS) in different waters (6 -9). These methods are based on the principle that organic substances adsorbed on the electrode surface influence the mass and charge transport across such a modified phase boundary. The recently developed electrochemical method using the o-nitrophenol (ONP) probe as a convenient tool for rough char...

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We characterised surface-active substances (SASs) in aqueous extract of atmospheric aerosols by using phase sensitive alternating current voltammetry. The electrochemical method has mainly been used for the quantification of surfactants in sea water but has not been applied to atmospheric aerosols yet. The advantage of the method is its simplicity and sensitivity that enables direct analysis of aerosol extracts without the need for sample concentration. Aerosol samples were collected at Middle Adriatic Martinska station influenced by different air masses as well as from urban (Zagreb, Croatia) and rural (K-puszta, Hungary) areas from late spring to early autumn in 2010. The highest SAS concentrations, expressed in equivalents of T-X-100, ranging from 0.34 to 0.91 µg m−3 were detected in urban samples. The SAS concentrations obtained for marine, regional and continental samples ranged from 0.14 to 0.31, 0.18 to 0.42 and 0.07 to 0.28 µg m−3, respectively. The SAS concentrations in K-puszta aerosols ranged from 0.13 to 0.46 µg m−3. Investigation of humic-like substances isolated from K-puszta samples (2008) confirmed their significant surfactant nature. Different SAS chemistry was noticed for urban and non-urban samples. Investigations at different pH revealed anionic character of SASs in aerosol samples

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The uptake of cytochrome c by charged and neutral lipid monolayers was studied by using reflection spectroscopy. The method was shown to be a very sensitive and useful technique in studies of lipid-protein interactions. It was found that cytochrome c is preferentially bound to monolayers of negatively charged monolayers in the solid phase. Polarized light under oblique incidence was used to determine the average orientation of chromophores in cytochrome c bound to lipid monolayer. The transition moments of chromophore are oriented parallel to the monolayer plane.

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Monolayer studies and a force-area quantification approach, in combination with electrochemical methods, are applied for physicochemical characterization of surface active substances (SAS) of the sea surface microlayers (MLs) from Middle Adriatic stations. Higher primary production during late spring-early autumn was reflected in the presence of MLs of higher surfactant activity containing on average molecules of lower molecular masses (M(w) = 0.65 ± 0.27 kDa) and higher miscibility (y = 6.46 ± 1.33) and elasticity (E(isoth) = 18.33 ± 2.02 mN m(-1)) modulus in comparison to structural parameters (average M(w) = 2.15 ± 1.58 kDa; y = 3.51 ± 1.46; E(isoth) = 6.41 ± 1.97 mN m(-1)) obtained for MLs from a period of lower production. A higher inhibition effect on the reduction process of Cd(2+) was observed for SAS abundant MLs from a more productive period. This kind of distribution is explained as the consequence of competitive adsorption of hydrophobic lipid-like substances of lower M(w) that act as end-members, highly influencing the surface structural properties of the natural air-water interface forming there segregated surface films during more productive period.