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A sensing system for the rapid on-line screening of lipids and biomembrane active compounds in water samples has been successfully developed. The sensor consists of a flow cell and incorporated wafer-based device with on-chip mercury on platinum (Pt/Hg) working electrode and platinum (Pt) auxiliary and pseudo-reference electrodes. To optimise system performance, the following experiments were carried out: (i) Deposition and removal of phospholipid layers on and from Pt/Hg electrodes respectively, (ii) Effect of electrode size on signal, (iii) Monitoring of different phospholipids deposited in flow cell and, (iv) Detection of phospholipid monolayer interaction with representative compounds. The results showed that: (i) Miniaturisation and ruggedisation of the mercury (Hg)/phospholipid system has been successfully achieved, (ii) Rapid cyclic voltammetry facilitates repetitive dioleoyl phosphatidylcholine (DOPC) monolayer formation on Hg from aqueous DOPC dispersion and, (iii) The device responds selectively to organic compounds injected into electrolyte flow.

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...

The northern Adriatic (NA) is a favorable basin for studying the adaptive strategies of plankton to a variety of conditions along the steep gradients of environmental parameters over the year. Earlier studies identified phosphorus (P)-limitation as one of the key stresses within the NA that shape the biological response in terms of biodiversity and metabolic adjustments. A wide range of reports supports the notion that P-limitation is a globally important phenomenon in aquatic ecosystems. In this study P stress of marine microphytoplankton was determined at species level along a trophic gradient in the NA. In P-limitation all species with considerable contributions to the diatom community expressed alkaline phosphatase activity (APA), compared to only a few marginal dinoflagellate species. Nevertheless, APA expressing species did not always dominate the phytoplankton community, suggesting that APA is also an important strategy for species to survive and maintain active metabolism outside of their mass abundances. A symbiotic relationship could be supposed for diatoms that did not express APA themselves and probably benefited from APA expressed by attached bacteria. APA was not expressed by any microphytoplankton species during the autumn when P was not limiting, while most of the species did express APA during the P-limitation. This suggests that APA expression is regulated by orthophosphate availability. The methods employed in this study allowed the microscopic detection of APA for each microphytoplankton cell with simultaneous morphologic/taxonomic analysis. This approach uncovered a set of strategies to compete in P-limited conditions within the marine microphytoplankton community. This study confirms the role of P-limitation as a shaping factor in marine ecosystems.