According to Liu and Dickut (1997) the sea surface microlayer (SML) is perhaps one of the most important but also most poorly characterised regions of the marine environment. The AIRWIN project consisted in a multidisciplinary approach to characterize processes occurring in the sea surface microlayer (SML) and their relevance to global change and effect on the marine environment and its living resources. This project brought together chemists, biologists and engineers
(i) To define the role of the SML in the entry and loss of organic and inorganic pollutants in the marine environment,
(ii) To identify those pollutants which are preferentially transfered to humans through marine food webs,
(iii) To identify and to isolate microorganisms that could be used for bioremediation and as cosmetic products to improve human's health,
(iv) To provide potential bioindicators of atmospheric pollution.
Appropriate set of three different sampling and analytical techniques were used to investigate the chemical and biological components of the SML in both oligotrophic and highly industrialized coastal areas. Special attention was devoted to compounds responsible for accumulation, aggregation and polymerisation processes mediated by UV radiation and microbial activity. Petrogenic and pyrolytic hydrocarbons (PAHs), chlorinated pesticides, PCBs, surfactants and trace metals were analyzed in marine samples (SML, underlying waters, surface particulate matter, neuston and sinking particles) to model the cycle of pollutants in the SML.
At the stage of our investigations, after three years of research activities carried out within the AIRWIN Project, we provide significant information on the structure and functioning of the sea surface microlayer (SML) as well as a collection of environmental bacterial strains.
The SML occurs at least 30-40% of the time at the open global ocean, but its occurrence can be higher in coastal regions were wind speeds are lower. In the Mediterranean Sea, the SML occurs mainly from late spring to early fall and, therefore, is a compartment to be taken into account from the standpoint of the inhabiting biological communities and their role in the transport and cycling of natural organic matter and xenobiotics.
A first evidence underscored by the project was the spatial and temporal variability of the SML itself, which constituted a major constraint for the assessment of the data obtained. Therefore, the short-term processes and the patchiness occurring at the SML are important features to be taken into account in field studies for an adequate interpretation of the results.
Experiments carried out during a diel cycle showed a great variability of the chemical composition and biological activity of the SML, although such a variability is difficult to analyse do to the patchiness of the surface microlayer.
Variations observed highlighted the potentiel role of lipids in the establishment and cohesion of the SML with slight enrichments in nutrients and natural organic matter, source of food supply for microorganisms. Indicators of various sources of organic matter such as amino acids and fatty acids suggested a permanent and quick adaptation of living organisms to the SML microenvironment. Nevertheless one question still persists : does the SML reflect the biological activity developing in the top ocean surface or is the SML a driving force for the surface ecosystem.
Future experiments could provide an answer to this fundamental question of high priority to assess the role of the SML in exchange processes betwen the ocean and the atmosphere. The coupling between characterization of the nature of both SML and UW and sinking particles collected in sediment traps moored just under the surface evidence that the SML is a source of biogenic particles with non negligible fluxes. These particles also reflect processes of aggregation/disaggregation as shown by measuring high molecular weight organic compounds.
An original and interesting result was the comparison of microbial food webs operating at the surface microlayer with those operating in underlying waters. The high enrichment of bacterial biomass and activity reported in the literature in the years 80th was never found. We showed that this is partly due to differences in the samplers, with some techniques such as membranes who have important drawbacks for quantifying microorganisms, and recommendations were addressed for optimizing the sampling of the surface microlayer. We also shown that the terms of « bacterioneuston » and « phytoneuston » are probably not appropriate as actually employed to define organisms which are specifically living in the surface microlayer. Most of these organisms are similar to those living in underlying waters. However, the occurrence of bacterial species highly resistant to UV radiations was increased in the surface layer. Both bacterial and phytoplanktonic cells accumulate in the surface microlayer by the passive and physical process of flotation, with an important part of senescent cells associated with degraded pigments. Interestingly, these cells which are not produced in the surface microlayer are grazed by autotrophic flagellates which accumulate in this layer. Additionally, the accumulation of degraded pigments together with the enrichment of autotrophic flagellates may contribute to some specific optical characteristics of the surface microlayer which may affect the satellite observation of the surface oceans. This interesting result address new questions on the potential influence of the surface microlayer on satellite data used to estimate primary production at the oceanic scale and this should be investigated in the future in different oligotrophic areas of oceans where contrasting situations may be encountered in the pigment concentration of SML and subsurface layers.
The surface microlayer of the sea represents the boundary layer between the atmosphere and the sea. There, hydrophobic compounds are accumulating, both naturally occurring and of anthropogenic origin. Microorganisms present in this layer are degrading these compounds along with ultraviolet radiation (UVR). This UVR acts, however, also exerts stress to the microorganisms. Particularly, membrane and DNA damage results in retarded prokaryotic activity when microorganisms are exposed to high levels of solar radiation. The bottom line of our investigation on the activity of microorganisms in this layer is that they are essentially as sensitive as the microbial plankton in the layers below the microlayer. Although we shown that a lot of bacterial species have developed very efficient mechanisms for DNA repair and if we except a few highly resistant species which were isolated in the SML, the bacterioneuston are apparently not specifically adapted to these high radiation levels. From an evolutionary point of view, it seems likely that this particular microenvironment is sufficiently stable to allow the development of specific (i.e., endemic) microbial communities. Thus, there is no indication that microorganisms inhabiting this microhabitat are specifically suited to degrade anthropogenically introduced compounds. It appears that solar radiation is more efficient in cleaving aromatic or hydrophobic compounds in this microlayer than the neuston biota. Therefore, anthropogenic compounds potentially introduced to the surface microlayer via the atmosphere or via direct release into the sea should be designed to be photosensitive so that they can be photolytically cleaved.
An important collection of environmental bacterial strains was developed and more than one hundred of different bacterial species were isolated, purified and stored in this collection. This collection represents an interesting output of the project for the European community at the time of development of genomic and proteomic studies. Some of these species should be of interest for biotechnological and bioremediation applications but this need further investigations to better characterize their physiological and metabolic properties. Some species were highly resistant to solar radiations and may have developed efficient DNA repair mechanisms and/or protection against natural radiations. These mechanisms should be investigated in the future and may be of great interest for the pharmaceutical industry. Some species were also highly sensitive to the presence of pollutants and should be used as bioindicators of the presence of low concentrations of pollutants. Again, these species should be studied in more details in the future.
The field work carried out within AIRWIN has demonstrated the enrichment of organic contaminants (e.g. PCBs, PAHs, NP, etc.) and heavy metals (e.g. Pb, Cu and Zn) in the sea surface microlayer, as an organic carbon driven process, both in the dissolved and particulate phases. In this respect, the trophic status of the coastal waters will determine the partitioning of xenobiotics between the particulate, colloidal and truly dissolved phases, which has important implications in assessing the corresponding bioavailability. POC explains the SML enrichment of PCBs and organochlorinated pesticides, which tends to be higher than in the dissolved phase. However, for the PAHs higher enrichments were found, suggesting that PAH partitioning is constrained not only by the organic carbon content of the particles but also by the particular form of this carbon. Presumably, soot carbon concentrations are higher in the SML due to accumulation of atmospherically deposited aerosol in the SML.
The occurrence of SML seems to be a very important driver enhancing dry deposition fluxes. This is true mainly for accumulation mode aerosols. The effect of a SML with lower surface tension and higher hydrophobicity would enhance the collision efficiency of aerosols to the air-water interface.
The field work, in combination with the modeling task, has evidenced that coastal regions are characterized by important net volatilization fluxes of POPs such as PAHs, NPs and PCBs. Conversely, net absorption fluxes are observed at open sea.
In summary, the AIRWIN project has confirmed the initial perspectives of the proposal in the sense that the SML, despite the methodological difficulties of sampling, is a natural integrator of information about ecological processes taking place in the water column. Without special SML studies, the dynamics of the chemical species in the water column and the significance of the anthopogenic inputs cannot be properly understood. The project has contributed to a significant increase of our knowledge on the structure and functions of the air-water interface and addressed some interesting questions and opened new fields of investigations for the European community in the future.