The complex mechanisms of marine mucilage formation by spectroscopic investigation of the structural characteristics of natural and synthetic mucilage samples

In this paper specific aspects concerning pathways of the anomalous aggregations of organic matter called mucilages were investigated by means of different analytical approaches applied to natural samples taken in Italian seas and synthetic samples obtained in laboratory under anoxic conditions. Samples of marine mucilages were previously treated by ultrafiltration by means of a 1 KDa cut off membrane to eliminate inorganic salts and then centrifugated to separate the macroaggregate fraction from the fraction of micro and nano particles of organic matter (MNO) present in the aqueous matrix as dissolved and particulate components. Then the obtained subsamples were analyzed by infrared spectroscopy in reflectance diffused mode (FTIR-DRIFT). FTIR spectroscopy supported our already discussed hypothesis that mucilages can be the evolutionary products of smaller organic matter aggregates because the two subfractions always showed high compositional and structural similarities. Synthetic mucilage samples obtained by several algae and molluscs placed in anoxic conditions were investigated by infrared spectroscopy in total attenuated reflectance mode (FTIR-ATR) and by visible ultraviolet spectroscopy (UV–VIS). The comparison between FTIR spectra of synthetic and marine mucilages performed by the multivariate statistic technique principal component analysis (PCA) showed the presence of wide zones of overlapped (similar) samples. This result allowed to use synthetic mucilages to study the characteristics of the mechanisms involved in mucilage formation. FTIR spectra of synthetic mucilages suggested that carbohydrates and proteins play a different role with respect to lipids in the formation of aggregates because they were always significant components of aggregates while lipids (fatty acids, ester fatty acids and aliphatic chains) were more evident during some specific steps of aggregation. Moreover, the examination of second derivative order FTIR spectra sometimes showed the presence of lignin and biogenic silica compounds in the aggregates. The examination of UV–VIS spectra confirmed the presence of aromatic and unsaturated compounds and chlorophyll pigments in mucilages obtained from green algae while brown and red algae showed only the presence of aromatic and unsaturated compounds. All the experimental results confirmed our previous hypothesis that mucilages are formed as a consequence of an altered mechanism of aggregation of organic matter when the degradation reactions become slower than those of polymerization and aggregation. At last, some specific studies on the formation of mucilages in presence of anomalous amounts of Cu and Pb showed that these metals can affect the mechanism of aggregation process.