Fractionation of surface active substances on the XAD-8 resin: Adriatic Sea samples and phytoplankton culture media

Sorption properties on the XAD-8 resin and adsorption behaviour at the mercury electrode surface of naturally present organic substances were studied in seawater samples and in a surface microlayer sample collected in the north Adriatic Sea, as well as in marine phytoplankton culture media. Fractionation of the complex mixture of organic substances into hydrophobic neutral, hydrophobic acid and hydrophilic components by sorption on the XAD-8 resin was followed by dissolved organic carbon (DOG) analysis and by adsorption measurements at the mercury electrode surface in influent and effluent solutions. Dissolved organic surface active substances (SAS) in seawater samples taken in June and September 1992 at different depths in the north Adriatic Sea possessed mostly hydrophilic properties at natural pH conditions, or conditionally hydrophobic nature in acidic medium. Strongly adsorbable hydrophobic basic and neutral substances were less abundant, which was also the case with the sea surface microlayer sample. Similarly, mixed phytoplankton culture medium containing predominantly diatoms showed a high content of hydrophilic organic substances (74.5% and 52.9% of total SAS, and 90.5% and 56.1% of DOG), while the content of hydrophobic basic and neutral substances was extremely low. In contrast, in the mixed phytoplankton culture media in which flagellates are the most abundant species, the predominance of hydrophobic basic and neutral SAS (44.4-48.7%) was found, while hydrophobic acid and hydrophilic surface active substances contributed in the less extent (30.3-31.9% and 21.0-23.7%, respectively). The distribution of the fractions in the mixed-culture medium with flagellates, calculated from DOC analysis, is quite different, i.e. hydrophobic basic and neutral fraction contained lower amounts (21.1%) of DOG, compared to the hydrophilic fraction with the highest amounts (70.4%) of the total DOG. The hydrophobic acid compounds represented only 8.5% of the DOC of the initial sample. The distribution of different fractions in the monocultures of flagellates and diatoms corresponds to the distribution obtained in the mixed-culture media, with the only exception that hydrophobic acid fraction in the diatom monoculture of Phaeodactilum tricornutum represented predominant surface active material (83.6%). Seasonal variations of SAS and DOC values in the north Adriatic Sea in the period 1984-1993 were presented and discussed with regard to seasonal features of phytoplankton communities and their release products with different hydrophobic and hydrophilic properties.