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 wer e studied in seawater samples and in a surface microlayer sample colle cted in the north Adriatic Sea, as well as in marine phytoplankton cul ture 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 carb on (DOG) analysis and by adsorption measurements at the mercury electr ode surface in influent and effluent solutions. Dissolved organic surf ace active substances (SAS) in seawater samples taken in June and Sept ember 1992 at different depths in the north Adriatic Sea possessed mos tly hydrophilic properties at natural pH conditions, or conditionally hydrophobic nature in acidic medium. Strongly adsorbable hydrophobic b asic and neutral substances were less abundant, which was also the cas e with the sea surface microlayer sample. Similarly, mixed phytoplankt on culture medium containing predominantly diatoms showed a high conte nt of hydrophilic organic substances (74.5% and 52.9% of total SAS, an d 90.5% and 56.1% of DOG), while the content of hydrophobic basic and neutral substances was extremely low. In contrast, in the mixed phytop lankton culture media in which flagellates are the most abundant speci es, the predominance of hydrophobic basic and neutral SAS (44.4-48.7%) was found, while hydrophobic acid and hydrophilic surface active subs tances contributed in the less extent (30.3-31.9% and 21.0-23.7%, resp ectively). The distribution of the fractions in the mixed-culture medi um 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 a mounts (70.4%) of the total DOG. The hydrophobic acid compounds repres ented only 8.5% of the DOC of the initial sample. The distribution of different fractions in the monocultures of flagellates and diatoms cor responds to the distribution obtained in the mixed-culture media, with the only exception that hydrophobic acid fraction in the diatom monoc ulture of Phaeodactilum tricornutum represented predominant surface ac tive material (83.6%). Seasonal variations of SAS and DOC values in th e north Adriatic Sea in the period 1984-1993 were presented and discus sed with regard to seasonal features of phytoplankton communities and their release products with different hydrophobic and hydrophilic prop erties.