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The role of sulfur species in the formation and stability of mucus aggregates in the Adriatic Sea was studied. Natural aggregates were compared with a mixed diatom culture. A sample from the sea bottom collected from the northern Adriatic Sea was cultivated and transferred to f/2 medium to isolate benthic diatoms. Diatom cultures were incubated at room temperature and irradiated with dim light (30 to 40 mmol m-2 s-1). Subcultured samples were incubated at room temperature under anaerobic conditions for up to 1 mo, with different concentrations of sulfide (10-4 and 10-3 mol l-1). In diatom cultures, interactions between sulfide and organic matter were investigated and followed for 10 d through: (1) formation of organosulfur species ; (2) changes in surfactant activity and (3) degradation of pennate diatoms followed by cell lysis and release of large amount of polysaccharides. A large increase in dissolved organic carbon (DOC) concentration was recorded in the ambient water of the sulfide-treated samples. A DOC concentration of 7.17 mg l-1 was determined in the ambient water of untreated diatom cultures increasing to 40 mg l-1 in sulfide-treated samples. Simultaneously, in the same sulfide-treated samples (both ambient water and aggregates of enriched diatom cultures), surfactant activity was observed to increase by 2 orders of magnitude. The concentration of surface-active substances (SAS, expressed as Triton-X-100 conc.) increased from 0.35 mg l-1 in the aggregate and 0.59 mg l-1 in the ambient water of the untreated diatom cultures to 20 mg l-1 in sulfide-incubated samples. Epifluorescence microscopy observations showed that diatoms decomposed under sulfide treatment and released a large amount of polysaccharides, as revealed by a specific molecular probe for glucose and mannose, i.e. Con-A lectin conjugated by fluorescein. These comparative experiments indicated that one of the factors triggering mucilage formation in the Adriatic Sea could be massive diatom lysis in bottom or anoxic microzones where, due to microbial activity, reduced sulfur species are prominent and have an important role in the formation and stability of mucilage development.

Mucopolysaccharide transformation by sulfide in diatom cultures and natural mucilage