Mucopolysaccharide transformation by sulfide in diatom cultures and natural mucilage

The role of sulfur species in the formation and stability of mucus in the Adriatic Sea was studied in natural aggregates and in a mixed diatom culture. Benthic diatoms were isolated from a sample collected from the sea bottom in the northern Adriatic Sea, cultivated in f/2 medium at room temperature, and irradiated with dim light (30 to 40 mumol m(-2) s(-1)). Subcultures were incubated at room temperature under anaerobic conditions with different concentrations of sulfide (10(-4) and 10(-3) mol l(-1)) for up to 1 mo. Interactions between sulfide and organic matter in the cultures were followed for 10 d through: (1) formation of organosulfur species; (2) changes in surfactant activity; (3) degradation of pennate diatoms followed by cell lysis and release of large amount of polysaccharides. Dissolved organic carbon (DOC) concentration increased from 7.17 mg l(-1) in the ambient water of untreated diatom cultures to 40 mg l(-1) in sulfide-treated samples. Surfactant activity increased by 2 orders of magnitude in the same sulfide-treated samples (both ambient water and aggregates of enriched diatom cultures). The concentration of surface-active substances (SAS, expressed as Triton-X-100 concentration) 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 showed that diatoms decomposed under sulfide treatment; a specific molecular probe for glucose and mannose (Con-A lectin conjugated by fluorescein) showed that decomposing diatoms released large amounts of polysaccharides. These experiments indicate that massive diatom lysis in bottom or anoxic microzones is one of the factors triggering mucilage formation in the Adriatic Sea, and reduced sulfur species due to microbial activity play an important role in the formation and stability of mucilage development.