AME 19:297-305 (1999)  -  doi:10.3354/ame019297

ABSTRACT: The immobilization and metabolism of dissolved organic carbon (DOC) in natural streambed biofilms from a Mediterranean (La Solana [SOL], Spain) and a temperate (Oberer Seebach [OSB], Austria) stream were investigated using plug-flow bioreactors. Bioreactors were subjected to short-term pulses of allochthonous DOC, i.e. extracted from stream adjacent soils, and of autochthonous DOC, i.e. extracted from stream periphyton. In SOL biofilms, relatively high concentrations of extracellular polymeric substances (EPS) co-occurred with retarded metabolic response to DOC pulses and relatively low respiration rates. In OSB biofilms, however, EPS concentrations were lower and respiration rates were elevated along with immediate metabolic responses to DOC pulses. Furthermore, the uptake rate of DOC from the bulk water increased linearly with the respiration rate in OSB biofilms, yet no such relationship existed in SOL biofilms. Carbon normalized respiration rates (μg O₂ mg^-1 DOC h^-1) generally revealed higher metabolic availability of autochthonous DOC than allochthonous DOC in both streams. Cross-feeding experiments (e.g. feeding OSB bioreactors with SOL DOC, and vice versa) revealed higher metabolic availability of terrigenous DOC from OSB than from SOL catchment soils. Cross feeding also suggested that autochthonous DOC from SOL inhibits the metabolism of OSB biofilms. We tentatively propose cyanobacterial toxins as the source of this inhibition. Contrasting patterns of biofilm EPS concentrations, DOC immobilization and metabolism suggest an adaptive response of the microbial biofilm community to major catchment-scale (e.g. climate, vegetation and diagenesis of terrigenous DOC) and channel (e.g. storm frequency and scouring) processes in temperate and Mediterranean streams.

KEY WORDS: Streambed biofilms · Dissolved organic carbon · Metabolism · Bioavailability · Bioreactors