Production of fructooligosaccharides (FOS) from sucrose was obtained using a bioprocess entirely performed in seawater. The halophilic fungus Cladosporium cladosporioides MUT 5506 was grown in a seawater-based medium and mycelium displayed an optimal activity in seawater at 50-60 degrees C, being stable up to 60 degrees C. Under optimized conditions in seawater (50 degrees C, sucrose 600 g/L, lyophilized mycelium 40 g/L), C. cladosporioides gave a maximum FOS yield of 344 g/L after 72 h with a preponderance of F-1-FOS derivatives (1-kestose 184 g/L, 1-nystose 98 g/L and 1-fructofuranosylnystose 22 g/L), and the noteworthy presence of the non-conventional disaccharide blastose (30 g/L after 144 h). Lyophilized mycelium exhibited good stability in seawater (76% of the initial activity was retained after 15 cycles of reutilization). This proof-of-concept application reports for the first time the production of FOS in a non-conventional medium such as seawater. (C) 2015 Elsevier Ltd. All rights reserved.