Caldicellulosiruptor kronotskyensis has gained interest for its ability to grow on various lignocellulosic biomass. The aim of this study was to investigate the growth profiles of C . kronotskyensis in the presence of mixtures of glucose-xylose. Recently, we characterized a diauxic-like pattern for C. saccharolyticus on lignocellulosic sugar mixtures. In this study we aimed to investigated further whether C . kronotskyensis has adapted to uptake glucose in the disaccharide form (cellobiose) rather than the monosaccharide (glucose).
Interestingly, growth of C . kronotskyensis on glucose and xylose mixtures did not display diauxic-like growth patterns. Closer investigation revealed that, in contrast to C. saccharolyticus , C . kronotskyensis does not possess a second uptake system for glucose. Both C. saccharolyticus and C . kronotskyensis share the characteristics of preferring xylose over glucose. Growth on xylose was twice as fast (μ max = 0.57 h -1 ) as on glucose (μ max = 0.28 h -1 ). It was found that C . kronotskyensis takes up glucose and xylose simultaneously with the same transporter. A study of the sugar uptake was made with different glucose-xylose ratios to find a kinetic relationship between the two sugars for transport into the cell. High concentrations of glucose inhibited xylose uptake and vice versa. The inhibition constants were estimated to be K I,glu = 0.01 cmol·L -1 and K I,xyl = 0.001 cmol·L -1 , hence glucose uptake was more severely inhibited by xylose uptake. Bioinformatic analysis indicated the lack of another sugar uptake system in C . kronotskyensis as compared to C. saccharolyticus . Therefore, it was investigated whether glucose uptake by C . kronotskyensis was in the form of cellobiose. Indeed, cellobiose is taken up faster than glucose, nevertheless, the growth rate on each sugar remained similar.
C . kronotskyensis possesses a xylose transporter that might take up glucose at an inferior rate even in the absence of xylose. Alternatively, glucose can be taken up in the form of cellobiose, but growth performance is still inferior to growth on xylose. Therefore, we propose that the catabolism of C . kronotskyensis has adapted more strongly to pentose rather than hexose thereby having obtained a specific survival edge in thermophilic lignocellulosic degradation communities.