Abstract: The present invention enables isomerization process flows at lower pH, lower temperature, and in the presence of certain inhibiting compounds by using the xylose isomerase enzyme from the microorganism Fulvimarina pelagi. The xylose isomerase from this marine bacterium not only is very active at the fermentation pH and temperature, it is also tolerant of xylitol and calcium in the amounts generated in biomass fermentation conditions. For the HFCS application, the xylose isomerase from Fulvimarina pelagi is pH compatible with the process and is tolerant of calcium in the amounts found in the HFCS process. The temperature optimum is similar to commercially available amylase/gluco-amylase products.