Carbohydrate and Amino Acid Fermentation in the Free-Living Primitive Protozoon Hexamita sp.

Hexamita sp. is an amitochondriate free-living diplomonad which inhabits O₂-limited environments, such as the deep waters and sediments of lakes and marine basins. ¹³C nuclear magnetic resonance spectroscopy reveals ethanol, lactate, acetate, and alanine as products of glucose fermentation under microaerobic conditions (23 to 34 μM O₂). Propionic acid and butyric acid were also detected and are believed to be the result of fermentation of alternative substrates. Production of organic acids was greatest under microaerobic conditions (15 μM O₂) and decreased under anaerobic (<0.25 μM O₂) and aerobic (200 to 250 μM O₂) conditions. Microaerobic incubation resulted in the production of high levels of oxidized end products (70% acetate) compared to that produced under anoxic conditions (20% acetate). In addition, data suggest that Hexamita cells contain the arginine dihydrolase pathway, generating energy from the catabolism of arginine to citrulline, ornithine, NH₄⁺, and CO₂. The rate of arginine catabolism was higher under anoxic conditions than under microaerobic conditions. Hexamita cells were able to grow in the absence of a carbohydrate source, albeit with a lower growth rate and yield.