The role of glycerol in the nutrition of halophilic archaeal communities: a study of respiratory electron transport

Respiratory electron transport activity in the Dead Sea and saltern crystallizer ponds, hypersaline environments inhabited by dense communities of halophilic archaea and unicellular green algae of the genus Dunaliella, was assayed by measuring reduction of 2-(p-iodophenyl)-3(p-nitrophenyl)-5-phenyl tetrazolium chloride (INT) to INT-formazan. Typical rates obtained were in the order of 5.5-17.7 nmol INT reduced h ^-1 per 10⁶ cells at 35 ° C. In Dead Sea water samples, respiratory activity was stimulated more than two-fold by addition of glycerol, but not by any of the other carbon compounds tested, including sugars, organic acids, and amino acids, or by addition of inorganic nutrients. Stimulation by glycerol had a half-saturation constant of 0.75 μM. A similar respiratory activity was also found when Dead Sea water samples were diluted with distilled water and incubated in the light. As Dunaliella cells did not reduce INT, it is suggested that photosynthetically produced glycerol leaking from the algae is the preferred carbon and energy source for the development of halophilic archaea in hypersaline environments. In samples from saltern crystallizer pond stimulation of INT reduction by glycerol was much less pronounced, probably because the community was less severely carbon-limited.