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 cr ystallizer ponds, hypersaline environments inhabited by dense communit ies of halophilic archaea and unicellular green algae of the genus Dun aliella, was assayed by measuring reduction of 2-(p-iodophenyl)-3(p-ni trophenyl)-5-phenyl tetrazolium chloride (INT) to INT-formazan. Typica l rates obtained were in the order of 5.5-17.7 nmol INT reduced h(-1) per 10(6) cells at 35 degrees C. In Dead Sea water samples, respirator y 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 mu M. A similar respiratory activity was also found when Dead Sea water sam ples were diluted with distilled water and incubated in the light. As Dunaliella cells did not reduce INT, it is suggested that photosynthet ically produced glycerol leaking from the algae is the preferred carbo n and energy source for the development of halophilic archaea in hyper saline environments. In samples from saltern crystallizer pond stimula tion of INT reduction by glycerol was much less pronounced, probably b ecause the community was less severely carbon-limited.