Strategies of sulfate-reducing bacteria to escape oxygen stress in a cyanobacterial mat

Abstract

Most probable number (MPN) counts of sulfate-reducing bacteria in the upper 3 mm of a cyanobacterial mat of Solar Lake (Sinai, Egypt) were 20 times lower during the day under oxic conditions than at night in the absence of oxygen. In order to study the oxygen-dependent behaviour of the predominant sulfate reducers four strains were isolated from the highest dilutions of the MPN series. Strain D22 was obtained from a sample taken during the day, while the strains N13, N33 and N24 were dominant in the upper layer at night. All isolates belonged to the genus Desulfovibrio. Strain N24 and strain N13 could be identified as Desulfovibrio oxyclinae. In capillary tubes filled with cell suspensions strain N13, but not strain D22, showed a negative aerotactic response by forming bands around oxygen bubbles at the oxic/anoxic interface. All isolates respired with oxygen using H 2, lactate or sulfide as electron donors. Ethanol, which could be used for sulfate reduction, was not metabolised with oxygen. Strain N13 grew with oxygen for 2 days. However, cell division was inhibited. While the protein content increased, the cell number remained constant and the cell length increased up to fivefold. Strain D22, which remained in the upper layer during the day, appeared not to be better adapted to oxic conditions than the strains isolated from the anoxic zone. It is concluded that possible strategies of sulfate-reducing bacteria to escape oxygen stress in a cyanobacterial mat include migration to deeper anoxic zones, formation of clumps, and oxygen removal by active respiration in bands.