Influence of growth phases and desiccation on the degrees of unsaturation of fatty acids and the survival rates of rhizobia

The influence of growth phase on the evolution of cellular fatty acids (CFA ) and survival of Sinorhizobium and Bradyrhizobium during desiccation and s torage at different levels of relative humidity (R.H.) was studied. Lactoba cillic, cis vaccenic and palmitic acids were the major fatty acids of S. me liloti RCR 2011, B. elkanii USDA 120 and B. japonicum 3.2, whatever the gro wth phase. An exchange of cis vaccenic with lactobacillic acid was observed during the course of growth. The degree of unsaturation (% unsaturated CFA /% saturated CFA = u/s ratio) was significantly higher during the mid logar ithmic phase of growth. Survival rates immediately after desiccation were u naffected by the growth phase and the R.H. Furthermore, no correlation was found between survival rate and u/s ratio. During the course of desiccation , the u/s ratio of rhizobia decreased but the decrease was largely independ ent of the R.H. Optimum R.H. values for storage were in the range 22-67.8%, and S. meliloti was significantly more tolerant than the bradyrhizobia. Ce lls of rhizobia harvested in the lag phase of growth were more resistant to protracted storage than cells at other growth phases. Again, no correlatio n was found between u/s ratio and survival rates, despite the expected prac tical significance for screening for drought-tolerant micro-organisms.