ABSTRACT
Biogas production has been shown to be inhibited by branched chain fatty acids (isobutyric, isovaleric) produced in the digester by cellulolytic organisms. Performance of these mixed cellulolytic cultures isolated at 25°C (C25) and at 35° (C35) in a batch digester using cattle manure confirmed that C35, which forms mainly straight chain fatty acids from cellulose was more suitable for use as an inoculum than C 25 which formed predominantly branched chain fatty acids. Reconstitution of cellulolytic culture C35 and mixed methanogens M35 almost doubled both the amount and rate of methane production. Cellulolytic culture was useful in pretreatment of water hyacinth prior to its use as a substrate for methane generation. A method for preservation and transportation of mixed cellulolytic culture for use as an bineodc. ulum in the digester is described.
ABSTRACT
The composition of volatile fatty acids in the biogas digester based on cattle manure as substrate and stabilised at 25°C showed that it contained 87-88% branched chain fatty acids, comprising of isobutyric and isovaleric acids, in comparison to 38 % observed in the digester operating at 35°C. Mixed cellulolytic cultures equilibrated at 25°C (C-25) and 35°C (C-35) showed similar properties, but rates of hydrolysis were three times higher than that observed in a standard biogas digester. The proportion of isobutyric and isovaleric were drastically reduced when C- 25 was grown with glucose or filter paper as substrates. The volatile fatty acids recovered from C-25 (at 25°C) inhibited growth of methanogens on acetate, whereas that from C-35 was not inhibitory. The inhibitory effects were due to the branched chain fatty acids and were observed with isobutyric acid at concentrations as low as 50 ppm. Addition of another micro-organism Rhodotorula selected for growth on isobutyric completely reversed this inhibition. Results indicate that the aceticlastic methanogens are very sensitive to inhibition by branched chain fatty acids and reduction in methane formation in biogas digester at lower temperature may be due to this effect.
ABSTRACT
Dipicolinic acid synthesis in Penicillium citreoviride strain 3114 was inhibited by Ca2+ ions, but not by Ba2+, Cu2+or Fe2+. Among the metals tested, only Zn2+ inhibited the synthesis of dipicolinic acid and promoted sporulation. None of these metals reversed the inhibition by Ca2+ or Zn2+ . Α mutant 27133-dpa-ca selected for resistance to feedback inhibition by dipicolinic acid: Ca2+ complex showed cross-resistance to inhibition by dipicolinic acid: Zn2+. Both 3114 and 27133-dpa-ca excreted a number of aliphatic and amino acids during secondary metabolism of dipicolinic acid. In the presence of 1000 ppm of Ca2+, accumulation of citric acid and α-aminoadipic acid was completely inhibited under conditions of inhibition of dipicolinic acid in parent strain 3114 but not in the mutant. Citric acid with or without Ca2+ did not inhibit the de novo synthesis of dipicolinic acid in the strain 3114. In fact, citric acid in the presence of Ca2+ improved significantly rate of dipicolinic acid synthesis. Apart from resistance to feed back inhibition by dipicolinic acid: Ca2+ complex, mutant differed from the parent in three other aspects viz. (i) dipicolinic acid synthesis was not subject to catabolite repression by glucose, (ii) sporulation as well as dipicolinic acid synthesis was dependent on the presence of Ca2+ ions in the medium and (iii) Mg2+ requirement for the mutant increased three fold. Higher requirement of the Mg2+ could be partially relieved by Ca2+ during secondary metabolism. The results support the inference that de novo synthesis of dipicolinic acid is regulated through feedback inhibition by dipicolinic acid: Ca2+ complex.
ABSTRACT
Synthesis of dipicolinic acid in Penicillium citreoviride showed typical kinetics of a secondary metabolite. Its synthesis resumed during idiophase and continued through stationary phase of growth. Total duration of synthesis was 100 h at the end of which its synthesis was arrested. Production of dipicolinic acid by the cells was subject to catabolite repression by glucose and was not subject to end product inhibition by exogenously added dipicolinic acid. Unlike the bacteria, dipicolinic acid synthesis in this mold was highly sensitive to inhibition by calcium ions in the growth medium. Calcium promoted sporulation but dipicolinic acid was not found to be present in detectable amounts in mold spores. Addition of dipicolinic acid and Ca2+ completely inhibited its de novo synthesis, an effect not observed when calcium was replaced by Mg2+ When the mold was grown in the presence of calcium alone, its inhibitory effects on de novo synthesis of dipicolinic acid were expressed only after some of this metabolite was first synthesised by the producer cells suggesting that the active feedback inhibitor is probably a Ca: dipicolinic acid complex. It is suggested that over-production of this metabolite is very important to the mold in increasing its survival potential in nature by retrieving the essential minerals from the environment through ligand: metal complex at a time when cells are in the process of dying, so that a proper mineral balance is maintained within the cells.