The effects of long chain fatty acids (LCFAs) on H₂-producing mixed anaerobic cultures were evaluated using kinetic, biochemical, molecular biology and statistical methods. The objectives of this study were to assess changes in microbial diversity and metabolic pathways by exposing the cultures to LCFAs. Batch experiments were conducted at 37°C and initial pH 5.0 containing a mixed culture fed glucose and LCFAs (linoleic, palmatic, myristic, lauric, or a 50:​50 mixture of myristic and palmatic acids). The H₂ yields for cultures from different sources were statistically the same when LA was added (3.3 mole H₂·mole-1 glucose;​ conversion efficiency = 82.5%). LA inhibited H₂-consumption, propionate production, and directed the electron flux towards acetate:​butyrate fermentation. Clostridia, Bacteroides, Flavobacteria, Syntrophus, Geobacillus, Actinobacillus, Citrobacter, Cytophaga, Enterobacter, Erwinia, E. coli, and Klebsiella dominated the LA-treated culture while Bacteroides, Bacillus, Clostridia, Acinetobacter, Falvobacteria, Eubacteria, and Rubrobacter dominated the control culture.

When LA was added at an initial pH of 5.0, the H₂ specific consumption rate decreased by approximately 99%;​ however, only a 24% reduction was detected in controls when the initial pH was set at pH 5.0. In the presence of LA, acetate, propionate, and butyrate degradation were inhibited. LA and its degradation by-products (PA, MA, and LUA) sustained higher H₂ yields for 30 days in suspended cultures but not in granulated cultures.

LA affected the diversity of the microbial communities differently in suspended and granular cultures from the same origin. After LA-treatment, the suspended culture showed a greater diversity of acidogens than the granular culture.

Adding PA increased the H₂ yield;​ however, LCFAs shorter than 16 carbons did not show any H₂ production. Thermoanaerovibrio, Geobacillus and Eubacteria dominated the control cultures while Clostridia comprised less than 1% of the biomass. LCFAs caused an increased in the abundance of Clostridia and Bacillus to differing degrees (PAThermoanaerovibrio, while increasing levels of Geobacter lovely and Bacteroides were detected.

Long-term adaptation to glucose eliminated Bacteroides, Parabaceroides, Lactobacillus, Fusobacterium and Syntrophobacter, and reduced the abundance of Deslfovibrio and Desulfobacter. H₂-consumption decreased and H₂ yield increased following LA-treatment and long-term adaptation to glucose./p>