This work examined the chiral inversion
of 2-arylpropionic acids
(2-APAs) under anaerobic conditions and the associated microbial community.
The anaerobic condition was simulated by two identical anaerobic digesters.
Each digester was fed with the substrate containing 11 either pure
(R)- or pure (S)-2-APA enantiomers.
Chiral inversion was evidenced by the concentration increase of the
other enantiomer in the digestate and the changes in the enantiomeric
fraction between the two enantiomers. Both digesters showed similar
and poor removal of 2-APAs (≤30%, except for naproxen) and
diverse chiral inversion behaviors under anaerobic conditions. Four
compounds exhibited (S → R) unidirectional inversion [flurbiprofen, ketoprofen, naproxen, and
2-(4-tert-butylphenyl)propionic acid], and the remaining
seven compounds showed bidirectional inversion. Several aerobic and
facultative anaerobic bacterial genera (Candidatus
Microthrix, Rhodococcus, Mycobacterium, Gordonia, and Sphingobium) were identified
in both digesters and predicted to harbor the 2-arylpropionyl-CoA
epimerase (enzyme involved in chiral inversion) encoding gene. These
genera presented at low abundances, <0.5% in the digester dosed
with (R)-2-APAs and <0.2% in the digester dosed
with (S)-2-APAs. The low abundances of these genera
explain the limited extent of chiral inversion observed in this study.