Formation of chloroform and tetrachloroethene by Sinorhizobium meliloti strain 1021

The mechanisms and organisms involved in the natural formation of volatile organohalogen compounds (VOX) are largely unknown. We provide evidence that the common and widespread soil bacterium Sinorhizobium meliloti strain 1021 is capable of producing up to 3338·6±327·8ngl^-1 headspace volume of chloroform (CHCl₃) and 807·8±13·5ngl^-1headspace volume of tetrachloroethene (C₂Cl₄) within 1h when grown in soil extract medium. Biotic VOX formation has been suggested to be linked to the activity of halogenating enzymes such as haloperoxidases. We tested if the observed VOX formation by S.meliloti can be attributed to one of its chloroperoxidases (Smc01944) that is highly expressed in the presence of H₂O_2. However, addition of 10mmoll^-1 H₂O₂ to the S.meliloti cultures decreased VOX formation by 52% for chloroform and 25% for tetrachloroethene, while viable cell numbers decreased by 23%. Interestingly, smc01944 gene expression increased 450-fold. The quantification of extracellular chlorination activity in cell suspension experiments did not provide evidence for a role of S.meliloti chloroperoxidases in the observed VOX formation. This suggests that a momentarily unknown mechanism which requires no H₂O₂ might be responsible for the VOX formation by S.meliloti. Regardless of the underlying mechanism our results suggest that the soil bacterium S.meliloti might be an important source of VOX in soils.