The HFC/HCFC breakdown product trifluoroacetic acid (TFA) and its effects on the symbiosis between Bradyrhizobium japonicum and soybean (Glycine max)

The environmental impact of hydrofluorocarbon (HFC) and hydrochlorofluorocarbon (HCFC) are under intense study due to the resistance of the breakdown product, trifluoroacetic acid (TFA), to further degradation. TFA has come under scrutiny due to its rapid and complete partitioning into aqueous phases of the environment, eventually allowing for deposition of TFA into soil via precipitation. Evidence exists that TFA may be toxic to soil microbes and plants, with little or no degradation occurring in soils. Uptake by plants and microorganisms and its similarity to acetate, implies its potential to effect the symbiotic nitrogen-fixing partners Bradyrhizobium japonicum and soybean (Glycine max). A preliminary study was performed in accordance with the Alternative Fluorocarbon Environmental Acceptability Study (AFEAS). Those results are presented here in addition to the findings of further experimentation on the initial interaction of B. japonicum with soybean. We used three levels of TFA (0.67, 6.74 and 67.40 μl TFA kg−1 soil; 0.003, 0.031 and 0.314 μl TFA l−1) for soil and hydroponics conditions and three levels (10, 100 μM and 1 mM) in bacterial culture. The results demonstrate that TFA affects growth of B. japonicum significantly, but does not affect PHB accumulation. Also no F− was found in cultures grown on TFA. Attachment of B. japonicum to soybean roots was enhanced with the lowest level of acetate or TFA and was significantly reduced with 1 mM acetate or TFA. Cultures grown on acetate or acetate with TFA do not attach well, with those grown with 1 mM TFA the least. Both effects may be attributed to pH. Soybean seedlings had significantly retarded development with levels of TFA at or above 6.74 μl TFA kg−1 soil and 0.031 μl TFA l−1 nutrient solution. No nodules formed on those plants treated with these levels of TFA except in the hydroponics trials. Nodule location was not affected regardless of the TFA level. At the lowest level used we found no effects on soybean or symbiotic nitrogen fixation. In some cases, nodulation was enhanced, but nodule weight reduced. Anaerobically isolated bacteroids had normal levels of acetylene reduction activity regardless of the level of TFA used. In summary, soybean is much more sensitive to low levels of TFA than its symbiotic counterpart B. japonicum. No detrimental effects on symbiotic nitrogen fixation in soybean should be expected unless large bioaccumulation of TFA occurs in agricultural areas.