The overlooked role of manganese in biodegradation studies of higher aminopolyphosphonates

Higher aminopolyphosphonates (APPs, with three or more C-P bonds) are common complexing agents, but their environmental fate and degradation/transformation pathways remain poorly understood. While abiotic degradation of higher APPs has been reported, recent studies have also proposed the potential for enzymatic breakdown based on laboratory model systems. However, herein, we show rapid transformation of two representative higher APPs - aminotris(methylenephosphonate) (ATMP, with three C-P bonds) and ethylenediaminetetramethylenephosphonate (EDTMP, with four C-P bonds) - in bacterial growth media both in the presence and the absence of Achromobacter insolitus strain Kg 19. The abiotic transformation was driven by trace levels of dissolved Mn(II) - typically present in mineral solutions used in growth media. Mn(II) catalyzed the abiotic oxidation of ATMP by molecular oxygen, yielding iminodimethylenephosphonate (IDMP) and orthophosphate (o-PO₄) as transformation products, via a known mechanism previously described in simpler matrices. The addition of ethylenediaminetetraacetic acid (EDTA) as metal complexing agent retarded but did not prevent the abiotic transformation. Bacterial growth under phosphorous (P)-limiting conditions in the presence of ATMP and EDTMP was thus supported by the formation of P-containing transformation products of the concurrent abiotic degradation rather than biotransformation of ATMP and EDTMP. These findings reveal a critical limitation in prior biodegradation studies of higher APPs, which likely overestimated biotransformation by overlooking Mn(II)-mediated abiotic degradation. Future research must redesign biodegradation approaches and disentangle biotic and abiotic mechanisms to accurately evaluate the actual biodegradability of higher APPs.