Electrochemically active bacteria (EAB) on the cathodes of microbial electrolysis cells (MECs) can remove metals from the catholyte, but the response of these indigenous EAB toward exotic metals has not been examined, particularly from the perspective of the co-presence of Cd(II) and Cr(VI) in a wastewater. Four known indigenous Cd-tolerant EAB of Ochrobactrum sp X1, Pseudomonas sp X3, Pseudomonas delhiensis X5, and Ochrobactrum anthropi X7 removed more Cd(II) and less Cr(VI) in the simultaneous presence of Cd(II) and Cr(VI), compared to the controls with individual Cd(II) or single Cr(VI). Response of these EAB toward exotic Cr(VI) was related to the associated subcellular metal distribution based on the sensing of fluorescence probes. EAB cell membrane harbored more cadmium than chromium and cytoplasm located more chromium than cadmium, among which the imaging of intracelluler Cr(III) ions increased over time, contrary to the decreased trend for Cd(II) ions. Compared to the controls with single Cd(II), exotic Cr(VI) decreased the imaging of Cd(II) ions in the EAB at an initial 2 h and negligibly affected thereafter. However, Cd(II) diminished the imaging of Cr (III) ions in the EAB over time, compared to the controls with individual Cr(VI). Current accelerated the harboring of cadmium at an initial 2 h and directed the accumulation of chromium in EAB over time. This study provides a viable approach for simultaneously quantitatively imaging Cd(II) and Cr (III) ions in EAB and thus gives valuable insights into the response of indigenous Cd-tolerant EAB toward exotic Cr(VI) in MECs.
