Multiheme c -Type Cytochrome-Dependent Arsenate Reduction Enhances Methanogenesis in Methanosarcina acetivorans

Abstract

Microorganism-mediated arsenate (As(V)) reduction plays a critical role in arsenic mobility and microbial adaptation in arsenic-contaminated anoxic environments. Current understanding of microbial As(V) reduction has primarily focused on two pathways: extracellular reduction mediated by membrane-bound As(V) respiratory reductase, and intracellular reduction catalyzed by cytoplasmic As(V) detoxification reductase. Here, we report an extracellular As(V) reduction process that operates independently of both known As(V) reductases and is mediated by methanogenic archaea that are recognized for obligate methanogenesis across diverse anoxic habitats. We demonstrate that a model methanogenic archaeon is capable of concurrently performing intracellular methanogenesis and extracellular As(V) reduction, with the latter process significantly enhancing methanogenic growth. Through a combination of microbial physiological, biochemical, and molecular genetic approaches, we identify multiheme c-type cytochromes (MHCs), which are widely distributed within the order Methanosarcinales and have been previously implicated in extracellular iron reduction, as the key molecular components responsible for extracellular As(V) reduction. Furthermore, employing density functional theory simulations, we propose plausible reduction pathways for As(V) reduction mediated by the heme prosthetic groups of MHCs and confirm their thermodynamic feasibility. This work not only provides new insights into the previously overlooked coupling between arsenic transformation and methanogenesis in anoxic environments on Earth, but also underscores the interconnected nature of iron and As(V) reduction, both of which can be mediated by MHCs.