A key step in the chlorine cycle is the reduction of perchlorate (ClO 4 -) and chlorate (ClO 3 -) to chloride by microbial respiratory pathways. Perchlorate-reducing bacteria and chlorate-reducing bacteria differ in that the latter cannot use perchlorate, the most oxidized chlorine compound.However, a recent study identified a bacterium with the chlorate reduction pathway dominating a community provided only perchlorate. Here we confirm a metabolic interaction between perchlorate-and chlorate-reducing bacteria and define its mechanism. Perchlorate-reducing bacteria supported the growth of chlorate-reducing bacteria to up to 90% of total cells in communities and co-cultures. Chlorate-reducing bacteria required the gene for chlorate reductase to grow in co-culture with perchlorate-reducing bacteria, demonstrating that chlorate is responsible for the interaction, not the subsequent intermediates chlorite and oxygen. Modeling of the interaction suggested that cells specialized for chlorate reduction have a competitive advantage for consuming chlorate produced from perchlorate, especially at high concentrations of perchlorate, because perchlorate and chlorate compete for a single enzyme in perchloratereducing cells. We conclude that perchlorate-reducing bacteria inadvertently support large populations of chlorate-reducing bacteria in a parasitic relationship through the release of the intermediate chlorate. An implication of these findings is that undetected chlorate-reducing bacteria have likely negatively impacted efforts to bioremediate perchlorate pollution for decades.

15A key step in the chlorine cycle is the reduction of perchlorate (ClO 4 -) and chlorate (ClO 3 -) to 16 chloride by microbial respiratory pathways. Perchlorate-reducing bacteria and chlorate-reducing 17 bacteria differ in that the latter cannot use perchlorate, the most oxidized chlorine compound. 18However, a recent study identified a bacterium with the chlorate reduction pathway dominating a 19 community provided only perchlorate. Here we confirm a metabolic interaction between 20 perchlorate-and chlorate-reducing bacteria and define its mechanism. Perchlorate-reducing 21 bacteria supported the growth of chlorate-reducing bacteria to up to 90% of total cells in 22 communities and co-cultures. Chlorate-reducing bacteria required the gene for chlorate reductase 23 to grow in co-culture with perchlorate-reducing bacteria, demonstrating that chlorate is 24 responsible for the interaction, not the subsequent intermediates chlorite and oxygen. Modeling of 25 the interaction suggested that cells specialized for chlorate reduction have a competitive 26 advantage for consuming chlorate produced from perchlorate, especially at high concentrations of 27 perchlorate, because perchlorate and chlorate compete for a single enzyme in perchlorate-28 reducing cells. We conclude that perchlorate-reducing bacteria inadvertently support large 29 populations of chlorate-reducing bacteria in a parasitic relationship through the release of the 30 intermediate chlorate. An implication of these findings is that undetected chlorate-reducing 31 bacteria have likely negatively impacted efforts to bioremediate perchlorate pollution for decades. 32 33 34 35 36 37 38 39 3