A little bit of light goes a long way: the role of phototrophs on mercury cycling

Among toxic metals, mercury (Hg) is a global priority contaminant due to the biomagnification of the most toxic form methylmercury (MeHg) in food webs, even in remote regions, such as the high Arctic. The importance of Hg as a chemical of major concern to human health was underscored by the recent adoption of the Minamata Convention on Mercury, a legally binding treaty that requires government agencies be equipped to monitor processes affecting global mercury transport and cycling. For several decades now, field and laboratory experiments have shown that phototrophs can directly interact with Hg and affect its speciation and fate. While an important body of work on the role of chemotrophic microbes on Hg cycling has been undertaken, the role of phototrophs is too often overlooked. Strikingly, what is known about phototroph- Hg interactions pertains mostly to oxygenic phototrophs with relatively little being known about anoxygenic phototrophs. Ongoing environmental change will no doubt affect the physical and chemical properties of aquatic ecosystems, which in turn will alter all phototrophic community dynamics. How these changes will affect the Hg cycle represent an important knowledge gap. After synthesizing what is currently known about chemotrophic Hg transformations, we describe the current state of knowledge on what is known about how phototrophs (bacteria and algae) affect Hg cycling (i. e., alteration of Hg redox state, Hg scavenging, potential for methylation) as well as describe the cellular and molecular targets of Hg toxicity in phototrophs. We discuss these interactions in an evolutionary context and provide recommendations for future research directions.