The effect of biotic iron cycling on mercury transformations in an estuarine salt marsh

Iron-rich coastal wetlands may act as sinks for heavy metal contaminants such as mercury, but may also remobilize this biotoxin, as well as mediate chemical changes that affect bioavailablity and toxicity. Walker Marsh is an iron-rich intertidal salt marsh dominated by the halophyte Salicornia sp. that lies between a remediated mercury mine and the San Francisco Bay Delta. Salicornia sp. rhizospheres are an ideal in situ model for studying mercury cycling because radial oxygen loss at their roots causes cycling between oxic and anoxic states, supporting active iron cycling, which in turn can affect mercury mobility. To understand these relationships, Salicornia sp. rhizosphere sediment was compared to adjacent bulk sediment for mercury and iron concentrations, microbial community composition, and mercury resistance biomarker genes. Reactive iron concentrations in rhizosphere sediments were significantly greater than bulk sediments, but average total mercury concentrations were nearly 10% lower. Microbial 16S rRNA gene sequence community analysis showed significant difference between microbial communities of rhizosphere and bulk sediment, and the physiology of highly connected taxa were aerobic in rhizosphere sediment and anaerobic in bulk sediment. Not all rhizosphere samples were iron-rich, however, those that were showed a strong inverse correlation of mercury and iron concentrations (r = -0.80, p ≤ 0.02) suggesting iron cycling supports mercury mobility. To see if biotic iron cycling increased mercury bioavailability in rhizospheres, the mercury resistance gene merA using was quantified by qPCR. This showed a strong presence of mercury-resistant organisms in all samples collected, but showed no difference between iron-rich rhizospheres and bulk sediment. These observations suggest biotic iron cycling promotes mercury mobility but not bioavailability, which may be controlled by abiotic factors.