Molecular assays advance understanding of sulfate reduction despite cryptic cycles

Abstract

Abiotic and secondary biotic reactions can contribute to the formation of cryptic biogeochemical cycles, resulting in an underestimation of carbon and nutrient budgets. This Texas coastal estuary sediment study provided a unique opportunity to use multidisciplinary RNA-based molecular and geochemical approaches to identify cryptic cycles associated with sulfate reduction, a commonly measured biogeochemical process considered to be the predominant anoxic terminal electron accepting process in shallow marine environments. Active sulfate reduction within an environment is typically determined by the detection of sulfides. However, a biologically driven cryptic cycle was determined by identifying metabolically active sulfate reducing and sulfur oxidizing lineages co-locating within the sediments, effectively masking sulfide production through re-oxidation back to sulfate. Similar co-location of sulfate and iron reducing lineages prevented the detection of sulfides through the formation of iron sulfide minerals, producing a geochemically driven cryptic cycle. We also showed that sulfate reduction rates determined by 35SO42− incubation analysis can be positively correlated with dsrA transcript abundance, and thus this molecular technique may be a proxy for the prohibitive radioactive method. Based on these results, support is given to a synergistic geochemical and molecular biological strategy to better provide an understanding of marine sulfur cycle.