Microcystin is one of the most common toxins associated with freshwater harmful algal blooms, but little is known about microcystin fate in the aquatic environment. Laboratory wave tank experiments were performed to determine whether exchange of surface water and pore water (benthic exchange) removes and dilutes microcystin‐LR (MC‐LR) at environmentally relevant concentrations in coastal waters overlying permeable sediments. Over the 100 h experiment, 60% of MC‐LR mass was removed due to interaction with sediment (via adsorption and/or biodegradation), while only 20% was removed in an experiment without sediment. The observed fate and transport of MC‐LR in sediments was adequately described with a one‐dimensional reactive transport model that uses an enhanced diffusion coefficient to represent benthic exchange of solutes. Numerical sensitivity studies showed that MC‐LR removal increases with hydraulic conductivity of sediment and wave height and decreases with water depth. For MC‐LR concentration at the WHO recreational guideline (20 ppb), sandy sediments can remove the equivalent MC‐LR mass in 1 m of surface water under typical nearshore wave conditions within tens of hours. In open water at large depths above a silty bed, removal times are much longer (on the order of weeks). Wave‐driven benthic exchange is therefore an important control on MC‐LR fate in energetic coastal areas but not in deep or calm settings where sediment–water interactions are greatly reduced. The nearshore fate of algal toxins is important to human health and socioeconomic vitality, since recreational activities and direct human exposures are concentrated along coasts.

Early Earth and Mars had analogous environments. While life developed on our planet, the question of whether it did on Mars remains to be answered. Hot spring deposits are compelling targets for exploration because of their high habitability and potential to retain morphological and chemical biosignatures. As a result in this study, we aim to better understand the potential for biosignature preservation in Fe-bearing hydrothermal systems. Understanding oxidation-reduction reactions involving Fe in hot springs is a key step in elucidating the preservation process. Fe reacts readily with reactive oxygen species (ROS), which are produced in hot spring surface waters through photochemical processes. Furthermore, Fe3+ can bind to cell membranes and preserve complex organic molecules (i.e., biomarkers). ROS formation is typically controlled by photoreactions with dissolved organic matter (DOM). However, Fe redox reactions more likely control ROS formation in these Fe-bearing systems. We deconvolved the relationship of ROS with Fe in hot springs and evaluated the role that DOM and dissolved organic sulfur (DOS) may have in ROS production. To better understand these coupled systems, field and laboratory experiments were conducted in hot springs of Yellowstone National Park. In situ H2O2 concentrations observed in these hot springs were comparable to, or higher than, those of other high-temperature systems. Reaction rates determined by measuring concentrations after specified time intervals varied based on water compositions and the presence of particulate or dissolved matter. Fe speciation (photochemical reactivity), concentration, and solubility further determined ROS cycling rates. Specifically, photochemically active Fe enhanced both ROS formation and decay rates depending on incident UV irradiance, and rates increased along with Fe concentration and solubility (i.e., in acidic conditions). Better understanding how ROS and Fe cycle in predominantly abiotic conditions will eventually aid in distinguishing between biosignatures and abiotic substances in the rock record.