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Arsenic (As) and mercury (Hg) were examined in the Yellowstone Lake food chain, focusing on two lake locations separated by approximately 20 km and differing in lake floor hydrothermal vent activity. Sampling spanned from femtoplankton to the main fish species, Yellowstone cutthroat trout and the apex predator lake trout. Mercury bioaccumulated in muscle and liver of both trout species, biomagnifying with age, whereas As decreased in older fish, which indicates differential exposure routes for these metal(loid)s. Mercury and As concentrations were higher in all food chain filter fractions (0.1‐, 0.8‐, and 3.0‐μm filters) at the vent‐associated Inflated Plain site, illustrating the impact of localized hydrothermal inputs. Femtoplankton and picoplankton size biomass (0.1‐ and 0.8‐μm filters) accounted for 30%–70% of total Hg or As at both locations. By contrast, only approximately 4% of As and <1% of Hg were found in the 0.1‐μm filtrate, indicating that comparatively little As or Hg actually exists as an ionic form or intercalated with humic compounds, a frequent assumption in freshwaters and marine waters. Ribosomal RNA (18S) gene sequencing of DNA derived from the 0.1‐, 0.8‐, and 3.0‐μm filters showed significant eukaryote biomass in these fractions, providing a novel view of the femtoplankton and picoplankton size biomass, which assists in explaining why these fractions may contain such significant Hg and As. These results infer that femtoplankton and picoplankton metal(loid) loads represent aquatic food chain entry points that need to be accounted for and that are important for better understanding Hg and As biochemistry in aquatic systems.Environ Toxicol Chem2023;42:225–241. © 2022 SETAC

 

Reports of aerobic biogenic methane (CH₄) have generated new views about CH₄sources in nature. We examine this phenomenon in the free‐flowing Yellowstone river wherein CH₄concentrations were tracked as a function of environmental conditions, phototrophic microorganisms (using chlorophylla, Chla, as proxy), as well as targeted methylated amines known to be associated with this process. CH₄was positively correlated with temperature and Chla, although diurnal measurements showed CH₄concentrations were greatest during the night and lowest during maximal solar irradiation. CH₄efflux from the river surface was greater in quiescent edge waters (71–94 μmol m⁻² d) than from open flowing current (~ 57 μmol m⁻² d). Attempts to increase flux by disturbing the benthic environment in the quiescent water directly below (~ 1.0 m deep) or at varying distances (0–5 m) upstream of the flux chamber failed to increase surface flux. Glycine betaine (GB), dimethylamine and methylamine (MMA) were observed throughout the summer‐long study, increasing during a period coinciding with a marked decline in Chla, suggesting a lytic event led to their release; however, this did not correspond to increased CH₄concentrations. Spiking river water with GB or MMA yielded significantly greater CH₄than nonspiked controls, illustrating the metabolic potential of the river microbiome. In summary, this study provides evidence that: (1) phototrophic microorganisms are involved in CH₄synthesis in a river environment; (2) the river microbiome possesses the metabolic potential to convert methylated amines to CH₄; and (3) river CH₄concentrations are dynamic diurnally as well as during the summer active months.

 

Reports of biogenic methane (CH 4 ) synthesis associated with a range of organisms have steadily accumulated in the literature. This has not happened without controversy and in most cases the process is poorly understood at the gene and enzyme levels. In marine and freshwater environments, CH 4 supersaturation of oxic surface waters has been termed the “methane paradox” because biological CH 4 synthesis is viewed to be a strictly anaerobic process carried out by O 2 -sensitive methanogens. Interest in this phenomenon has surged within the past decade because of the importance of understanding sources and sinks of this potent greenhouse gas. In our work on Yellowstone Lake in Yellowstone National Park, we demonstrate microbiological conversion of methylamine to CH 4 and isolate and characterize an Acidovorax sp. capable of this activity. Furthermore, we identify and clone a gene critical to this process (encodes pyridoxylamine phosphate-dependent aspartate aminotransferase) and demonstrate that this property can be transferred to Escherichia coli with this gene and will occur as a purified enzyme. This previously unrecognized process sheds light on environmental cycling of CH 4 , suggesting that O 2 -insensitive, ecologically relevant aerobic CH 4 synthesis is likely of widespread distribution in the environment and should be considered in CH 4 modeling efforts.