Diurnal Floc Generation from Neuston Biofilms in Two Contrasting Freshwater Lakes

Selective adaptation of biofilm-forming bacteria to the nutrient-rich but environmentally challenging conditions of the surface microlayer (SML) or neuston layer was evident in littoral regions of two physically and geochemically contrasting freshwater lakes. SML bacterial communities (bacterioneuston) in these systems were depleted in <i>Actinobacteria</i>, enriched in either <i>Betaproteobacteria</i> or <i>Gammaproteobacteria</i>, and either unicellular <i>Cyanobacteria</i> were absent or microbial mat forming <i>Cyanobacteria</i> enriched relative to communities in the underlying shallow water column (0.5 m depth). Consistent with the occurrence of biofilm-hosted, geochemically distinct microhabitats, As-, Fe-, and S-metabolizing bacteria including anaerobic taxa were detected only in the SML in both systems. Over diurnal time scales, higher wind speeds resulted in the generation of floc from SML biofilms, identifying a transport mechanism entraining SML accumulated microorganisms, nutrients, and contaminants into the underlying water column. The energy regime experienced by the SML was more important to floc generation as larger flocs were more abundant in the larger, oligotrophic lake (higher relative energy regime) compared to the sheltered, smaller lake, despite relatively higher concentrations of bacteria, organic carbon, Fe, and PO₄^3– in the latter system

Microbial Engineering of Floc Fe and Trace Element Geochemistry in a Circumneutral, Remote Lake

Evaluation of lacustrine floc Fe, Pb, and Cd biogeochemistry over seasonal (summer, winter) and water column depth (metalimnetic, hypolimnetic) scales reveals depth-independent seasonally significant differences in floc Fe biominerals and trace element (TE: Pb, Cd) sequestration, driven by floc microbial community shifts. Winter floc [TE] were significantly lower than summer [TE], driven by declining abundance and reactivity of floc amorphous Fe^(III)-(oxy)­hydroxide (FeOOH) phases under ice ([FeOOH]^summer = 37–77 mgg^–1 vs [FeOOH]^winter = 0.3–7 mgg^–1). Further, while high summer floc [FeOOH] was observed at both water column depths, winter floc was dominated by Fe^(II) phases. However, the observed seasonal change in the nature and concentrations of floc Fe-phases was independent of water column [Fe], O₂, and pH and, instead, significantly correlated to floc bacterial community membership. Bioinformatic modeling (Unifrac, PCA analyses) of in situ and experimental microcosm results identified a temperature-driven seasonal turnover of floc microbial communities, shifting from dominantly putative Fe metabolisms within summer floc to wintertime ancillary Fe reducing and S metabolizing bacteria. This seasonal shift of floc microbial community functioning, significantly the wintertime loss of microbial Fe^(II)-oxidizing capability and concomitant increases of sulfur-reducing bacteria, alters dominant floc Fe minerals from Fe^(III) to Fe^(II) phases. This resulted in decreased winter floc [TE], not predicted by water column geochemistry

pH and Organic Carbon Dose Rates Control Microbially Driven Bioremediation Efficacy in Alkaline Bauxite Residue

Bioremediation of alkaline tailings, based on fermentative microbial metabolisms, is a novel strategy for achieving rapid pH neutralization and thus improving environmental outcomes associated with mining and refining activities. Laboratory-scale bioreactors containing bauxite residue (an alkaline, saline tailings material generated as a byproduct of alumina refining), to which a diverse microbial inoculum was added, were used in this study to identify key factors (pH, salinity, organic carbon supply) controlling the rates and extent of microbially driven pH neutralization (bioremediation) in alkaline tailings. Initial tailings pH and organic carbon dose rates both significantly affected bioremediation extent and efficiency with lower minimum pHs and higher extents of pH neutralization occurring under low initial pH or high organic carbon conditions. Rates of pH neutralization (up to 0.13 mM H⁺ produced per day with pH decreasing from 9.5 to ≤6.5 in three days) were significantly higher in low initial pH treatments. Representatives of the <i>Bacillaceae</i> and <i>Enterobacteriaceae</i>, which contain many known facultative anaerobes and fermenters, were identified as key contributors to 2,3-butanediol and/or mixed acid fermentation as the major mechanism(s) of pH neutralization. Initial pH and salinity significantly influenced microbial community successional trajectories, and microbial community structure was significantly related to markers of fermentation activity. This study provides the first experimental demonstration of bioremediation in bauxite residue, identifying pH and organic carbon dose rates as key controls on bioremediation efficacy, and will enable future development of bioreactor technologies at full field scale

Comparative Floc-Bed Sediment Trace Element Partitioning Across Variably Contaminated Aquatic Ecosystems

Significantly higher concentrations of Ag, As, Cu, Ni and Co are found in floc compared to bed sediments across six variably impacted aquatic ecosystems. In contrast to the observed element and site-specific bed sediment trace element (TE) partitioning patterns, floc TE sequestration is consistently dominated by amorphous oxyhydroxides (FeOOH), which account for 30–79% of floc total TE concentrations, irrespective of system physico-chemistry or elements involved. FeOOH consistently occur in significantly higher concentrations in floc than within bed sediments. Further, comparative concentration factors indicate significantly higher TE reactivity of floc-FeOOH relative to sediment-FeOOH in all systems investigated, indicating that both the greater abundance and higher reactivity of floc-FeOOH contribute to enhanced floc TE uptake. Results indicate that floc-organics (live cells and exopolymeric substances, EPS) directly predict floc-FeOOH concentrations, suggesting an organic structural role in the collection/templating of FeOOH. This, in turn, facilitates the sequestration of TEs associated with floc-FeOOH formation, imparting the conserved FeOOH “signature” on floc TE geochemistry across sites. Results demonstrate that the organic rich nature of floc exerts an important control over TE geochemistry in aquatic environments, ultimately creating a distinct solid with differing controls over TE behavior than bed sediments in close proximity (<0.5 m)