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Abstract Microbial communities in Arctic coastal lagoons mediate nitrogen and carbon cycling at the terrestrial–marine interface of these rapidly changing ecosystems. To investigate these microbial processes, we measured gene expression, nitrification, and inorganic carbon assimilation in waters of Elson Lagoon on the Beaufort Sea coast under ice cover (April), during spring break‐up (July), and in open water (August). Quantitative metatranscriptomics with internal controls quantified per‐liter transcript abundances alongside in situ light and dark¹⁵N‐ammonium,¹⁵N‐urea, and¹³C‐bicarbonate stable isotope tracer incubations. Nitrification was only detectable during ice cover, showing high rates for Arctic coastal systems and evidence of light inhibition. Although carbon assimilation was relatively low during ice cover, dark carbon assimilation accounted for nearly half of total uptake, matching estimates of chemoautotrophic potential based on nitrification. Microbial gene expression also shifted seasonally in abundance and function. Transcripts for nitrification peaked during ice cover when genes for ammonia oxidation and 3‐HP/4‐HB carbon fixation were primarily expressed by archaeal genusNitrosopumilus, while those for nitrite oxidation and reverse TCA carbon fixation were expressed by bacterial phylum Nitrospinota. During break‐up and open water, expression shifted toward urea metabolism, nitrogen assimilation, Calvin Cycle carbon fixation, and anaplerotic pathways. These shifts highlight the seasonality of microbial metabolic strategies and reveal distinct functional shifts across the Arctic lagoon seasonal cycle. The findings suggest that ongoing warming and declining ice cover may reduce chemoautotrophic activity and alter nitrogen and carbon cycling under future conditions, with implications for nutrient dynamics and primary production in Arctic coastal ecosystems. 

Climate change is causing larger wildfires and more extreme precipitation events in many regions. As these ecological disturbances increasingly coincide, they alter lateral fluxes of sediment, organic matter, and nutrients. Here, we report the stream chemistry response of watersheds in a semiarid region of Utah (USA) that were affected by a megafire followed by an extreme precipitation event in October 2018. We analyzed daily to hourly water samples at 10 stream locations from before the storm event until three weeks after its conclusion for suspended sediment, solute and nutrient concentrations, water isotopes, and dissolved organic matter concentration, optical properties, and reactivity. The megafire caused a ~2,000-fold increase in sediment flux and a ~6,000-fold increase in particulate carbon and nitrogen flux over the course of the storm. Unexpectedly, dissolved organic carbon (DOC) concentration was 2.1-fold higher in burned watersheds, despite the decreased organic matter from the fire. DOC from burned watersheds was 1.3-fold more biodegradable and 2.0-fold more photodegradable than in unburned watersheds based on 28-day dark and light incubations. Regardless of burn status, nutrient concentrations were higher in watersheds with greater urban and agricultural land use. Likewise, human land use had a greater effect than megafire on apparent hydrological residence time, with rapid stormwater signals in urban and agricultural areas but a gradual stormwater pulse in areas without direct human influence. These findings highlight how megafires and intense rainfall increase short-term particulate flux and alter organic matter concentration and characteristics. However, in contrast with previous research, which has largely focused on burned-unburned comparisons in pristine watersheds, we found that direct human influence exerted a primary control on nutrient status. Reducing anthropogenic nutrient sources could therefore increase socioecological resilience of surface water networks to changing wildfire regimes. 

The transcriptional anti-silencing and DNA-binding protein, VirB, is essential for the virulence of Shigella species and, yet, sequences required for VirB-DNA binding are poorly understood. While a 7-8 bp VirB-binding site has been proposed, it was derived from studies at a single VirB-dependent promoter, icsB. Our previous in vivo studies at a different VirB-dependent promoter, icsP, found that the proposed VirB-binding site was insufficient for regulation. Instead, the required site was found to be organized as a near-perfect inverted repeat separated by a single nucleotide spacer. Thus, the proposed 7-8 bp VirB-binding site needed to be re-evaluated. Here, we engineer and validate a molecular tool to capture protein-DNA binding interactions in vivo. Our data show that a sequence organized as a near-perfect inverted repeat is required for VirB-DNA binding interactions in vivo at both the icsB and icsP promoters. Furthermore, the previously proposed VirB-binding site and multiple sites found as a result of its description (i.e., sites located at the virB, virF, spa15, and virA promoters) are not sufficient for VirB to bind in vivo using this tool. The implications of these findings are discussed.