Hypersaline lakes harbor more active bacterial communities

ABSTRACT Extremophiles employ a diverse array of resistance strategies to thrive under harsh 18 environmental conditions but maintaining these adaptations comes at an energetic cost. If energy reserves to drop too low, extremophiles may enter a dormant state of reduced 20 metabolic activity to survive. Dormancy is frequently offered as a plausible explanation for the persistence of bacteria under suboptimal environmental conditions with the 22 prevalence of this mechanism only expected to rise as stressful conditions intensify. We estimated dormancy in ten hypersaline and freshwater lakes across the Western United 24 States. To our surprise, we found that extreme environmental conditions did not induce higher levels of bacterial dormancy. Based on our approach using rRNA:rDNA gene 26 ratios to estimate activity, halophilic and halotolerant bacteria were classified as inactive at a similar percentage as freshwater bacteria, and the proportion of the community 28 exhibiting dormancy was considerably lower (16%) in hypersaline than freshwater lakes across a range of cutoffs estimating activity. Of the multiple chemical characteristics we 30 evaluated, salinity and, to a lesser extent, total phosphorus concentrations influenced activity. But instead of dormancy being more common as stressful conditions intensified, 32 the percentage of the community residing in an inactive state decreased with increasing salinity in freshwater and hypersaline lakes, suggesting that salinity acts as a strong 34 environmental filter selecting for bacteria that persist and thrive under saltier conditions. Within the compositionally distinct and less diverse hypersaline communities, abundant 36 taxa were disproportionately active and localized in families Microbacteriacea

DU Undergraduate Showcase: Research, Scholarship, and Creative Works

DU Undergraduate Showcase: Research, Scholarship, and Creative Work

High Salinity Stabilizes Bacterial Community Composition and Activity

Dormancy is offered as an explanation for how bacteria are able to survive temporal fluctuations in resource availability or adverse environmental conditions. Bacteria may exit this reversible, low-metabolic state and resuscitate, becoming metabolically active in response to favorable environmental cues. In extreme environments, the prevalence of dormancy is only expected to rise as stressful conditions intensify, however, the overriding effects of a constant stress in extreme environments may overshadow other environmental cues from influencing activity. We evaluated shifts in microbial community composition and dormancy and related these variables to water chemistry over one year in six lakes in UT, USA. The lakes represented a salinity gradient ranging from 303.22 PSU in the North Arm of the Great Salt Lake to 0.52 PSU in Deer Creek Reservoir. We analyzed 16S rDNA-based communities (i.e., all bacteria present in the community) and 16S rRNA-based communities (i.e., only active bacteria) with target metagenomics, observed and analyzed changes in bacterial community composition over time, and calculated dormancy based on rRNA to rDNA ratios of the relative recovery of individual operational taxonomic units. We measured changes in lake chemistry (i.e., temperature, pH, dissolved oxygen, total nitrogen, total phosphorus, dissolved organic carbon) and linked them to dormancy and community composition with multiple regression models and metric distance matrices in R. We found that rDNA-based community composition was driven primarily by salinity and dissolved oxygen concentrations (P \u3c .001); that is, where these variables were most extreme, very little change in community composition occurred. rRNA-based community composition, however, was additionally highly associated with fluctuations in phosphorus concentrations (P \u3c .001). Furthermore, the prevalence of total dormancy in a lake was positively associated with the proportion of rare taxa (OTU’s representing \u3c0.1% of the total bacteria) in the system. These findings suggest that high salinity and low oxygen prevent the introduction and prosperity of taxa that are not predisposed to such extreme conditions. Without a stabilizing extreme variable, the freshwater lakes catered to a more diverse range of bacteria, which were then subject to changes in multiple variables (i.e., phosphorus availability), high levels of competition, and increased use of dormancy as a life history strategy

\u3ci\u3eDrosophila\u3c/i\u3e Muller F Elements Maintain a Distinct Set of Genomic Properties Over 40 Million Years of Evolution

The Muller F element (4.2 Mb, ~80 protein-coding genes) is an unusual autosome of Drosophila melanogaster; it is mostly heterochromatic with a low recombination rate. To investigate how these properties impact the evolution of repeats and genes, we manually improved the sequence and annotated the genes on the D. erecta, D. mojavensis, and D. grimshawi F elements and euchromatic domains from the Muller D element. We find that F elements have greater transposon density (25–50%) than euchromatic reference regions (3–11%). Among the F elements, D. grimshawi has the lowest transposon density (particularly DINE-1: 2% vs. 11–27%). F element genes have larger coding spans, more coding exons, larger introns, and lower codon bias. Comparison of the Effective Number of Codons with the Codon Adaptation Index shows that, in contrast to the other species, codon bias in D. grimshawi F element genes can be attributed primarily to selection instead of mutational biases, suggesting that density and types of transposons affect the degree of local heterochromatin formation. F element genes have lower estimated DNA melting temperatures than D element genes, potentially facilitating transcription through heterochromatin. Most F element genes (~90%) have remained on that element, but the F element has smaller syntenic blocks than genome averages (3.4–3.6 vs. 8.4–8.8 genes per block), indicating greater rates of inversion despite lower rates of recombination. Overall, the F element has maintained characteristics that are distinct from other autosomes in the Drosophila lineage, illuminating the constraints imposed by a heterochromatic milieu