Unveiling shrouded oceans on temperate sub-Neptunes via transit signatures of solubility equilibria vs. gas thermochemistry

Abstract The recent discovery and initial characterization of sub-Neptune-sized exoplanets that receive stellar irradiance of approximately Earth's raised the prospect of finding habitable planets in the coming decade. Some of these temperate planets may support liquid water oceans, if they do not have massive H2/He envelopes and are thus not too hot at the bottom of the envelopes. For planets larger than Earth, and especially planets in the 1.7-3.5 R_Earth population, the mass of the H2/He envelope is typically not sufficiently constrained to assess the potential habitability. Here we show that the solubility equilibria vs. thermochemistry of carbon and nitrogen gases results in observable discriminators between small H2 atmospheres vs. massive ones. On temperate sub-Neptunes, the condition to form a liquid-water ocean and that to achieve the thermochemical equilibrium are mutually exclusive. The dominant carbon and nitrogen gases are typically CH4 and NH3 due to thermochemical recycling in a massive atmosphere of a temperate planet, and those in a small atmosphere overlying a liquid-water ocean are most likely CO2 and N2, followed by CO and CH4 produced photochemically. NH3 is depleted in the small atmosphere by dissolution into the liquid-water ocean. These gases lead to distinctive features in the planet's transmission spectrum, and a moderate number of repeated transit observations with the James Webb Space Telescope should readily tell apart a small atmosphere vs. a massive one via these spectral features on planets like K2-18 b. This method thus provides a way to use near-term facilities to constrain the atmospheric mass and habitability of temperate sub-Neptune exoplanets.

Download Full-text

The Sonora Substellar Atmosphere Models. II. Cholla: A Grid of Cloud-free, Solar Metallicity Models in Chemical Disequilibrium for the JWST Era

The Astrophysical Journal ◽

10.3847/1538-4357/ac3140 ◽

2021 ◽

Vol 923 (2) ◽

pp. 269

Author(s):

Theodora Karalidi ◽

Mark Marley ◽

Jonathan J. Fortney ◽

Caroline Morley ◽

Didier Saumon ◽

...

Keyword(s):

Near Infrared ◽

Chemical Species ◽

Brown Dwarfs ◽

Eddy Diffusion ◽

Giant Planets ◽

James Webb Space Telescope ◽

Solar Metallicity ◽

Composition Profiles ◽

Thermochemical Equilibrium

Abstract Exoplanet and brown dwarf atmospheres commonly show signs of disequilibrium chemistry. In the James Webb Space Telescope (JWST) era, high-resolution spectra of directly imaged exoplanets will allow the characterization of their atmospheres in more detail, and allow systematic tests for the presence of chemical species that deviate from thermochemical equilibrium in these atmospheres. Constraining the presence of disequilibrium chemistry in these atmospheres as a function of parameters such as their effective temperature and surface gravity will allow us to place better constraints on the physics governing these atmospheres. This paper is part of a series of works presenting the Sonora grid of atmosphere models. In this paper, we present a grid of cloud-free, solar metallicity atmospheres for brown dwarfs and wide-separation giant planets with key molecular species such as CH4, H2O, CO, and NH3 in disequilibrium. Our grid covers atmospheres with T eff ∈ [500 K, 1300 K], log g ∈ [3.0, 5.5] (cgs) and an eddy diffusion parameter of log K zz = 2 , 4 and 7 (cgs). We study the effect of different parameters within the grid on the temperature and composition profiles of our atmospheres. We discuss their effect on the near-infrared colors of our model atmospheres and the detectability of CH4, H2O, CO, and NH3 using the JWST. We compare our models against existing MKO and Spitzer observations of brown dwarfs and verify the importance of disequilibrium chemistry for T dwarf atmospheres. Finally, we discuss how our models can help constrain the vertical structure and chemical composition of these atmospheres.

Download Full-text

The selenophosphate synthetase, selD, is important for Clostridioides difficile physiology

Journal of Bacteriology ◽

10.1128/jb.00008-21 ◽

2021 ◽

Author(s):

Kathleen N. McAllister ◽

Andrea Martinez Aguirre ◽

Joseph A. Sorg

Keyword(s):

Treatment Options ◽

Metabolic Process ◽

Carbon And Nitrogen ◽

Clostridioides Difficile ◽

Selenophosphate Synthetase ◽

Antibiotic Associated Diarrhea ◽

Initial Characterization ◽

Significant Burden ◽

Gut Pathogen

The endospore-forming pathogen, Clostridioides difficile, is the leading cause of antibiotic-associated diarrhea and is a significant burden on the community and healthcare. C. difficile, like all forms of life, incorporates selenium into proteins through a selenocysteine synthesis pathway. The known selenoproteins in C. difficile are involved in a metabolic process that uses amino acids as the sole carbon and nitrogen source (Stickland metabolism). The Stickland metabolic pathway requires the use of two selenium-containing reductases. In this study, we built upon our initial characterization of the CRISPR-Cas9-generated selD mutant by creating a CRISPR-Cas9-mediated restoration of the selD gene at the native locus. Here, we use these CRISPR-generated strains to analyze the importance of selenium-containing proteins on C. difficile physiology. SelD is the first enzyme in the pathway for selenoprotein synthesis and we found that multiple aspects of C. difficile physiology were affected (e.g., growth, sporulation, and outgrowth of a vegetative cell post-spore germination). Using RNAseq, we identified multiple candidate genes which likely aid the cell in overcoming the global loss of selenoproteins to grow in medium which is favorable for using Stickland metabolism. Our results suggest that the absence of selenophosphate (i.e., selenoprotein synthesis) leads to alterations to C. difficile physiology so that NAD+ can be regenerated by other pathways. Importance C. difficile is a Gram-positive, anaerobic gut pathogen which infects thousands of individuals each year. In order to stop the C. difficile lifecycle, other non-antibiotic treatment options are in urgent need of development. Towards this goal, we find that a metabolic process used by only a small fraction of the microbiota is important for C. difficile physiology – Stickland metabolism. Here, we use our CRISPR-Cas9 system to ‘knock in’ a copy of the selD gene into the deletion strain to restore selD at its native locus. Our findings support the hypothesis that selenium-containing proteins are important for several aspects of C. difficile physiology – from vegetative growth to spore formation and outgrowth post-germination.

Download Full-text

The selenophosphate synthetase, selD, is important for Clostridioides difficile physiology

10.1101/2021.01.06.425661 ◽

2021 ◽

Author(s):

Kathleen McAllister ◽

Andrea Martinez Aguirre ◽

Joseph A. Sorg

Keyword(s):

Amino Acids ◽

Metabolic Pathway ◽

Metabolic Process ◽

Carbon And Nitrogen ◽

Clostridioides Difficile ◽

Selenophosphate Synthetase ◽

Antibiotic Associated Diarrhea ◽

Initial Characterization ◽

Significant Burden

Download Full-text

Characterization of the amdR-controlled lamA and lamB genes of Aspergillus nidulans.

Genetics ◽

10.1093/genetics/122.2.331 ◽

1989 ◽

Vol 122 (2) ◽

pp. 331-339

Author(s):

M E Katz ◽

M J Hynes

Keyword(s):

Aspergillus Nidulans ◽

Recombinant Dna ◽

Regulatory Gene ◽

Carbon And Nitrogen ◽

Amino Butyric Acid ◽

Endogenous Gaba ◽

Initial Characterization ◽

New Gene ◽

Recombinant Dna Techniques

Abstract Four Aspergillus nidulans genes are known to be under the control of the trans-acting regulatory gene amdR. We describe the isolation and initial characterization of one of these amdR-regulated genes, lamA. The lam locus, however, was found to consist of two divergently transcribed genes, the lamA gene, and a new gene, also under amdR control, which we have designated lamB. Using recombinant DNA techniques we have constructed a strain of A. nidulans lacking a functional lamB gene. Experiments conducted with this strain demonstrate that lamB, like lamA, is involved in utilization of 2-pyrrolidinone in A. nidulans. Metabolism of a related compound, gamma-amino butyric acid (GABA) is not affected. We also provide evidence that the conversion of exogenous 2-pyrrolidinone to endogenous GABA requires a functional lamB gene. The expression of both lamA and lamB is subject to carbon and nitrogen metabolite repression in addition to amdR-mediated induction by omega-amino acids.

Download Full-text