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1. Roadmap for naming uncultivated Archaea and Bacteria

   https://doi.org/10.1038/s41564-020-0733-x  

   Murray, Alison E.; Freudenstein, John; Gribaldo, Simonetta; Hatzenpichler, Roland; Hugenholtz, Philip; Kämpfer, Peter; Konstantinidis, Konstantinos T.; Lane, Christopher E.; Papke, R. Thane; Parks, Donovan H.; et al (June 2020, Nature Microbiology)

   Abstract The assembly of single-amplified genomes (SAGs) and metagenome-assembled genomes (MAGs) has led to a surge in genome-based discoveries of members affiliated with Archaea and Bacteria, bringing with it a need to develop guidelines for nomenclature of uncultivated microorganisms. The International Code of Nomenclature of Prokaryotes (ICNP) only recognizes cultures as ‘type material’, thereby preventing the naming of uncultivated organisms. In this Consensus Statement, we propose two potential paths to solve this nomenclatural conundrum. One option is the adoption of previously proposed modifications to the ICNP to recognize DNA sequences as acceptable type material; the other option creates a nomenclatural code for uncultivated Archaea and Bacteria that could eventually be merged with the ICNP in the future. Regardless of the path taken, we believe that action is needed now within the scientific community to develop consistent rules for nomenclature of uncultivated taxa in order to provide clarity and stability, and to effectively communicate microbial diversity. 

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2. A genomic catalog of Earth’s microbiomes

   https://doi.org/10.1038/s41587-020-0718-6  

   Nayfach, Stephen; Roux, Simon; Seshadri, Rekha; Udwary, Daniel; Varghese, Neha; Schulz, Frederik; Wu, Dongying; Paez-Espino, David; Chen, I-Min; Huntemann, Marcel; et al (November 2020, Nature Biotechnology)

   null (Ed.)

   Abstract The reconstruction of bacterial and archaeal genomes from shotgun metagenomes has enabled insights into the ecology and evolution of environmental and host-associated microbiomes. Here we applied this approach to >10,000 metagenomes collected from diverse habitats covering all of Earth’s continents and oceans, including metagenomes from human and animal hosts, engineered environments, and natural and agricultural soils, to capture extant microbial, metabolic and functional potential. This comprehensive catalog includes 52,515 metagenome-assembled genomes representing 12,556 novel candidate species-level operational taxonomic units spanning 135 phyla. The catalog expands the known phylogenetic diversity of bacteria and archaea by 44% and is broadly available for streamlined comparative analyses, interactive exploration, metabolic modeling and bulk download. We demonstrate the utility of this collection for understanding secondary-metabolite biosynthetic potential and for resolving thousands of new host linkages to uncultivated viruses. This resource underscores the value of genome-centric approaches for revealing genomic properties of uncultivated microorganisms that affect ecosystem processes. 

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3. Elucidation of trophic interactions in an unusual single-cell nitrogen-fixing symbiosis using metabolic modeling

   https://doi.org/10.1371/journal.pcbi.1008983  

   Sarkar, Debolina; Landa, Marine; Bandyopadhyay, Anindita; Pakrasi, Himadri B.; Zehr, Jonathan P.; Maranas, Costas D. (May 2021, PLOS Computational Biology)

   Hatzimanikatis, Vassily (Ed.)

   Marine nitrogen-fixing microorganisms are an important source of fixed nitrogen in oceanic ecosystems. The colonial cyanobacterium Trichodesmium and diatom symbionts were thought to be the primary contributors to oceanic N 2 fixation until the discovery of the unusual uncultivated symbiotic cyanobacterium UCYN-A ( Candidatus Atelocyanobacterium thalassa ). UCYN-A has atypical metabolic characteristics lacking the oxygen-evolving photosystem II, the tricarboxylic acid cycle, the carbon-fixation enzyme RuBisCo and de novo biosynthetic pathways for a number of amino acids and nucleotides. Therefore, it is obligately symbiotic with its single-celled haptophyte algal host. UCYN-A receives fixed carbon from its host and returns fixed nitrogen, but further insights into this symbiosis are precluded by both UCYN-A and its host being uncultured. In order to investigate how this syntrophy is coordinated, we reconstructed bottom-up genome-scale metabolic models of UCYN-A and its algal partner to explore possible trophic scenarios, focusing on nitrogen fixation and biomass synthesis. Since both partners are uncultivated and only the genome sequence of UCYN-A is available, we used the phylogenetically related Chrysochromulina tobin as a proxy for the host. Through the use of flux balance analysis (FBA), we determined the minimal set of metabolites and biochemical functions that must be shared between the two organisms to ensure viability and growth. We quantitatively investigated the metabolic characteristics that facilitate daytime N 2 fixation in UCYN-A and possible oxygen-scavenging mechanisms needed to create an anaerobic environment to allow nitrogenase to function. This is the first application of an FBA framework to examine the tight metabolic coupling between uncultivated microbes in marine symbiotic communities and provides a roadmap for future efforts focusing on such specialized systems. 

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4. Identifying the active microbes driving organosulfur cycling from taurine and methionine in marine sediment

   Coskun, Ömer K; Orsi, William D; D’Hondt, Steven; Gomez-Saez, Gonzalo V (February 2025, ISME communications)

   Studies on microbial sulfur cycling in marine sediment have primarily centered on the cycling of inorganic sulfur. The microbial diversity underlying the cycling of organosulfur compounds is largely unexplored. In this study, we present the first quantification of dissolved organic sulfur (DOS) microbial assimilation in marine surface sediments using 13C-DOS quantitative DNA stable isotope probing (qSIP). We sampled marine sediment from 493 m water depth on the Puerto Rico continental slope, measured 13C-assimilation from two DOS substrates (13C-taurine and 13C-methionine), and compared the 13C-DOS assimilation to 13C-glucose uptake. Taurine utilization was confined to bacteria, whereas methionine was degraded by bacteria and archaea, including methanogenic Methanococcoides. Globally widespread uncultivated clades of Gammaproteobacteria and Deltaproteobacteria were the main drivers of DOS cycling and exhibited increased assimilation of carbon from taurine and methionine, compared to glucose. Only one operational taxonomic unit (OTU) affiliated with Neptuniibacter was found to assimilate taurine and methionine, but not glucose, implying that microbes exclusively utilizing both DOS substrates as a carbon source in marine sediments are rare. Still, a substantial number of bacterial taxa exhibited a higher assimilation of 13C from taurine or methionine, compared to glucose, indicating their preference for both DOS substrates over glucose as a carbon source in the sediment. These results represent the first quantitative assessment of organosulfur cycling from taurine and methionine by uncultivated microbes in a marine benthic environment. 

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5. Host population diversity as a driver of viral infection cycle in wild populations of green sulfur bacteria with long standing virus-host interactions

   https://doi.org/10.1038/s41396-020-00870-1  

   Berg, Maureen; Goudeau, Danielle; Olmsted, Charles; McMahon, Katherine D.; Yitbarek, Senay; Thweatt, Jennifer L.; Bryant, Donald A.; Eloe-Fadrosh, Emiley A.; Malmstrom, Rex R.; Roux, Simon (June 2021, The ISME Journal)

   Abstract Temperate phages are viruses of bacteria that can establish two types of infection: a lysogenic infection in which the virus replicates with the host cell without producing virions, and a lytic infection where the host cell is eventually destroyed, and new virions are released. While both lytic and lysogenic infections are routinely observed in the environment, the ecological and evolutionary processes regulating these viral dynamics are still not well understood, especially for uncultivated virus-host pairs. Here, we characterized the long-term dynamics of uncultivated viruses infecting green sulfur bacteria (GSB) in a model freshwater lake (Trout Bog Lake, TBL). As no GSB virus has been formally described yet, we first used two complementary approaches to identify new GSB viruses from TBL; one in vitro based on flow cytometry cell sorting, the other in silico based on CRISPR spacer sequences. We then took advantage of existing TBL metagenomes covering the 2005–2018 period to examine the interactions between GSB and their viruses across years and seasons. From our data, GSB populations in TBL were constantly associated with at least 2-8 viruses each, including both lytic and temperate phages. The dominant GSB population in particular was consistently associated with two prophages with a nearly 100% infection rate for >10 years. We illustrate with a theoretical model that such an interaction can be stable given a low, but persistent, level of prophage induction in low-diversity host populations. Overall, our data suggest that lytic and lysogenic viruses can readily co-infect the same host population, and that host strain-level diversity might be an important factor controlling virus-host dynamics including lytic/lysogeny switch. 

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