Evolution of Oxygenic Photosynthesis as Preserved in Melainabacterial Genomes from Lake Vanda, Antarctica
Atmospheric oxygen rose suddenly approximately 2.4 billion years ago after Cyanobacteria evolved the ability to produce oxygen through photosynthesis (oxygenic photosynthesis). This change permanently altered the future of life on Earth, yet little is known about the evolutionary processes leading to it. The Melainabacteria were first discovered in 2013 and are closely related non-photosynthetic relatives of the first group of organisms capable of oxygenic photosynthesis. This project will utilize existing data on metagenomes from microbial mats in Lake Vanda, an ice-covered lake in Antarctica where many sequences of Melainabacteria have been previously identified. From this genetic information, we identified a new cyanobacterium, named Aurora vandensis, that is sister to all other Cyanobacteria, providing evolutionary insights. In addition, we assessed the metabolic capabilities of the Melainabacteria with good genomic coverage to identify their potential ecological roles. None contain photosynthetic genes, and we are evaluating the evolutionary relationships among the Cyanobacteria and Melainabacteria, particularly with respect to metabolic genes that will allow an advancement in understanding of the evolutionary path that lead to oxygenic photosynthesis on Earth. The project will focus on extracting evolutionary information from the genomic data of Melainabacteria and Sericytochromatia, recently-described groups closely related to but basal to the Cyanobacteria. The characterization of novel members of these groups in samples from Lake Vanda, Antarctica, provide insights into the path and processes involved in the evolution of oxygenic photosynthesis. The research identified a novel cyanobacterial genus that is sister to all other Cyanobacteria, is most closely related to Gloeobacter, and shares evolutionary differences with that genus. Results also show that characterized Melainabacteria lack photosynthesis genes, but their respiration genes provide insight into evolutionary relationships among Melainabacteria and Cyanobacteria. Results provide unexpected constraints. The project focuses on 12 metagenomes, from which Melainabacteria and novel Cyanobacteria bins are annotated and preliminary metabolic pathways will be constructed. The project utilizes full-length sequences of marker genes from across the bacterial domain with a particular focus on taxa that are oxygenic or anoxygenic phototrophs and use the marker genes, to build a rooted "backbone" tree. Incomplete or short sequences from the metagenomes are added to the tree using the Evolutionary Placement Algorithm. The researchers built a corresponding phylogenetic tree using a Bayesian framework and compare their topologies. By doing so, the project aims to improve the understanding of the evolution of oxygenic photosynthesis, which caused the most significant change in Earth's surface chemistry. Specifically, we document a novel and basal cyanobacterium, significantly broader metabolic diversity within the Melainabacteria than has been previously identified, gain significant insights into their metabolic evolution, their evolutionary relationships with the Cyanobacteria, and the evolutionary steps leading to the origin of oxygenic photosynthesis. This research is constraining key evolutionary processes in the origin of oxygenic photosynthesis. It provides the foundation for future studies by indicating where a genomic record of the evolution of oxygenic photosynthesis may be preserved. Results will are being shared with middle school children through the development of scientific lesson plans in collaboration with teachers. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.
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