Distribution and functional potential of photoautotrophic bacteria in alkaline hot springs

ABSTRACT Alkaline hot springs in Yellowstone National Park (YNP) provide a framework to study the relationship between photoautotrophs and temperature. Previous work has focused on cyanobacteria (oxygenic phototrophs), but anoxygenic phototrophs are critical parts of the evolutionary history of life on Earth and and are abundant across temperature gradients in alkaline hot springs. However, many questions remain regarding the ecophysiology of anoxygenic photosynthesis due to the taxonomic and metabolic diversity of these taxa. Here, we examined the distribution of genes involved in phototrophy and carbon and nitrogen fixation in eight alkaline (pH 7.3-9.4) hot spring sites approaching the upper temperature limit of photosynthesis (~72°C) in YNP using metagenome sequencing. Genes associated with cyanobacteria are abundant throughout our data and more diverse at temperatures > 63°C, genes for autotrophic Chloroflexi are more abundant in sites > 63°C and genes associated with phototrophic Chloroflexi are abundant throughout. Additionally, we recovered deep branching nitrogen fixation genes from our metagenomes, which could inform the evolutionary history of nitrogen fixation. Lastly, we recovered 25 metagenome assembled genomes of Chloroflexi. We found distinct differences in carbon fixation genes in Roseiflexus and Chloroflexus bins, in addition to several novel Chloroflexi bins. Our results highlight the physiological diversity and evolutionary history of the understudied, anoxygenic autotrophic Chloroflex. Furthermore, we provide evidence that genes involved in nitrogen fixation in Chloroflexi is more widespread than previously assumed. IMPORTANCE Photosynthetic bacteria in hot springs are of great importance to both microbial evolution and ecology because they are responsible for the rise of oxygen and are critical to nutrient cycling. While a large body of work has focused on the oxygenic photosynthesis in cyanobacteria, many questions remain regarding the metabolic potential of anoxygenic phototrophs but are further compounded by their metabolic and taxonomic diversity. Here, we have recovered several novel metagenome bins and quantified the distribution of key genes involved in carbon and nitrogen metabolism in both oxygenic and anoxygenic phototrophs. Together, our results add to the body of work focusing on photosynthetic bacteria in hot springs in Yellowstone National Park.