Temperature impacts community structure and function of phototrophic Chloroflexi and Cyanobacteria in two alkaline hot springs in Yellowstone National Park

Abstract

Summary Photosynthetic bacteria are abundant in alkaline, terrestrial hot springs and there is a long history of research on phototrophs in Yellowstone National Park (YNP). Hot springs provide a framework to examine the ecophysiology of phototrophs in situ because they provide natural gradients of geochemistry, pH and temperature. Phototrophs within the Cyanobacteria and Chloroflexi groups are frequently observed in alkaline hot springs. Decades of research has determined that temperature constrains Cyanobacteria in alkaline hot springs, but factors that constrain the distribution of phototrophic Chloroflexi remain unresolved. Using a combination of 16S rRNA gene sequencing and photoassimilation microcosms, we tested the hypothesis that temperature would constrain the activity and composition of phototrophic Cyanobacteria and Chloroflexi. We expected diversity and rates of photoassimilation to decrease with increasing temperature. We report 16S rRNA amplicon sequencing along with carbon isotope signatures and photoassimilation from 45 to 72°C in two alkaline hot springs. We find that Roseiflexus, Chloroflexus (Chloroflexi) and Leptococcus (Cyanobacteria) operational taxonomic units (OTUs) have distinct distributions with temperature. This distribution suggests that, like phototrophic Cyanobacteria, temperature selects for specific phototrophic Chloroflexi taxa. The richness of phototrophic Cyanobacteria decreased with increasing temperature along with a decrease in oxygenic photosynthesis, whereas Chloroflexi richness and rates of anoxygenic photosynthesis did not decrease with increasing temperature, even at temperatures approaching the upper limit of photosynthesis (~72–73°C). Our carbon isotopic data suggest an increasing prevalence of the 3‐hydroxypropionate pathway with decreasing temperature coincident with photoautotrophic Chloroflexi. Together these results indicate temperature plays a role in defining the niche space of phototrophic Chloroflexi (as has been observed for Cyanobacteria), but other factors such as morphology, geochemistry, or metabolic diversity of Chloroflexi, in addition to temperature, could determine the niche space of this highly versatile group.

Distribution and functional potential of photoautotrophic bacteria in alkaline hot springs

“…Similar to psb genes, we recovered more copies of both psc and puf genes in sites < 63ºC, suggesting diversity (or taxa) abundance increases with decreasing temperature. These results are consistent with our previous work that suggest phototrophs with type-I RCs are less abundant at high temperatures, while the type-II phototrophic Chloroflexi are highly abundant and diverse in 60-72ºC hot springs (8,12).…”

“…However, in sites < 63ºC, we recovered more copies of psb genes, suggesting higher alpha diversity within the cyanobacteria population or that single species harbor multiple copies of the psb genes. Previous work has shown that cyanobacterial diversity in alkaline hot springs decreases with increasing temperature and that Leptococcus species, which contain up to three copies of psbA (4) are most abundant in these conditions (8,9,12 Leptococcus A and B 'ecotypes, which respectively occupy temperature ranges of ~55-68ºC and ~50-62ºC, as either A-type or B'-type. Each of the YNP Leptococcus genomes contain two to three variants of the psbA gene, but the environmental conditions that select for strains that harbor multiple copies is unknown.…”

“…The distribution of cyanobacterial species in Mushroom and Octopus Springs, two alkaline hot springs in YNP, has been described at ~ 1ºC resolution (4)(5)(6)(7), revealing ecotypes that are highly adapted to specific temperatures. Additionally, cyanobacterial diversity increases with decreasing temperature (8). While anoxygenic phototrophs are also present in high temperature alkaline springs (5,6,(8)(9)(10)), many questions remain regarding their metabolic potential and contribution to carbon and nitrogen cycling in situ, which is complicated by their taxonomic and metabolic diversity.…”

“…Additionally, cyanobacterial diversity increases with decreasing temperature (8). While anoxygenic phototrophs are also present in high temperature alkaline springs (5,6,(8)(9)(10)), many questions remain regarding their metabolic potential and contribution to carbon and nitrogen cycling in situ, which is complicated by their taxonomic and metabolic diversity.…”

“…Of these phyla, Chloroflexi are the most abundant anoxygenic phototrophs in alkaline hot springs > 60ºC, while other anoxygenic phototrophs (e.g. Chlorobi and Candidatus Chloracidobacteium thermophilum), are usually present but less abundant (8,9,11,12). At present, phototrophy in hot spring Chloroflexi is limited to class Chloroflexales, with the exception of the novel photoheterotroph, Candidatus Roseilineae in class Anaerolineae (13).…”