TEMPERATURE AFFECTS THE DISTRIBUTION AND FUNCTIONAL POTENTIAL OF PHOTOAUTOTROPHS IN ALKALINE HOT SPRINGS

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 extant phototrophs in situ because they harbor natural gradients of geochemistry, pH and temperature. Phototrophs within the Cyanobacteria (oxygenic phototrophs) and Chloroflexi (anoxygenic phototrophs) groups are frequently observed in alkaline hot springs where temperature, sulfide, and pH constrain the distribution of Cyanobacteria. In contrast to phototrophic Cyanobacteria, which are obligate photoautotrophs, Chloroflexi are a diverse phylum including facultative photoheterotrophic and photoautotrophic growth as well as mixotrophy. As a result, the factors that constrain the taxonomy and physiology of phototrophic Chloroflexi are not well known. These studies are further hindered by a paucity of characterized phototrophic Chloroflexi isolates.

We recently reported a role for temperature in constraining specific groups of Chloroflexi and Cyanobacteria in two alkaline hot springs in YNP: Rabbit Creek and Rosette Geyser. Photoassmilaiton and carbon stable isotope data suggest that, in addition to taxonomy, temperature also controls the distribution of prevalent carbon fixation pathways. Inferring physiology from 16 rRNA gene sequences can be informative for major metabolic pathways when taxa are closely related to their nearest cultured relative. Inferring metabolism from 16S rRNA gene sequences becomes more difficult as the number of available genomes from similar environments decreases as is the case for phototrophic Chloroflexi. To overcome the limitations of 16S rRNA sequencing in the absence of isolates, we have analyzed metagenomes collected along the temperature gradients in Rabbit Creek (45ºC to 70ºC). We aimed to determine the distribution and functional potential of phototrophic Chloroflexi and to further examine mechanisms underlining the co-occurrence of these two groups in alkaline hot springs. We found that puf genes (anoxygenic photosynthesis) followed a similar distribution to psb genes (oxygenic photosynthesis) and that genes for different carbon fixation pathways in photoautotrophs had different distributions with temperature. Our data suggest that, while genes for phototrophy follow a similar pattern with temperature, specific carbon fixation pathways may have an temperature limits that contribute to their distribution.