Cyanobacteria and Algae Meet at the Limits of Their Habitat Ranges in Moderately Acidic Hot Springs

Microbial oxygenic photosynthesis in thermal habitats is thought to be performed by Bacteria in circumneutral to alkaline systems (pH > 6) and by Eukarya in acidic systems (pH < 3), yet the predominant oxygenic phototrophs in thermal environments with pH values intermediate to these extremes have received little attention. Sequencing of 16S and 18S rRNA genes was performed on samples from twelve hot springs in Yellowstone National Park (Wyoming, USA) with pH values from 3.0 to 5.5, revealing that Cyanobacteria of the genus Chlorogloeopsis and algae of the genus Cyanidioschyzon (phylum Rhodophyta) coexisted in ten of these springs. Cyanobacterial 16S rRNA genes were more abundant than rhodophyte 18S rRNA genes by 1–7 orders of magnitude, with rhodophyte template abundance approaching that of Cyanobacteria only at the most acidic sites. The ketocarotenoids echinenone and canthaxanthin were identified in samples from all but one spring yielding cyanobacterial sequences and are attributed to pigment synthesis by Cyanobacteria, whereas the absence of detectable chloroplast sequences affiliated with Cyanidioschyzon, pH and temperatures in excess of its limits for growth, and other observations collectively suggested these algae were inactive in many of the springs at the time of sampling. Fluctuations in the supply of meteoric water likely contribute to physicochemical variability in these springs, leading to transitions in microbial community composition. Spatial overlap, but perhaps not temporal overlap, in the habitat ranges of bacterial and eukaryal oxygenic phototrophs indicates that the notion of a sharp transition between these lineages with respect to pH is unwarranted.