Cyanobacteria and Algae Meet at the Limits of their Habitat Ranges in
Moderately Acidic Hot Springs
Abstract
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. Cyanobacteria
were detected at pH values as low as 3.0, challenging the paradigm of
Cyanobacteria being excluded below pH values of 4.0. Cyanobacterial 16S
rRNA genes were more abundant than rhodophyte 18S rRNA genes by up to 7
orders of magnitude, with rhodophyte template abundance approaching that
of Cyanobacteria only at the most acidic sites. Light-driven carbon
fixation was observed at two sites where chlorophyll a was
detected but not at two other sites where chlorophyll a was not
detected. Collectively, these observations suggest that many of the
rhodophyte 18S rRNA gene sequences were from inactive cells.
Fluctuations in the supply of meteoric water likely contributes to
physicochemical variability in these springs, leading to transitions in
photosynthetic community composition. Spatial, 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.