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.

Thermodynamic calculations provide valuable insights into the reactions that drive the profound fluid transformations during serpentinization, where surface fluids are transformed into some of the most reduced and alkaline fluids on Earth. However, environmental observations usually deviate from thermodynamic predictions, especially those occurring at low temperatures where equilibrium is slowly reached. In this work, we sampled and analyzed >100 low-temperature (<40°C) fluids from the Samail ophiolite in Oman to test thermodynamic predictions with environmental observations. Additional simulations (e.g., fluid mixing, mineral leaching) were also conducted to account for deviations from equilibrium expectations. Type 1 circumneutral (pH 7 to 9) fluids result from fluid interactions with completely serpentinized rocks common in the shallow subsurface. Type 2 hyperalkaline (pH >11) fluids approach equilibrium with diopside, and serpentine and brucite actively forming during advanced stages of serpentinization. We also investigated fluids with pH values of 9 to 11 to test whether these fluids are indicative of intermediate stages of serpentinization or mixing between the above end-member fluids. Fluids at intermediate stages of serpentinization and fluids derived from mixing can have the same pH, but the former have considerably lower dissolved Si that can be attributed to concomitant subsurface serpentinization and mineral carbonation processes. Overall, this work demonstrates that predicted and measured compositions of serpentinization-derived fluids can be successfully reconciled using a combination of equilibrium and fluid-transport simulations. This work substantiates these calculations as useful tools in exploring serpentinization reactions in deep subsurface aquifers on Earth as well as those beyond our own planet.