SEASONAL GEOCHEMICAL CYCLING OF YELLOWSTONE HOT SPRING FLUID DRIVES CHEMOSYNTHETIC ENERGY SUPPLY FLUX

Hot springs in Yellowstone National Park (YNP) are at the interface between subsurface volcanic processes and the shallow hydrologic system and provide discrete aquatic ecosystems, ranging in pH from <2 to >9 and up to 93°C, for hyperdiverse chemosynthetic microbial communities. The compositions of YNP hot springs result from mixing of fluids that include deep hydrothermally (DH) altered fluids, enriched in Cl^- and ¹⁸O due to extensive subsurface water-rock interaction, and shallow meteoric (SM) fluids that preserve precipitation δ¹⁸O signatures and orders of magnitude lower concentrations of dissolved ions. These fluids are inferred to originate from local precipitation but DH fluids are estimated to have been recharged by ancient snowmelt with a residence time on the scale of hundreds of years.

A seasonal sampling campaign was conducted to test the hypothesis that DH-signature hot springs exhibit consistent geochemical behavior throughout the seasons while SM-signature hot springs exhibit cyclical geochemical behavior in response to spring snowmelt, similar to the hydrologic hysteresis of river systems. Two YNP hot springs, with signatures predominantly of DH fluid (spring RW1) and SM fluid (spring RS1), were targeted for sampling approximately monthly for 16 months. RW1 and RS1 were above 85°C in July 2019 and RW1 stayed above 85°C throughout the sampling period while RS1 cooled to 62°C in February 2020 and returned above 85°C by July 2020. RW1 pH remained nearly constant (8.1±0.26 2σ) but RS1 cycled almost a whole pH unit, starting and ending around 4.9 with a minimum of 4.1. Stable isotopes of water, dissolved organic carbon, and specific conductance show similar behavior.

Geochemical changes drive variations in chemosynthetic energy supply. As an example, during the maximum temperature phases of the RS1 cycle, the energy available from the methanotrophic reaction: CH₄(g) + 4NO₃^- -> HCO₃^- + 4NO₂^- + H⁺ + H₂O is low, but during the low temperature phase, it yields nearly 10x as much energy and surpasses the constant energy yield of RW1 by about 3x. Seasonal geochemical cycling can provide energy fluxes for chemosynthetic metabolisms, and evidence that some hot springs are more sensitive to seasonal hydrologic patterns suggests that changing climatic patterns will differentially influence hot spring ecosystems.