Fig. 8: Conceptual depiction of microbial community changes concurrent with distinct states of methane dynamics at Storfjordrenna gas hydrate mounds.

Panels a, b, and c illustrate data shown in Figs. 3–5, respectively. Methane and sulfate profiles are shown in gray and blue lines, respectively, with microbial community changes indicated by blowup circles. ANME and SRB represent red and green ovals, with other bacteria and archaea in gray (shapes representing the diversity of other taxa). a Steady-state sulfate profiles suggest a weak methane influx, and low rates of AOM are observed at the SMT. Stable conditions may allow for higher microbial diversity in sediment communities and the growth of macroscopic biofilms. b A concave-up bend in porewater sulfate suggests recent methane migration into the sulfate reduction zone consistent with a pulse of methane beginning hundreds of years ago (approx. 290 years ago at GC1045, Fig. 2f, g). Methane travels upward throughout the sediment column, and ANME/SRB growth follows with less than a year of lag time, driving down alpha diversity. c Methane seepage stimulates high rates of AOM and densities of ANME/SRB. Sulfide fluxes from AOM-coupled sulfate reduction are sufficient to support frenulate siboglinid tubeworms, which may distribute sulfate across the upper several cm of sediment, increase redox gradients in underlying sediments, and further ANME/SRB growth.