Extreme temperature exposure has negative demographic consequences for Sulfolobus acidocaldarius

Abstract Microorganisms inhabiting geothermal springs and volcanic systems experience fluctuating temperatures that can periodically exceed their upper thermal limits, but the demographic consequences of such exposure remain poorly understood. Here, we investigated demographic responses of the thermophilic archaeon Sulfolobus acidocaldarius to an extreme temperature (94.1°C) under two regimes: sustained exposure varying in duration, and episodic exposure interspersed with recovery at a permissive temperature (75 °C). Under sustained exposure, populations showed no detectable loss of viability after 15 min but declined thereafter, decreasing by approximately five orders of magnitude after 120 min. Under episodic exposure, populations remained viable across nine exposure-recovery cycles but declined in density with successive cycles. Similar responses were observed for three strains, including a DNA mismatch repair knockout (Δ nucS ), indicating that mismatch repair deficiency did not affect viability or recovery. Together, these results demonstrate that S. acidocaldarius can withstand brief and repeated exposure to near-boiling temperatures, with mortality determined primarily by cumulative exposure duration rather than a fixed thermal threshold. Importance This study shows that microbial tolerance to extreme temperature depends not only on the temperature reached, but also on how long and how often that stress occurs. In Sulfolobus acidocaldarius , brief near-boiling exposure was tolerated, whereas prolonged or repeated exposure reduced population recovery. This contrasts with previous experimental work in which repeated heat exposure promoted adaptation, showing that recurrent thermal stress does not necessarily lead to increased tolerance. These findings highlight the importance of temporal environmental variation in shaping survival and show why fixed thermal limits alone may not capture microbial persistence in fluctuating extreme environments.