The hydrothermal mats, mounds, and chimneys of the southern Guaymas Basin are the surface expression of complex subsurface hydrothermal circulation patterns. In this overview, we document the most frequently visited features of this hydrothermal area with photographs, temperature measurements, and selected geochemical data; many of these distinct habitats await characterization of their microbial communities and activities. Microprofiler deployments on microbial mats and hydrothermal sediments show their steep geochemical and thermal gradients at millimeter-scale vertical resolution. Mapping these hydrothermal features and sampling locations within the southern Guaymas Basin suggest linkages to underlying shallow sills and heat flow gradients. Recognizing the inherent spatial limitations of much current Guaymas Basin sampling calls for comprehensive surveys of the wider spreading region.
Extreme thermal gradients and compressed metabolic zones limit the depth range of microbial colonization in hydrothermally active sediments at Guaymas Basin. We investigated the physicochemical characteristics of this ecosystem and their influence on microbial community structure. Temperature‐related trends of δ13C values of methane and dissolved inorganic carbon from 36 sediment cores suggest in situ thermal limits for microbial anaerobic methane oxidation and organic carbon re‐mineralization near 80°C and 100°C respectively. Temperature logging probes deposited in hydrothermal sediments for 8 days demonstrate substantial thermal fluctuations of up to 25°C. Putative anaerobic methanotroph (ANME) populations dominate the archaeal community, transitioning from ANME‐1 archaea in warm surficial sediments towards ANME‐1 Guaymas archaea as temperatures increase downcore. Since ANME archaea performing anaerobic oxidation of methane double on longer time scales (months) compared with relatively rapid in situ temperature fluctuations (hours to days), we conclude that ANME archaea possess a high tolerance for short‐term shifts in the thermal regime.