Fig. 1

A Shaded relief map showing the location of Alum Rock springs, CA, USA. Insets show the location of Alum Rock and of the MS4 and MS11 springs. Photographs of (B) MS4 and (C) MS11 biofilms. Thin white streamers (5–10 cm) are mostly found attached to the surfaces of rocks. Hydrogeological properties (D) discharge, (E) δ¹⁸Ο, and (F) temperature are steady over periods greater than a decade, except following large regional earthquakes. A discharge increase in late 2007 followed a magnitude 5.6 earthquake with an epicenter 4 km from the springs (vertical red line), neither δ¹⁸Ο nor the temperature changed indicating that fluid sources did not change. The horizontal lines show averages of plotted quantities over the entire sampling period, except discharge for which the average excludes the first 2 years after the earthquake. Vertical gray lines show dates of biofilm and planktonic sampling. G Microbial community composition at the class and order levels, respectively, highlighting the top 15 most abundant groups in each category. Each bar represents a sample collected from different biotopes (bulk/biofilm, 0.1-µm filter) in the MS4 and MS11 springs over several years (2015, 2019, and 2020). The stacked bar plots illustrate the relative abundance of each microbial group, with each color corresponding to a different group, from top to bottom in decreasing order of overall abundance across all samples