The subsurface is among Earth’s largest biomes, but the extent to which microbial communities vary across tectonic plate boundaries or interact with subduction-scale geological processes remains unknown. Here we compare bacterial community composition with deep-subsurface geochemistry from 21 hot springs across the Costa Rican convergent margin. We find that cation and anion compositions of the springs reflect the dip angle and position of the underlying tectonic structure and also correlate with the bacterial community. Co-occurring microbial cliques related to cultured chemolithoautotrophs that use the reverse tricarboxylic acid cycle (rTCA) as well as abundances of metagenomic rTCA genes correlate with concentrations of slab-volatilized carbon. This, combined with carbon isotope evidence, suggests that fixation of slab-derived CO₂ into biomass may support a chemolithoautotrophy-based subsurface ecosystem. We calculate that this forearc subsurface biosphere could sequester 1.4 × 10⁹ to 1.4 × 10¹⁰ mol of carbon per year, which would decrease estimates of the total carbon delivered to the mantle by 2 to 22%. Based on the observed correlations, we suggest that distribution and composition of the subsurface bacterial community are probably affected by deep tectonic processes across the Costa Rican convergent margin and that, by sequestering carbon volatilized during subduction, these chemolithoautotrophic communities could in turn impact the geosphere.

地下是地球上最大的生物群落之一，但微生物群落在构造板块边界上的变化程度或与俯冲尺度地质过程相互作用的程度仍然未知。在这里，我们将来自哥斯达黎加会聚边缘的 21 个温泉的细菌群落组成与深层地下地球化学进行比较。我们发现泉水的阳离子和阴离子成分反映了下伏构造结构的倾角和位置，也与细菌群落相关。与使用反向三羧酸循环 (rTCA) 的培养的化能自养生物相关的共生微生物群落以及宏基因组 rTCA 基因的丰度与平板挥发碳的浓度相关。这与碳同位素证据相结合，表明平板衍生的 CO 的固定₂转化为生物质可以支持基于化学自养的地下生态系统。我们计算出这个弧前地下生物圈每年可以隔离 1.4 × 10 ⁹到 1.4 × 10 ¹⁰  mol 的碳，这将使输送到地幔的总碳的估计值减少 2% 到 22%。基于观察到的相关性，我们认为地下细菌群落的分布和组成可能受到哥斯达黎加会聚边缘的深层构造过程的影响，并且通过隔离俯冲过程中挥发的碳，这些化学自养群落可能反过来影响地圈。