Microbial carbon fixation in a partially submerged caldera driven by the effect of CO₂ degassing

Calderas are subsided volcanic terrains formed by the destructive power of some of the largest volcanic eruptions on Earth. Globally, many such depressions are today covered by crater lakes, or seawater, rendering them less accessible to scientific scrutiny and hence more complicated to monitor during unrest. One such system is the restless, partly submerged, Campi Flegrei caldera near Naples, Italy, where escalating hydrothermal activity during ongoing unrest demands an improved understanding of the caldera offshore. Here, we present a high-resolution, system-wide mapping of CO2 degassing encompassing the offshore and onshore sectors of the caldera. Combining vertical seawater profiles with groundwater composition results, we obtain a refined delimitation of the anomalous CO2 degassing zones, showing overall structural control on volatile transport and surface/seafloor discharge. Intense CO2 degassing on the seafloor also causes ocean acidification, sustains diverse chemolithotrophic communities near the hydrothermal emissions, and influences the near-vent environment as demonstrated by taxonomic and functional diversity data. Submerged calderas are extremely dynamic environments whose volcanological significance and biogeochemistry processes deserve further investigation. Volcanic CO₂ emissions shape fluid pathways, drive seawater acidification, and sustain microbial carbon fixation near seafloor hydrothermal vents, according to a high-resolution map of CO₂ degassing across the onshore and offshore sectors of Campi Flegrei caldera. Primary Handling Editors: Alireza Bahadori.