Primary productivity below the seafloor at deep-sea hot springs

TLDR

Mixing of geothermal fluids with seawater at deep‑sea hot springs supports a potentially vast microbial ecosystem, yet the impact of subseafloor microorganisms on deep‑ocean biogeochemical cycles remains uncertain without quantitative metabolic measurements. The study aims to quantitatively constrain subseafloor primary productivity, biomass standing stock, and turnover time by incubating subseafloor fluids under in situ conditions. Incubations of subseafloor fluids were conducted under in situ temperature, pressure, and chemical conditions to measure primary productivity, biomass, and turnover. Campylobacteria dominated carbon fixation, with oxygen and temperature driving niche partitioning, and the subseafloor biosphere fixed carbon at up to 321 µg C L⁻¹ d⁻¹, turned over within tens of hours, rivaling surface chemosynthetic productivity and markedly influencing deep‑ocean biogeochemical cycles.

Abstract

Below the seafloor at deep-sea hot springs, mixing of geothermal fluids with seawater supports a potentially vast microbial ecosystem. Although the identity of subseafloor microorganisms is largely known, their effect on deep-ocean biogeochemical cycles cannot be predicted without quantitative measurements of their metabolic rates and growth efficiency. Here, we report on incubations of subseafloor fluids under in situ conditions that quantitatively constrain subseafloor primary productivity, biomass standing stock, and turnover time. Single-cell-based activity measurements and 16S rRNA-gene analysis showed that Campylobacteria dominated carbon fixation and that oxygen concentration and temperature drove niche partitioning of closely related phylotypes. Our data reveal a very active subseafloor biosphere that fixes carbon at a rate of up to 321 μg C·L−1·d−1, turns over rapidly within tens of hours, rivals the productivity of chemosynthetic symbioses above the seafloor, and significantly influences deep-ocean biogeochemical cycling.