Primary productivity below the seafloor at deep-sea hot springs

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.

Keywords: Campylobacteria; NanoSIMS; chemosynthesis; deep-sea hydrothermal vents; ecophysiology.

Fig. 1.

Bacterial community composition at the end of incubations. (A) Taxonomic composition inferred from CARD-FISH counts, and (B) Nonmetric multidimensional scaling (NMDS) plot showing the similarity of Sulfurimonas 97% OTU composition between experimental treatments. Each dot represents a different biological replicate for incubations carried out at 24 °C and is colored according to the initial P_O2. All CARD-FISH data are averaged by treatment, and errors are presented as SDs (n = 3) or ranges (n = 2) except for the Nautiliales probe in the 110 μM O₂ treatment (n = 1). Validation of newly designed probes (Nautiliales = NAUT921 and Sulfurimonas = SFMN287; SI Appendix, Table S2) are described in the Materials and Methods, and specificity tests are shown in SI Appendix, Figs. S5 and S6. Campylobacteria in A corresponds to the combined probes EPSI549 and EPSI914.

Fig. 2.

Metabolic activity of Campylobacteria cells from Crab Spa fluids after short-term incubations at in situ pressure as quantified by HISH-SIMS. Rows represent different experimental treatments as follows: (A–C) control treatment (10% H¹³CO₃⁻) and (D–I) oxygen amendments (110 μM O₂ + 10% H¹³CO₃⁻). Cells were hybridized with general Campylobacteria probe (A–F) and with a specific Nautiliales probe (G–I), using Fluorine-containing tyramides. Columns display parallel secondary ion images of ¹²C¹⁴N as total biomass indicator (A, D, and G), ¹⁹F as a marker for cell identity (B, E, and H) and the ¹³C enrichment inferred from secondary ions (¹³C−, ¹²C−) given as atomic percentage [100 × ¹³C/(¹²C + ¹³C; at %)], as indicator of cell activity. [Scale bar, 2 μm (A–F) and 3 μm (G–I).]

Fig. 3.

Relative estimations of primary productivity in incubations of hydrothermal vent fluids at in situ temperature and pressure determined by HISH-SIMS. Bars represent relative volumetric rates of campylobacterial CO₂ assimilation during incubations. Errors are SDs (n = 3) or ranges (n = 2). Values are not corrected for the influence of CARD-FISH procedure (20).