The pH and pCO2 dependence of sulfate reduction in shallow-sea hydrothermal CO2 - venting sediments (Milos Island, Greece)

Abstract

Microbial sulfate reduction (SR) is a dominant process of organic matter mineralization in sulfate-rich anoxic environments at neutral pH. Recent studies have demonstrated SR in low pH environments, but investigations on the microbial activity at variable pH and CO2 partial pressure are still lacking. In this study, the effect of pH and pCO2 on microbial activity was investigated by incubation experiments with radioactive (35)S targeting SR in sediments from the shallow-sea hydrothermal vent system of Milos, Greece, where pH is naturally decreased by CO2 release. Sediments differed in their physicochemical characteristics with distance from the main site of fluid discharge. Adjacent to the vent site (T ~40-75°C, pH ~5), maximal sulfate reduction rates (SRR) were observed between pH 5 and 6. SR in hydrothermally influenced sediments decreased at neutral pH. Sediments unaffected by hydrothermal venting (T ~26°C, pH ~8) expressed the highest SRR between pH 6 and 7. Further experiments investigating the effect of pCO2 on SR revealed a steep decrease in activity when the partial pressure increased from 2 to 3 bar. Findings suggest that sulfate reducing microbial communities associated with hydrothermal vent system are adapted to low pH and high CO2, while communities at control sites required a higher pH for optimal activity.

Keywords: extreme environment; microbial activity; pCO2 effect; pH effect; shallow-seahydrothermal vents; sulfate reduction; sulfate reduction rate.

FIGURE 1

(A) Location of Milos Island and other calc-alkaline volcanoes (shaded) along the Aegean Island Arc (dotted line). (B) Milos Island and the location of Palaeochori Bay. Stippled offshore areas around the island are mapped gas emissions by echo sounding (Dando et al., 1995). Maps modified from Price et al., 2012.

FIGURE 2

Transect through the different zones of Paleochori Bay marked with sample bottles. Zones were divided in yellow-orange, white, transition, and brown. The yellow-orange zone was located adjacent to the venting site.

FIGURE 3

Residual sulfate reduction rate of Paleochori Bay sediments after incubation for 35 h at a temperature of (A) 40°C and (B) 75°C.

FIGURE 4

Sulfate reduction rates after incubations for 16 h of Paleochori Bay, white, transition, and brown zone sediment incubated in not amended and VFA-supplemented media as a function of pH. Incubation temperature was 40°C. Horizontal error bars show development of pH value over time of incubation. Vertical error bars show the standard errors of rates (n = 3). (A) White zone sediment. (B) Transition zone sediment. (C) Brown zone sediment.

FIGURE 5

Sulfate reduction rates after incubations for 16 h of Paleochori Bay, white (A), transition (B), and brown zone sediment (C) incubated in not amended and VFA-supplemented media as a function of changing pCO₂. Incubation temperature was 40°C. Vertical error bars show the standard errors of rates (n = 3). Symbols represent different pCO₂ conditions, starting with circles for no pCO₂, triangles for 1 bar, squares for 2 bar, and diamonds for 3 bar.

FIGURE 6

Buffering capacity of white, transition, and brown sediment depicted by the pH of each sediment type to the corresponding pCO₂ value.