Microbial heterotrophy coupled to Fe-S-As cycling in a shallow-sea hydrothermal system
Abstract
To date, there are only a few known heterotrophic arsenite oxidizers and arsenate reducers. They utilize organic compounds as their carbon source and/or as important electron donors in the transfer arsenic in high temperature environments. Arsenic in hydrothermal vent systems can be immobilized at low temperatures through (ad)sorption on iron oxide and other iron-bearing minerals. Interactions with sulfur species can also affect the redox state of arsenic species. A better understanding of microbially-catalyzed reactions involving carbon, arsenic, iron and sulfur would provide constraints on the mobility of arsenic in a wide variety of natural and engineered systems. The aim of this study is to establish links between microbial distribution and in situ Fe-S-As cycling processes in a shallow-sea hydrothermal vent system. We investigated three shallow-sea hydrothermal vents, Champagne Hot Spring (CHS), Soufriere Spring (SOU) and Portsmouth Spring (PM), located off the western coast of Dominica, Lesser Antilles. CHS and SOU are characterized by moderate temperatures (46oC and 55oC, respectively), and PM is substantially hotter (~90-111 oC). Two sediment cores (one close to and one far from the thermal source) were collected from CHS and from SOU. Porewaters in both background cores had low concentrations of arsenic (mostly As3+, to a lesser extent As5+, DMA, MMA) and ferrous iron. The arsenic concentrations (predominantly As3+) in the CHS high temperature core were 30-90 nM, tracking with dissolved iron. Similar to CHS, the arsenic concentration in the SOU high temperature core was dominated by As3+ and controlled by ferrous iron. However, the arsenic concentration at SOU is comparatively higher, up to 1.9 mM. At the hotter and deeper PM site, highly elevated arsenic levels (1-2.5 mM) were measured, values that are among the highest arsenic concentrations ever reported in a marine hydrothermal system. Several autotrophic and heterotrophic media at two pHs (5.5 and 8.0) were designed to target microbial reductive and oxidative metabolisms of arsenic, iron and sulfur. Incubations were carried out at four temperatures (30, 50, 70, 90 oC), covering the mesophilic to hyperthermophilic range. Sediment and biofilm samples from all three sites were used as inocula in enrichments targeting heterotrophic arsenite oxidation and arsenate reduction. From these enrichments, multiple pure strains were isolated at 30, 50 and 70 oC. From the 50oC enrichments on oxic, heterotrophic media inoculated with both SOU sediment and biofilm, we isolated aerobic thermophilic sulfate reducers producing high concentration of sulfide. The produced sulfide transforms ferrihydrite to an amorphous arsenic-metal-sulfide mineral. The current culturing results not only expand the diversity of arsenic oxidizing, arsenate reducing, and aerobic sulfate reducing organisms, but also show that the microbes influence in situ Fe-S-As transformation. More molecular and physiological tests are underway to better characterize the microbe-mineral interactions in laboratory enrichments and natural environments to determine the biological effects on the cycling of As-Fe-S in shallow-sea hydrothermal systems.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2013
- Bibcode:
- 2013AGUFM.B13B0476L
- Keywords:
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- 0456 BIOGEOSCIENCES Life in extreme environments;
- 0461 BIOGEOSCIENCES Metals;
- 0414 BIOGEOSCIENCES Biogeochemical cycles;
- processes;
- and modeling;
- 0450 BIOGEOSCIENCES Hydrothermal systems