Roy E. Price

Prokaryotic Populations in Arsenic-Rich Shallow-Sea Hydrothermal Sediments of Ambitle Island, Papua New Guinea

This study is the first to investigate the microbial ecology of the Tutum Bay (Papua New Guinea) shallow-sea hydrothermal system. The subsurface environment was sampled by SCUBA using push cores, which allowed collection of sediments and pore fluids. Geochemical analysis of sediments and fluids along a transect emanating from a discrete venting environment, about 10 mbsl, revealed a complex fluid flow regime and mixing of hydrothermal fluid with seawater within the sediments, providing a continuously fluctuating redox gradient. Vent fluids are highly elevated in arsenic, up to ∼1 ppm, serving as a “point source” of arsenic to this marine environment. 16S rRNA gene and FISH (fluorescence in situ hybridization) analyses revealed distinct prokaryotic communities in different sediment horizons, numerically dominated by Bacteria. 16S rRNA gene diversity at the genus level is greater among the Bacteria than the Archaea. The majority of taxa were similar to uncultured Crenarchaea, Chloroflexus, and various heterotrophic Bacteria. The archaeal community did not appear to increase significantly in number or diversity with depth in these sediments. Further, the majority of sequences identifying with thermophilic bacteria were found in the shallower section of the sediment core. No 16S rRNA genes of marine Crenarchaeota or Euryarchaeota were identified, and none of the identified Crenarchaeota have been cultured. Both sediment horizons also hosted “Korarchaeota”, which represent 2–5% of the 16S rRNA gene clone libraries. Metabolic functions, especially among the Archaea, were difficult to constrain given the distant relationships of most of the community members from cultured representatives. Identification of phenotypes and key ecological processes will depend on future culturing, identification of arsenic cycling genes, and RNA-based analyses.

Archaeal and bacterial diversity in an arsenic-rich shallow-sea hydrothermal system undergoing phase separation

Phase separation is a ubiquitous process in seafloor hydrothermal vents, creating a large range of salinities. Toxic elements (e.g., arsenic) partition into the vapor phase, and thus can be enriched in both high and low salinity fluids. However, investigations of microbial diversity at sites associated with phase separation are rare. We evaluated prokaryotic diversity in arsenic-rich shallow-sea vents off Milos Island (Greece) by comparative analysis of 16S rRNA clone sequences from two vent sites with similar pH and temperature but marked differences in salinity. Clone sequences were also obtained for aioA-like functional genes (AFGs). Bacteria in the surface sediments (0–1.5 cm) at the high salinity site consisted of mainly Epsilonproteobacteria (Arcobacter sp.), which transitioned to almost exclusively Firmicutes (Bacillus sp.) at ~10 cm depth. However, the low salinity site consisted of Bacteroidetes (Flavobacteria) in the surface and Epsilonproteobacteria (Arcobacter sp.) at ~10 cm depth. Archaea in the high salinity surface sediments were dominated by the orders Archaeoglobales and Thermococcales, transitioning to Thermoproteales and Desulfurococcales (Staphylothermus sp.) in the deeper sediments. In contrast, the low salinity site was dominated by Thermoplasmatales in the surface and Thermoproteales at depth. Similarities in gas and redox chemistry suggest that salinity and/or arsenic concentrations may select for microbial communities that can tolerate these parameters. Many of the archaeal 16S rRNA sequences contained inserts, possibly introns, including members of the Euryarchaeota. Clones containing AFGs affiliated with either Alpha- or Betaproteobacteria, although most were only distantly related to published representatives. Most clones (89%) originated from the deeper layer of the low salinity, highest arsenic site. This is the only sample with overlap in 16S rRNA data, suggesting arsenotrophy as an important metabolism in similar environments.

Energy sources for chemolithotrophs in an arsenic‐ and iron‐rich shallow‐sea hydrothermal system

The hydrothermally influenced sediments of Tutum Bay, Ambitle Island, Papua New Guinea, are ideal for investigating the chemolithotrophic activities of micro-organisms involved in arsenic cycling because hydrothermal vents there expel fluids with arsenite (As(III)) concentrations as high as 950 μg L(-1) . These hot (99 °C), slightly acidic (pH ~6), chemically reduced, shallow-sea vent fluids mix with colder, oxidized seawater to create steep gradients in temperature, pH, and concentrations of As, N, Fe, and S redox species. Near the vents, iron oxyhydroxides precipitate with up to 6.2 wt% arsenate (As(V)). Here, chemical analyses of sediment porewaters from 10 sites along a 300-m transect were combined with standard Gibbs energies to evaluate the energy yields (-ΔG(r)) from 19 potential chemolithotrophic metabolisms, including As(V) reduction, As(III) oxidation, Fe(III) reduction, and Fe(II) oxidation reactions. The 19 reactions yielded 2-94 kJ mol(-1) e(-) , with aerobic oxidation of sulphide and arsenite the two most exergonic reactions. Although anaerobic As(V) reduction and Fe(III) reduction were among the least exergonic reactions investigated, they are still potential net metabolisms. Gibbs energies of the arsenic redox reactions generally correlate linearly with pH, increasing with increasing pH for As(III) oxidation and decreasing with increasing pH for As(V) reduction. The calculated exergonic energy yields suggest that micro-organisms could exploit diverse energy sources in Tutum Bay, and examples of micro-organisms known to use these chemolithotrophic metabolic strategies are discussed. Energy modeling of redox reactions can help target sampling sites for future microbial collection and cultivation studies.

A natural laboratory to study arsenic geobiocomplexity

Research on seafloor hydrothermal activity has focused primarily on deep-sea black smoker-type locations, which are found along volcanically active portions of the mid-ocean ridges and in deep back-arc basins. Submarine hydrothermal activity, however, is not confined to deepwater environments. Hydrothermal vents have been documented on the tops of seamounts, on the flanks of volcanic islands, and in other near-shore environments characterized by high heat flow. Their easy accessibility, relative to deep-sea hydrothermal systems, makes them excellent natural laboratories to study a wide range of chemical, physical, and biological processes.

Changes in Benthic Macrofauna Associated with a Shallow-Water Hydrothermal Vent Gradient in Papua New Guinea

Abstract: Infaunal macroinvertebrates were characterized along an environmental gradient from a shallow-water hydrothermal vent located at Tutum Bay, Ambitle Island, Papua New Guinea. Samples were collected at three sites located at 7.5, 60, and 150 m from the vent and from a nonhydrothermal reference site located to the north. Temperature and arsenic concentration were found to decrease and pH increased with distance away from the vent. At each site, five replicate core samples were taken randomly from a 1 m2 sampling grid. All infaunal invertebrates > 500 µm were sorted, identified to the lowest practical taxonomic level and counted. Results from the macrofauna data show a strong trend of increasing abundance, species richness and diversity relative to distance away from the vent, but even at 150 m the benthic macrofauna appeared to be depressed relative to the reference site. Mollusks were completely absent 7.5 m from the vent, rare at 60 m, and abundant at 150 m, suggesting that the low pH values associated with the hydrothermal activity play an important role in the benthic community structure.

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

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 35S 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.