2.5 IRON OXIDES AND BIOMINERALIZATION
2.5a The Role of Bacterial Surfaces in Fe-Oxide Formation - Danielle Fortin, University of Ottawa
Bacteria possess a very reactive cell wall which is composed of numerous surface binding sites, such as carboxyl, hydroxyl, phosphate and amine groups. These binding sites show acid-base properties and interact strongly with dissolved elements such as metals. Bacterial cell walls are known to sorb large amounts of metals, but can also nucleate various minerals under solution saturation conditions. Such minerals are often called biogenic minerals and include Fe- and Mn-oxides, silicates, sulfides and carbonates. Biogenic minerals are generally small (1-50 nm in diameter), poorly ordered and in close association with the outer layer of the cell wall. Due to their small size and poorly crystalline structure, they are believed to be highly reactive toward other dissolved species. The question is then, how do biogenic minerals form and how different are they from abiotic minerals, i.e., minerals formed in the absence of bacteria?
Bacteria have been shown to exist under a wide range of physico-chemical conditions in hydrothermal sea vent environments. They are observed in the vents where they are involved in redox reactions and away from the vents, at much lower temperature. Fe-oxides are also widely present in such environments. They are found on high temperature sulfide chimneys, low temperature chimneys, in close association with microbial mats and even on pillow lava on the sea floor. How do they form and what is the role of bacteria in their formation? We hope to answer these questions by studying the formation of Fe-oxides under two sets of laboratory experiments. The first experiment will deal with systems showing chemical conditions mimicking the aqueous chemistry of the sample location and use "in situ" bacterial strains. We have inoculated a basic marine growth medium with each Fe-oxide sample collected at Axial (see table), in order to obtain "in situ" strains that can be chemically characterized (through acid-base titration) and used as bacterial surfaces for mineral nucleation. Results from this experiment will be compared to the mineralogical, physical and chemical characteristics of Fe-oxides collected at Axial, a study by Chris Kennedy from the University of Toronto. The second experiment will use the same bacterial strains to study basalt weathering. Sample collection at Axial has indeed shown that some old pillow lava show signs of surface weathering and that Fe-oxides are formed 1to 2 mm below the surface of the rock. Sterile small fragments of old lava collected at Castle and Coquilles will be exposed to the "in situ" bacterial strains and compared to abiotic systems. Both systems will be analyzed overtime for mineral dissolution (i.e., Fe release) and Fe-oxide formation, in order to assess the microbial weathering. Results from this experiment will be compared to SEM (scanning electron microscopy) and X-ray diffraction analysis performed on the weathered basalt samples from both sites.
2.5b Bacterial Biomineralization: Fe-Oxides - Chris Kennedy, University of Toronto
What initially was seen as an orange "fluff" sitting on freshly formed basalt was later shown through SEM and TEM studies from the 1998 and 1999 NeMO cruises to have a microbial component to it. The composition was determined to be iron oxyhydroxide, more specifically ferrihydrite, a mostly amorphous mineral. It is formed on bacterial surfaces and is found in many different environments such as acid mine drainage sites, soils and now around hydrothermal vents. Fe-oxides are capable of absorbing many types of environmentally toxic chemicals, and as a result, there is much interest into the formation of Fe-oxides. Their presence around vent systems generates both curiosity and many questions as to their formation which this study hopes to provide some answers for.
Ferrihydrite is thought to form on nucleation sites provided by bacterial cell walls, but whether bacteria are playing a direct or indirect role is still unknown in hydrothermal vent systems. Past studies have shown bacteria's metabolism to be involved in the formation of Fe-oxides (i.e. in acid mine drainage sites at low pH) and as a result, the question arises as to whether bacteria at hydrothermal vents are metabolically driving the nucleation of minerals. Danielle Fortin will attempt to answer this question as a part of her research at the University of Ottawa, but regardless of the bacteria's metabolic role in mineral formation, an association exists between bacteria and minerals and it is this association that will be the focus of this study.
The first objective will be to determine the chemical and mineralogical nature of the samples obtained (see attached table). The samples recovered contained both a "fluffy" Fe-oxide (found as deposits on the seafloor) and a weathering Fe-oxide product on pillow lava. Are they formed through the same processes, is there even a microbial origin for basalt weathering? TEM and SEM will be done to provide an indication as to the extent of the bacteria present in the Fe-oxide samples. The mineralogical structure will be analyzed by XRD to determine if indeed all the Fe-oxide is the amorphous ferrihydrite, or if the Fe-oxides are more crystalline. For any samples with a crystalline structure, it will be important to determine if any bacteria are present. ICP-MS will be done to determine the relationship between the Fe-oxides and the trace elements sorbed to them.
From here, other questions will attempted to be answered based on the initial characterization of the obtained samples. Are there differences in Fe-oxide minerals related to the age of the samples? Fe-oxides at Ashes have been around for years while at Magnesia, Fe-oxides are only a year old. What inhibits or enhances the formation of Fe-oxides? The origins of the Fe-oxides would also be interesting to look at as some Fe-oxides form in mounds such as at Fe-city and Fe-hyde while some Fe-oxides are the by-product of Fe-sulfide oxidation. Another potentially interesting idea is bacterial fossilization. As these bacteria are forming iron type sheaths around them, the potential for fossilization is quite good. From obtaining an understanding as to the extent of preserved bacteria in these samples, future work could include looking at other modern day hydrothermal sulfide deposits and determining if bacteria have been preserved within. If positive results are obtained, looking at other older deposits for bacteria would be attempted. From here the imagination begins to wander as to the possibilities this could lead to, who knows, this study could even assist in the search for evidence of life in our solar system....