Final Report Summary - IRONGEOBIOVENT (Iron geobiology at deep-ocean hydrothermal vents)
Animals have been found at undersea hydrothermal systems, where seawater seep underneath oceanic crust via tectonic rifting and the heated fluids would become loaded with metals from leaching rocks along its path. Among these megafauna, a type of shrimps, Rimicaris exoculata, are dominated in many Mid-Atlantic Ridge vent sites, swarming on the surface of the chimneys harvesting food. These shrimps are characterized with enlarged gill chamber, filled with red-colored iron minerals (ferric oxyhydroxide or ferrihydrite). Iron precipitates in shrimps’ carapace are attached to the surface of bacteria that live inside the shrimps and throughout their molting cycles. In other words, these shrimps were pale at the beginning and gradually became redder until carapaces were fully packed with iron minerals about 10 days later. If we look closely at micro-scale, the ferric minerals at early stages are coated like pairs of little socks on the bacterial filaments. This process gradually ‘suffocated’ the shrimps such that seawater cannot flow through gill chambers, and induced a molting event to get rid of mineral precipitates.
Bacteria found inside the carapace of R. exoculata are living in symbiosis with the shrimp, meaning they rely on each other for host and food supply. Because no sunlight can be reached for photosynthesis at hydrothermal ecosystem, food chains here rely on chemosynthetic bacteria. Symbiotic bacteria get their energy and nutrient for reproduction via metabolism, in which microbiologists have identified two major pathways: via sulphur, methane oxidizing and, another lately discovered pathway (Peterson et al., 2011), hydrogen. However, with so much iron accumulated inside the shrimp, one cannot help to wonder if iron oxidation has also contributed energy gain. The reduced iron (Fe2+) emitted from the vent fluid can also serve as an electron donor and has been a confirmed metabolism pathway, though the energy yield is far less than that from hydrogen and it had never been found in any symbiotic bacteria. Fe isotopes were proposed for this specific question. The researcher, Dr. Chu, has experiences in isotope geochemistry. The multidisciplinary collaboration with microbiologist was one of the objectives for her to expand her research repertoire. Dr. Chu first developed the protocol to measure Fe isotopes, which has not been done in the host institute before she arrives and has become a successful knowledge transfer to the laboratory at IFREMER. The protocol includes both chemistry and analytical parts. The results of the chemistry set up are satisfactory. Because microbiology was an unfamiliar subject to the researcher, Dr. Chu took initiatives in organizing small seminars, took part in microbiology conferences and study literatures. After these preparation works, she later designed an experiment on symbiotic bacteria. For this experiment, she first cultivated bacteria found in the shrimps’ gill chamber. With the help from the staff in the microbiology department at IFREMER, she prepared culture medium and inoculated with Fe(II) solutions (as mimic the Fe(II) from the vent fluids) and bacteria. Sampling was carried out over a course of 2 weeks in an oxygen-free glove box. The results from the culture experiment (Fig A&B) show that the 56Fe/54Fe of solid phase fractionate (+0.5‰) from original Fe solution then decreases to equilibrium with incubation days. This is a typical fractionation pattern with Fe oxidizing bacteria (Croal et al, 2004), in which heavier isotope precipitated faster than lighter one at onset of bacterial Fe(II) oxidizing. However, their results were obtained with anaerobic bacteria, which no oxygen is involved in their growth environment. Dr. Chu learned that the bacteria enrichment in this study, neither isolated nor identified at the end of the project, is actually survived in a micro-oxygenated (2%) environment (they would perish if oxygen is below this level). This implies that Fe(II), even without bacterial activities, could precipitate with time to Fe(III) with the small amount of oxygen existing, which is shown in the inoculated sample (open square) in FigB. Therefore, to answer microbiologists’ question on Fe metabolism pathway in this particular case, Dr. Chu thinks it necessary to explore another tools or proxies to further clarify the process and that Fe isotope alone is not enough to prove such mechanism exist. Finally, she and colleagues dissected several shrimps at different molting stages (different redness) from several locations and collected the iron mineral deposits or isotope analyses. Results have shown that the Fe isotopes in the real shrimp samples are much more enriched (high 56Fe/54Fe) and did not show strong correlation with the molding stages. Dr. Chu suggested to measure hydrothermal fluids collected at the same sites for comparison. Unfortunately, she cannot have access to these samples. Moreover, she suggests to analyses the shrimp iron precipitates in a micro scale. The field results she obtained were bulk samples, which obviously include iron hydroxide at different stages of oxidation. Without a technique suitable to identify the stage of mineral growth, the results would be difficult for interpretation. Dr. Chu also suggested that a laser technique for Fe isotope would be a potential tool for measurements at micro scales.
Apart from the Fe isotope studies, other objectives of this individual training project have included graduate student supervision, science communication via science outreach activities and, finally, having been working in a renowned French oceanographic institute, to develop visions in maritime policy and research at European level. To achieve these, Dr. Chu supervised two master students during this project. Mr. Fabien Quéroué (2009) was Master Year-1 student at Institute Universitaire Européen de la Mer (IUEM). He was introduced to the clean lab, carried out chemistry calibration and measurements on the calibration results. His 2-month project, alongside with this proposal, was to refine the chemistry method previously used by Dr. Chu and to adjust it for future sample matrix. The supervision was satisfying and Mr Quéroué became highly motivated in research after this experience. He is now a PhD student studying marine chemistry under a joint programme between University of Tasmania and University of Western Brittany. Mr. Emmanuel Favreau (2010), a Master Year-2 student also at IUEM, was under Dr. Chu’s supervision for his master thesis report. His 6-month project was on applying stable Sr isotopes in marine authigenic carbonates, including cold seep carbonates at the continental margins and Lost City hydrothermal vent carbonate chimney. The idea of the project was new even in the isotope geochemistry community and required refined analytical skills and data processing. Dr. Chu has later realized a problem in the chemistry procedures for certain sample matrix after this master project terminated. She took the project over and revised the methods for the results to be finally presented in international conferences. This has later led to collaborations with other institutes for inter-laboratory standards comparisons. Throughout this individual training fellowship, via discussion with scientists from different disciplines and student supervision, Dr. Chu has developed an interest in science communication. Her host organisation is a world-leading expert/explorer in hydrothermal activities. Thanks to such working environment and her hydrothermal-related projects, she won the 3rd prize in a Science Writing competition organized by National Museum of Natural Sciences and Scientific American (Taiwan) with an article on the Lost City Hydrothermal Vents. Afterwards, she continues to write articles on various scientific subjects and published in various media (in print and website). Thanks to her new network in microbiology, she could interview a colleague on bio-plastics made from bacteria found at extreme marine environments. Dr. Chu was also actively participated in community science outreach events, such as Fête de la Science (2010), a national science fair in major French cities. During this event, she volunteered to present themes on ‘marine carbonates and ocean acidification’ with some on-site pedagogic experiment. She also attended European Science Open Forum (ESOF) in 2010, which has expanded her perspective on science and has made an important impact on her later career decision. Figure/Plate captions: 1) Shrimps grazing on hydrothermal vent chimney. 2) White versus red R. exoculata. 3) SEM micrographs of mineral deposits on bacteria filaments. Left: socks-like mid-stage encrustation; right: packed mineral crusts. A) Three isotope plots to control isotopic data quality (i.e. natural stable isotopic fractionation follow mass difference) B) δ56Fe values vs time in solid and liquid phase of the culture experiments. δ56Fe of shrimp’s iron precipitates are more enriched. References: Corbari et al (2008) Aquatic Biology, 2008. 1(3): p. 225-238. Petersen, J.M. et al., Nature, 2011. 476(7359): p. 176-180
Bacteria found inside the carapace of R. exoculata are living in symbiosis with the shrimp, meaning they rely on each other for host and food supply. Because no sunlight can be reached for photosynthesis at hydrothermal ecosystem, food chains here rely on chemosynthetic bacteria. Symbiotic bacteria get their energy and nutrient for reproduction via metabolism, in which microbiologists have identified two major pathways: via sulphur, methane oxidizing and, another lately discovered pathway (Peterson et al., 2011), hydrogen. However, with so much iron accumulated inside the shrimp, one cannot help to wonder if iron oxidation has also contributed energy gain. The reduced iron (Fe2+) emitted from the vent fluid can also serve as an electron donor and has been a confirmed metabolism pathway, though the energy yield is far less than that from hydrogen and it had never been found in any symbiotic bacteria. Fe isotopes were proposed for this specific question. The researcher, Dr. Chu, has experiences in isotope geochemistry. The multidisciplinary collaboration with microbiologist was one of the objectives for her to expand her research repertoire. Dr. Chu first developed the protocol to measure Fe isotopes, which has not been done in the host institute before she arrives and has become a successful knowledge transfer to the laboratory at IFREMER. The protocol includes both chemistry and analytical parts. The results of the chemistry set up are satisfactory. Because microbiology was an unfamiliar subject to the researcher, Dr. Chu took initiatives in organizing small seminars, took part in microbiology conferences and study literatures. After these preparation works, she later designed an experiment on symbiotic bacteria. For this experiment, she first cultivated bacteria found in the shrimps’ gill chamber. With the help from the staff in the microbiology department at IFREMER, she prepared culture medium and inoculated with Fe(II) solutions (as mimic the Fe(II) from the vent fluids) and bacteria. Sampling was carried out over a course of 2 weeks in an oxygen-free glove box. The results from the culture experiment (Fig A&B) show that the 56Fe/54Fe of solid phase fractionate (+0.5‰) from original Fe solution then decreases to equilibrium with incubation days. This is a typical fractionation pattern with Fe oxidizing bacteria (Croal et al, 2004), in which heavier isotope precipitated faster than lighter one at onset of bacterial Fe(II) oxidizing. However, their results were obtained with anaerobic bacteria, which no oxygen is involved in their growth environment. Dr. Chu learned that the bacteria enrichment in this study, neither isolated nor identified at the end of the project, is actually survived in a micro-oxygenated (2%) environment (they would perish if oxygen is below this level). This implies that Fe(II), even without bacterial activities, could precipitate with time to Fe(III) with the small amount of oxygen existing, which is shown in the inoculated sample (open square) in FigB. Therefore, to answer microbiologists’ question on Fe metabolism pathway in this particular case, Dr. Chu thinks it necessary to explore another tools or proxies to further clarify the process and that Fe isotope alone is not enough to prove such mechanism exist. Finally, she and colleagues dissected several shrimps at different molting stages (different redness) from several locations and collected the iron mineral deposits or isotope analyses. Results have shown that the Fe isotopes in the real shrimp samples are much more enriched (high 56Fe/54Fe) and did not show strong correlation with the molding stages. Dr. Chu suggested to measure hydrothermal fluids collected at the same sites for comparison. Unfortunately, she cannot have access to these samples. Moreover, she suggests to analyses the shrimp iron precipitates in a micro scale. The field results she obtained were bulk samples, which obviously include iron hydroxide at different stages of oxidation. Without a technique suitable to identify the stage of mineral growth, the results would be difficult for interpretation. Dr. Chu also suggested that a laser technique for Fe isotope would be a potential tool for measurements at micro scales.
Apart from the Fe isotope studies, other objectives of this individual training project have included graduate student supervision, science communication via science outreach activities and, finally, having been working in a renowned French oceanographic institute, to develop visions in maritime policy and research at European level. To achieve these, Dr. Chu supervised two master students during this project. Mr. Fabien Quéroué (2009) was Master Year-1 student at Institute Universitaire Européen de la Mer (IUEM). He was introduced to the clean lab, carried out chemistry calibration and measurements on the calibration results. His 2-month project, alongside with this proposal, was to refine the chemistry method previously used by Dr. Chu and to adjust it for future sample matrix. The supervision was satisfying and Mr Quéroué became highly motivated in research after this experience. He is now a PhD student studying marine chemistry under a joint programme between University of Tasmania and University of Western Brittany. Mr. Emmanuel Favreau (2010), a Master Year-2 student also at IUEM, was under Dr. Chu’s supervision for his master thesis report. His 6-month project was on applying stable Sr isotopes in marine authigenic carbonates, including cold seep carbonates at the continental margins and Lost City hydrothermal vent carbonate chimney. The idea of the project was new even in the isotope geochemistry community and required refined analytical skills and data processing. Dr. Chu has later realized a problem in the chemistry procedures for certain sample matrix after this master project terminated. She took the project over and revised the methods for the results to be finally presented in international conferences. This has later led to collaborations with other institutes for inter-laboratory standards comparisons. Throughout this individual training fellowship, via discussion with scientists from different disciplines and student supervision, Dr. Chu has developed an interest in science communication. Her host organisation is a world-leading expert/explorer in hydrothermal activities. Thanks to such working environment and her hydrothermal-related projects, she won the 3rd prize in a Science Writing competition organized by National Museum of Natural Sciences and Scientific American (Taiwan) with an article on the Lost City Hydrothermal Vents. Afterwards, she continues to write articles on various scientific subjects and published in various media (in print and website). Thanks to her new network in microbiology, she could interview a colleague on bio-plastics made from bacteria found at extreme marine environments. Dr. Chu was also actively participated in community science outreach events, such as Fête de la Science (2010), a national science fair in major French cities. During this event, she volunteered to present themes on ‘marine carbonates and ocean acidification’ with some on-site pedagogic experiment. She also attended European Science Open Forum (ESOF) in 2010, which has expanded her perspective on science and has made an important impact on her later career decision. Figure/Plate captions: 1) Shrimps grazing on hydrothermal vent chimney. 2) White versus red R. exoculata. 3) SEM micrographs of mineral deposits on bacteria filaments. Left: socks-like mid-stage encrustation; right: packed mineral crusts. A) Three isotope plots to control isotopic data quality (i.e. natural stable isotopic fractionation follow mass difference) B) δ56Fe values vs time in solid and liquid phase of the culture experiments. δ56Fe of shrimp’s iron precipitates are more enriched. References: Corbari et al (2008) Aquatic Biology, 2008. 1(3): p. 225-238. Petersen, J.M. et al., Nature, 2011. 476(7359): p. 176-180