B23G-01 INVITED
Neutrophilic Iron-Oxidizing Microbes In The Marine Environment
Neutrophilic iron-oxidizing bacteria (FeOB) have been recognized, described, enriched for and isolated from terrestrial aquatic and soil habitats for over one hundred years. Microbiologists and geoscientists alike have appreciated the important role FeOB play in processes such as corrosion and mineral deposition. However, recognition of their role and activities has lagged considerably behind in marine realms. Over approximately the past twenty years, however, there has been mounting interest and recognition of the presence and ubiquity of marine FeOB, particularly in the deep sea . Their role in rock and mineral weathering and alteration, mineral deposition, and biomass production in the deep sea has come into focus and is the subject of intense study. This paper will present an overview of marine FeOB including discussion of their habitats, diversity, and role in geochemical processes. In an effort to specifically target this elusive class of microbes, an observatory project for the study of FeOB and the marine microbial iron cycle has recently been launched--the Iron Microbial Observatory "FeMO", at the Loi'hi seamount, Hawaii. FeMO and FeOB isolated from Loi'hi are being used as model systems and testing grounds for studying mineral deposition and rock colonization in laboratory/field settings.
B23G-02
Spatial and Temporal Variability in Microbial Communities from Pre- and Post-Eruption Microbial Mats Collected from Loihi Seamount, Hawaii: An Update
Loihi Seamount is an active submarine volcano that marks the southernmost extent of the Hawaiian hotspot. Loihi rises over 3000 meters from the seafloor and summits nearly 1000 meters below sea level. Hydrothermal activity was discovered at Loihi in 1987, yielding diffuse vent effluent (Tmax 37°C) with associated high CO2 and Fe(II) concentrations and luxuriant microbial mats located near the summit of the volcano. Loihi erupted most recently in 1996 forming a new 300 meter deep caldera (Pele's Pit) with hydrothermal venting up to 200°C. Pele's Pit has cooled and now contains multiple hydrothermal vents with hydrothermal fluids ranging from 8-58°C with concentrations of Fe(II) remaining between 50 and 750 μM. Community fingerprints from over 75 microbial mat samples have now been collected from Loihi Seamount from 1993 to 2006, with temperatures ranging from ambient (~4°C) up to nearly ~200°C. These samples were analyzed using Pearson product-moment coupled with UPGMA cluster analysis of terminal- restriction fragment length polymorphisms (T-RFLP) coupled with traditional clone library and sequence analysis to identify the primary populations within each community. These mat samples form two distinct community clusters (Loihi Cluster Group 1 and Group 2) representing a combined 90% of all mat samples collected. Loihi Cluster Group 1 is by far the largest group (n = 45) and contains the most mat samples collected over time. Group 1 is dominated by phylotypes closely related to the recently described zeta- Proteobacteria that includes the type strain Mariprofundus ferrooxydans, an obligately lithotrophic, Fe-oxidizing bacterium. Loihi Cluster Group 2 is comprised of only post-eruption communities (n = 18) that generally contain greater diversity (in terms of richness) than Group 1 communities. Group 2 communities are primarily dominated by a unique array of phylotypes belonging to the Nitrospira division and by the class epsilon- Proteobacteria, including many putative sulphur-oxidizing bacteria. Interestingly, we have recently witnessed a shift away from the Group 2 communities back to the Group 1 communities. Only 2 of 11 microbial mat samples collected in 2004 exhibited the Group 2 community structure. All 10 of the mat samples collected in 2006 from multiple locations exhibited the Group 1 community structure, indicating a reduced overall diversity within Pele's Pit and a return to the microbial mats being dominated by Fe-oxidizing bacteria. http://fire.biol.wwu.edu/cmoyer/research.html
B23G-03
Iron Cycling at Loihi Seamount
Loi"hi is an active submarine volcano located close to the island of Hawai"i at a depth around 1100 m. The summit is host to a variety of focused and diffuse flow hydrothermal vent sites, which have in common a chemistry that is dominated by the presence of Fe(II) often in concentrations of 10's to 100's of μM at circumneutral pH. An outcome of these conditions is the deposition of substantial quantities of rust-colored microbial mats that are dominated by Fe-oxidizing bacteria (FeOB). These mat communities were first sampled in the late 1980s, and have subsequently been the subject of continued investigation, to the extent that Loih'i is now a model system for understanding deep-sea microbial communites that are primarily driven by iron-based lithotrophic metabolism. While the role of FeOB is becoming well-established, the short-term fate of the iron oxyhydroxides that are produced is less well understood, and specifically that role that Fe-reducing bacteria may have in re-cycling Fe(III) to Fe(II). A recent cruise to Loihi in 2006 found that one vent site, which has been slowly decreasing in activity, was essentially "dead", i.e. there was no sign of fluid flow or temperature anomaly. Yet, iron mats several cm thick persisted and chemical analysis suggested O2 penetrated the mat and that no Fe(II) was present, i.e. there no sign of active Fe-reduction. This raises the question of how active is the iron cycle in Loih'i iron mats? Experiments to measure Fe- reduction potential in Lohi"i mats do show activity and it is possible to enrich Fe-reducing bacteria. We have recently isolated a novel Geothermobacter sp. HR-1 from a Loih'i mat, that is an Fe-reducer and grows at mesophilic temperatures. This work will report on continued investigations into the iron cycle at Loih'i to further constrain the chemical conditions, further elucidate the potential for Fe-reduction in natural mat samples, and present the outcome of growing native isolates of Fe- oxidizers and reducers from Loih'i in co-culture.
B23G-04
Biogeochemical Cycling of Iron Isotopes at Loihi Seamount
It is now well recognized that seafloor hydrothermal systems support diverse and unique biological communities capable of using dissolved chemical species, such as Iron (Fe), as well as mineral substrates as sources of metabolic energy. Deep-sea hydrothermal systems such as the Loihi Seamount hydrothermal field are important examples of environments where both chemical and biological oxidation of Fe can occur simultaneously and provide an ideal system in which to test hypotheses on biotic vs. abiotic origin of iron-oxide formation. Here, we applied Fe isotope systematics of hydrothermal fluids and Fe-oxide precipitates to study biogeochemical cycling of iron and the formation of microbial mats at Loihi seamount. Warm hydrothermal fluids (<60°C) and iron oxide precipitates were recovered using the DSV Jason II during FeMO 2006 cruise. Fe-isotope composition of warm hydrothermal fluids yielded δ56Fe values near 0.1‰ and are indistinguishable from basalt values defined at 0.09‰. Suspended iron oxide particles in the fluids and seafloor iron oxide sediments (microbial mats) recovered in the vicinity of the vents yielded systematically positive δ56Fe values. The enrichment in heavy isotopes between 1.05 to 1.43‰ relative to Fe(II) in vent fluids is slightly higher than those obtained for abiotic Fe oxidation (around 0.9‰) and slightly lower than for bacterial Fe oxidation at circum neutral pH (around 1.5‰). Mass balance considerations also imply that the extent of Fe(II) oxidation is very limited in the vicinity of the vents (<20%) and that most Fe(II) is oxidized later in the water column. These results are consistent with the low oxygen content of seawater (i.e. summit of Loihi is located in the OMZ) and resultant slow kinetics of abiotic Fe oxidation. In contrast, mats supported by very diffuse fluids recovered at the base of the Loihi Seamount (~ 5000m depth) have distinctly negative Fe-isotope values between -0.3 to -1.5‰. These negative values are best explained by near-complete oxidation of isotopically light Fe(II) source. Negative δ56Fe values in the source fluid are likely generated by subsurface precipitation of isotopically heavy Fe-oxides during partial Fe(II) oxidation. These results, together with the significant enrichment in Mn-oxides relative to the warm mats, are consistent with the higher oxygen level in deep seawater and suggest extensive microbial Fe(II) oxidation below seafloor. Fe-isotope compositions of microbial mats at Loihi Seamount display a remarkable range between -1.2 to 1.6‰ which enlarges considerably the range of δ56Fe values for other hydrothermal Fe-oxide deposits at mid-oceanic ridges (δ56Fe values between -0.8 to 0‰). This unique feature at Loihi indicates that Fe isotope compositions of hydrothermal Fe-oxide precipitates are particularly sensitive to oxygen levels in the local environment where they form, and are less sensitive to abiotic vs. biotic origins. The Loihi hydrothermal ecosystem provides an important modern analogue for testing hypotheses about the biogeochemical cycling of Fe-isotopes on early Earth.
B23G-05
Widespread Iron Oxidizing Bacterial Communities in a late Paleoproterozoic Marine Environment
Discovery of exceptionally well-preserved microfossils in the Gunflint Iron Formation provided the first evidence for diverse and abundant life in the Precambrian oceans. However, whether the Gunflint microbial communities were dominated by cyanobacteria or represent an archaic, lithotrophic ecosystem is a matter of longstanding debate. Here, we present Fe isotope and rare earth element (REE) data for microfossiliferous stromatolites that are consistent with the Gunflint biota being an iron-oxidizing bacterial community. The lack of or positive Ce anomalies in REE data indicate that the benthic community grew below a redoxcline and positive iron isotope compositions reveal that the microbial community only oxidized a fraction of the dissolved iron load. This depositional setting and Fe isotope evidence for incomplete iron oxidation are inconsistent with a cyanobacterial interpretation and provide strong support that the Gunflint biota was indeed a microaerophilic lithotrophic microbial ecosystem. An apparently restricted temporal distribution of the Gunflint biota coupled with a widespread spatial distribution signals a significant ecosystem evolutionary event in the late Paleoproterozoic when the ocean was redox-stratified and Fe-rich anoxic waters impinged onto the shallow shelf.
B23G-06
Microbially Mediated Glass Alteration in the Geological Record: Textural clues for Microbial Functions.
Fe and Mn oxidizing microbes interact with their environment through the microbially mediated formation of Fe/Mn oxides and through the corrosion textures they may leave behind in the solids they colonize and from which they extract nutrients. Understanding the geo-biology of Fe and Mn oxidation may focus on the study of the microbes themselves, the mineral products, its biocorrosion features and the relationships between these types of observations. We have reviewed our own data on glass bio-corrosion and in particular the wider literature on microbial mineral tunneling to develop a two stage biocorrosion model for volcanic glass that offers feedback for our understanding of the mechanisms and the dynamics of microbial dissolution. Traces of microbially mediated dissolution of volcanic glass are commonly observed in volcanic glass found in submarine volcanoes on the seafloor, and in uplifted submarine volcanoes of almost any geological age back to the origin of life. Two main bioalteration textures care observed, granular and tubular. Based on a comparison of these features in particular with tunneling by ectomycorrhizal fungi, we propose two distinct types of biocorrosion that affects glass: (1) Granular alteration textures, made up of colonies of microbe-sized, near spherical mineral - filled cavities that form irregular clusters ranging to a tens of micron thick bands at the glas surfaces. These granular textures are interpreted as the result of microbial colonization. accompanied by dissolution of the glass in their contact surface, deposition of authigenic minerals and the formation of a biofilm, that eventually seals the glass from easy access by seawater for hydration, or from microbes accessing Fe (II) in the glass. (2) The most spectacular bioalteration feature, repesented by the formation of tubes cannot be easily formed by the former mechanism because near spherical, individual microbes are likely not to produce the directionality that is required to produce the near linear or sometimes coiled tubes. Instead, we envision the activity of hyphae-like organelles or filaments, that may radiate out from a host body located in direct contact with circulating water, possibly penetrating a biofilm and entering/drilling into the fresh glass. Such microdrilling is well described in soils, where hyphae can slowly drill into silicates, in a process that takes about 1000 years to become visible as tunnels.
B23G-07
The Distribution and Stabilisation of Dissolved Fe in Deep-sea Hydrothermal Plumes
Iron (Fe) is an essential micronutrient for oceanic phytoplankton, yet the debate over its sources and sinks persists. Dissolved Fe(II) in hydrothermal vent fluids is enriched ca. 106-fold over open ocean values, but as vent-fluids enter the base of the water column, abundant polymetallic particulate phases are formed: predominantly Fe-rich sulfides and Fe oxyhydroxides. More recently in hydrothermal plumes, deviation from first order Fe oxidation kinetics has suggested the occurrence of organically stabilised dissolved Fe. Such stabilisation could have implications on the hydrothermal Fe flux to the deep-ocean. To study Fe stabilisation in non-buoyant hydrothermal plumes, we have investigated the plume system at 5°S, Southern Mid-Atlantic Ridge, for dissolved Fe concentrations and Fe-complexing ligands. Six CTD stations were occupied for this study that intercepted non-buoyant plumes. Along-axis flow dominated the dispersion of plume material within the ridge segment and, 2.5 km down-plume from the nearest vent-site, high concentrations of dissolved Fe (20 nM) were still present. These high levels of "dissolved" Fe could be due to the presence of Fe colloids and/or organic Fe complexes. With the use of Competitive Ligand Exchange- Cathodic Stripping Voltammetry (CLE-CSV), we have detected stabilised dissolved Fe complexes within this "dissolved" Fe fraction, on the edges of a plume. We calculate that such a stabilised Fe fraction in hydrothermal plumes, 2.9 to 5.6 times greater than deep-ocean dissolved Fe concentrations (0.7 nM), could be sufficient to provide 10-20% of the global deep-ocean dissolved Fe budget.
B23G-08
Particulate Organic Carbon and Iron Speciation within Deep-Sea Hydrothermal Plumes
Geophysical and geochemical models indicate that mid-ocean ridge hydrothermal venting may be a quantitatively significant component of the global Fe budget. Direct measurements demonstrate that seawater interactions with hydrothermal plumes exert quantitative controls on the elemental oceanic cycling of numerous trace elements and isotopes including essential nutrients and micronutrients. The chemical reactivity of hydrothermal plumes has been attributed, but not firmly linked, to the formation and surface reactivity of Fe oxide minerals. Despite the overwhelming evidence that plume particulate Fe is a key factor in ocean chemistry, essentially nothing is known of Fe speciation within plumes, outside of sulfide mineralogy. In addition, due to the analytically challenging nature of the plume particles, little is known about the mechanisms behind the biogeochemical processes catalyzed by these materials. Although a role for organic C (Corg) in Fe speciation and mobility in the deep-sea has been hypothesized, Corg sources and speciation in plumes remain largely unknown. The present study examines Fe and C speciation in plume particles at the nanometer scale using scanning transmission X- ray microscopy (STXM) combined with C 1s and Fe 2p near edge X-ray absorption fine structure (NEXAFS) spectroscopy. The results offer evidence for direct interactions between Fe and C in plume particles; Fe(II), Fe(III), and C occur in close spatial proximity within plume particle aggregates. Iron(II) is co-located with C, and is stable in the presence of dissolved O2 on timescales that exceed the calculated "half-life" (with respect to oxidation) for Fe(II) in seawater. Organic C matrices coat micrometer scale Fe-rich particles and physically entrap nanometer scale Fe-rich particles. Our results suggest a new and broader conceptual model for the source of trace-element reactive surfaces in hydrothermal plumes is required, one that includes Corg in addition to the long-hypothesized Fe oxide particles. Our study has wide ranging implications for long-range Fe oceanic transport, oceanic trace element distributions, and deep-sea and sub-seafloor microbial C fixation.