B24C-01 INVITED
Elements and the Origin of Life. Boron and Molybdenum
The central paradox surrounding the origin of life is not presented by a scarcity of organic compounds (which
are abundant in the cosmos), but rather the ease with which organic compounds become tar when they are
exposed to energy. One emerging solution to this problem is the interaction of minerals with organic
compounds in ways that no only guides their reactivity, but also stabilizes end products having biological
value. One breakthrough in this area is the discovery that ribose, the "R" in "RNA", is formed in a guided
process in the presence of boron-containing minerals, and is stabilized by boron once it is formed. This
process may have been coupled with molybdenum-guided pathways on early Earth. These observations,
confirmed in detail in the laboratory, are guiding those who model planetary formation to consider how these
two elements, neither particularly abundant in the Earth's crust, might have been made available to organic
molecules evolving early in Earth's history to give chemical systems capable of Darwinian evolution.
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B24C-02 INVITED
Trace Metals in the Early Ocean, Biological Implications, and Evolving Biospheric Redox
Dissolved molybdenum abounds in modern, oxygen-rich seawater as the highly soluble molybdate ion. But this trace metal, also a nutrient essential to phytoplankton productivity, was not always so readily available. Several periods of the geologic past are noted for being euxinic (i.e., oxygen-poor and sulfide-rich) at regional or perhaps even global scales. During the Proterozoic in particular, numerous euxinic settings may have persisted in the ocean for a billion years or more following the initial rise of oxygen in the atmosphere. Under such conditions, Mo is sequestered efficiently, and corresponding limitation in the oceanic inventory may have impacted nitrogen fixation by prokaryotes, as well as uptake of fixed nitrogen by early eukaryotes. Mo is essential for nitrogen fixation and nitrate assimilation as a cofactor for nitrogenase and assimilatory nitrate reductase enzymes, respectively. Evidence for analogous limitations is emerging for portions of the Paleozoic and Mesozoic, but the potential impacts have not yet been considered. In each case, the critical arguments hang on our ability (1) to estimate oceanic extents of oxygen-poor, sulfide-rich settings for the time slice of interest and (2) to discriminate Mo drawdown in the global ocean from that occurring locally within individual basins. Traditional and emerging approaches for distinguishing local from global ancient anoxia/euxinia will be highlighted with an eye toward estimating the impact on availability of Mo and other bioessential, redox-sensitive transition metals. The context for this exploration will be the long history of evolving redox in the ocean and atmosphere from the Archean forward. The Phanerozoic examples are also associated with major biotic extinction events, and our understanding of the relationships among the primary drivers of extinction, widespread euxinic conditions, and the impact of micronutrient availability during extinction and recovery remains nascent.
B24C-03 INVITED
Trace element requirements of anoxygenic phototrophic Fe(II)-oxidizers: what we can learn about early earth evolution and the role of these bacteria in BIF deposition
Banded Iron Formations (BIFs) are Precambrian sedimentary deposits of alternating iron and silica mineral layers. The mechanism of BIF deposition is still unclear. While younger BIFs (2.3-2-5 Ga and younger) probably were deposited due to chemical or biological Fe(II) oxidation involving O2 as electron acceptor, BIFs that are older than 2.5-2.7 Ga were likely deposited in an O2-poor or even O2-free environment. Two formation processes have been suggested for Fe(III) mineral deposition under anoxic conditions. The first mechanism, photooxidation of Fe(II) via UV light, was recently found to have a negligible contribution to BIF formation. Here we focus on the second anoxic mechanism: the biological oxidation of Fe(II) via anoxygenic Fe(II)-oxidizing photoautotrophs. In the work presented, we determined trace element requirements of anoxygenic phototrophic Fe(II)-oxidizing bacteria (Mo, Co, V) and compared the results to trace element abundance and distribution in BIFs deposited before and after the 2.3-2.5 oxidation event. The trace element abundance in the Archean ocean during the time of BIF deposition is linked to the present redox conditions. Additionally, the trace element requirements of these phototrophic Fe(II)-oxidizing strains, in combination with the information about the abundance of these elements in the Archean ocean, constrain the abundance and activity of phototrophic Fe(II)-oxidizing microbes in these oceans. Therefore, the analysis of trace metal distributions in these formations may offer a new tool for investigating the microbial role in mineral formation and BIF deposition. In this way, these analyses may allow the potential activity of these organisms in Precambrian oceans to be constrained and the atmospheric and ocean water-dissolved concentrations of O2 to be inferred.
B24C-04 INVITED
Neoproterozoic Oxygenation of Earth Surface Environments Reflected in the Late Evolution of the O2-Dependent Vitamin B12 Biosynthesis Pathway
There are multiple lines of evidence for a significant rise of O2 in the Earth's atmosphere ~2.4 Ga. A second oxygenation event in the Neoproterozoic is not as well constrained. These changes in environmental redox affected the abundances of bioessential elements. Trace elements such as Co, Fe, and Ni were likely favored in the early evolution of metalloenzymes, prior to the first oxidation event. Consistent with this expectation, vitamin B12 is a Co-containing biomolecule whose biosynthesis is thought to have evolved prior to the origin of oxygenic photosynthesis and the first rise in O2. However, biochemical characterization of the many enzymes involved in B12 biosynthesis has revealed two distinct pathways: an O2-independent pathway and an O2-dependant pathway. The major difference between these pathways involves the timing of the insertion of Co. We examined the amino acid sequences of enzymes in the B12 biosynthesis pathway from a set of 100 phylogenetically diverse microbial genomes, focusing on enzymes exclusive to each pathway as well as enzymes shared by both. Molecular clock and phylogenetic analyses were performed on alignments of the sequences obtained from these study genomes. This approach focused on functional genes rather than the phylogeny of microbes in an attempt to understand the evolution of the pathway itself, rather than its presence in individual phylogenetic groups. Clear differences in age are apparent between representatives of each pathway. The O2-independent pathway and enzymes shared in both pathways show the most ancient last common ancestors. In contrast, the enzymes associated exclusively with the O2-dependent pathway diverged from a common ancestor less than a billion years ago. Phylogenetic analysis suggests that these enzymes were recruited from other biochemical pathways. From these results it seems likely that the evolution of the O2-dependent pathway occurred long after the initial evolution of the B12 biosynthesis. This conclusion provides evidence independent of the geological record as to the timing and existence of a second great oxidation event in Earth history.
B24C-05 INVITED
The Co-Evolution of Phytoplanton and Trace Element Cycles in the Oceans
The composition of the oceans and of its biota have influenced each-other through Earth history. Of all the biologically essential elements, N is the only one whose seawater concentration is clearly controlled biologically; this is presumably the main reason why the stoichiometry of N (defined as its mol ratio to P), but not that of the trace nutrients Mn, Fe, Co, Ni, Cu, Zn and Cd, is the same in seawater and in the plankton. Like the major nutrients, the trace nutrients are depleted in surface seawater as a result of quasi-complete utilization by the biota. This is made possible in part by the ability of marine phytoplankton to replace one trace metal by another in various biochemical functions. This replacement also results in an equalization of the availability of most essential trace metals in surface seawater. The difference in the stoichiometric composition of the plankton and of deep seawater, which is the dominant source of new nutrients to the surface, indicates that some nutrients are likely recycled with different efficiencies in the photic zone. The difference in the composition of the ocean and its biota provides insight into the coupling of biochemistry and biogeochemistry in seawater.
B24C-06 INVITED
Sulfur Substitution in Oxyanions.
Sulfide can react with oxyanions in two ways. In anions such as chromate, iodate or permanganate, the central metal(loid) is reduced rapidly. In anions such as molybdate, arsenate or antimonate, sulfur atoms substitute for oxygen atoms in the first coordination sphere. In the latter cases, the central metal(loid) often retains its high oxidation state in the final thioanion; however lower valent species, which tend to be coordinatively more labile, may be important reaction intermediates. Replacement of oxygen by sulfur "softens" oxyanions, in some cases making them strong binders of soft metals, like Cu, Ag, Au and Hg. These changes also can profoundly affect the geochemical fate of the metal(loids). Sulfur substitution in oxyanions can be extremely sluggish. Recently, computational chemistry has begun to yield information about sulfur substitution reactions that are too slow to be studied experimentally but yet are potentially important in geochemistry. Thioperrhenates, thiovanadates and thiotungstates are species whose geochemical roles, if any, remain to be determined. It is possible that sulfur substitution reactions are more important under hydrothermal conditions than at ambient temperatures. For example, germanate dominates the ambient-temperature chemistry of Ge, but in hydrothermal deposits this element occurs commonly in sulfide minerals.
B24C-07 INVITED
Immobilization and Natural Attenuation of Arsenic in Surface and Subsurface Sediments
Understanding of molecular-scale biogeochemical processes that control the mobilization and distribution of As and other oxyanions can be used to develop remediation strategies that take advantage of natural geochemical and hydrologic gradients. Arsenic and other toxic oxyanions can be mobilized at low bulk sediment concentrations (ppm range) and thus, treatment technologies are challenged by low contaminant concentrations, widespread sources, variable pH and Eh conditions, and inaccessibility of subsurface environments. In situ chemical amendments to soils and sediments can be used to decrease the mobility and bioaccessibility of As and oxyanions through sorption to, or precipitation with, stabilizing phases. At a site near San Francisco Bay (CA, USA), treatment of As-contaminated soils with sulfate-cement amendments has effectively immobilized As. Laboratory experiments with field soils and spectroscopic characterizations showed that in high pH cement-type treatments, As is precipitated in ettringite-type phases (Ca-Al sulfates), whereas in low pH ferrous sulfate treatments, As is associated with an iron-arsenate phase (angellelite). The presence of As-associated ettringite-type phases in field sediments amended more than a decade ago indicates long-term stability of these neophases, as long as environmental conditions are relatively constant. At sites of subsurface contamination, monitored natural attenuation (MNA) as a remediation approach for As is gaining interest and acceptance. Successful implementation of MNA requires a mechanistic understanding of As sequestration processes and of the subsurface conditions that may enhance or reduce long-term effectiveness. At a former military site (MA, USA), naturally occurring As was mobilized from sediments as a result of reducing conditions from addition of organic carbon as a biodegradation treatment of chlorinated solvents. Elevated As concentrations were not detected further than about 30 m downgradient of the injection, indicating that As sequestration was also occurring by natural processes in the aquifer. Laboratory experiments with aquifer sediments and spectroscopic characterization of reaction products were used to quantify the extent of As(III) sorption and abiotic oxidation to As(V), probably by Mn(III,IV) present in sediment minerals. Interrogation by XANES spectroscopy and analysis of uptake data indicated that sediments have a limited abiotic oxidation capacity for As(III), which did not exceed 30% of the total amount of As sorbed and was estimated at 0.025 to 0.4 mmol/kg sediment. Results indicate that pH-controlled sorption is the primary mechanism for As uptake and sediment capacity for oxidative sorption is limited. As such, MNA may be temporarily effective at this site, depending on the size of the contaminant plume and the rate of groundwater flow.
B24C-08 INVITED
Arsenic, Anaerobes, and Autotrophy.
That microbes have resistance to the toxic arsenic oxyanions arsenite [As(III)] and arsenate [As(V)] has been recognized for some time. More recently it was shown that certain prokaryotes can demonstrate As- dependent growth by conserving the energy gained from the aerobic oxidation of As(III) to As(V), or from the reduction of As(V) to As(III) under anaerobic conditions. During the course of our field studies of two alkaline, hypersaline soda lakes (Mono Lake and Searles Lake, CA) we have discovered several new anaerobic chemo- and photo-autotrophic bacteria that can center their energy gain around the redox reactions between As(III) and As(V). Alkalilimnicola ehrlichii, isolated from the water column of Mono Lake is a nitrate-respiring, As(III)-oxidizing chemoautotroph of the gamma-proteobacteria that has a highly flexible metabolism. It can function either as a facultative anaerobe or as a chemo-autotroph, or as a heterotroph (Hoeft et al., 2007). In contrast, strain MLMS-1 of the delta-proteobacteria was also isolated from Mono Lake, but to date is the first example of an obligate As(V)-respirer that is also an obligate chemo-autotroph, gaining its energy via the oxidation of sulfide to sulfate (Hoeft et al., 2004). Strain SLAS-1, isolated from salt-saturated Searles Lake is a member of the Halananerobiales, and can either grow as a heterotroph (lactate e-donor) or chemo- autotroph (sulfide e-donor) while respiring As(V). The fact that it can achieve this feat at salt-saturation (~ 340 g/L) makes it a true extremophile (Oremland et. al., 2005). Finally, strain PHS-1 isolated from a hot spring on Paoha island in Mono Lake is the first example of a photosynthetic bacterium of the gamma- proteobacteria able to link its growth to As(III)-dependent anoxygenic photosynthesis (Kulp et al., 2008). These novel microbes give us new insights into the evolution of arsenic-based metabolism and their role in the biogeochemical cycling of this toxic element. Hoeft, S.E., et al. 2007. Int. J. Syst. Evol. Microbiol. 57: 514 – 512. Hoeft, S.E, et al. 2004. Appl. Environ. Microbiol. 70: 2741 - 2747. Oremland, R.S., et al. 2005. Science 308: 1305 – 1308. Kulp, T.R. et al. 2008. Science 321: 967 - 970.