ALUMNI NEWSLETTER - Earth Sciences - University of Minnesota

Bacterial Polyphosphate Metabolism-continued from page 1settings associated with upwelling systems where nutrient-richwaters fuel the blooming of photosynthetic algae. The increasedbiologicaloxygen demand that accompanies the decay of thisalgal biomass often results in low-oxygen conditions in shelf sedimentsand bottom waters. One might infer that phosphogenesis ispromoted by the increased export of phosphorus from the watercolumn into sediments that accompanies these algal blooms, andthis may in fact be an important aspect of phosphogenesis. Butthe concentration of phosphate in certain sedimentary pore waterhorizons where apatite is actively precipitating, cannot be easilyexplained by the simple the breakdown of organic detritus. Gatheringevidence suggests that microbial processes may be importantfor the concentration of phosphate that leads to sedimentaryapatite precipitation.In shelf settingsoff of Namibia, and inother phosphogenicsediments, pore waterphosphate concentrationsand precipitationof apatite mineralsis closely associatedwith accumulations ofbacteria that acquiretheir metabolic energyprimarily through thechemical oxidation ofhydrogen sulfide, whichthey use to fix CO2 intobiomass in a mannersimilar to plants. Someof these bacteria aregiants of the microbialworld, with individual cells reaching nearly a millimeter in diameter.These giant cells are adapted to dynamic changes in bottomwater geochemistry that can oscillate between anoxic, sulfidicconditions and oxygenated conditions over periods of months.Part of that adaptation involves the storage of various metabolitesin granules and vacuoles within the cell. One of these types ofinclusions is known to store polyphosphate. Polyphosphates arelinear chains of phosphate linked by phosphoanhydride bonds(Figure 2). These polyphosphate molecules have diverse functionsin organisms from all three domains of life, including serving asan energy reserve.The role, or roles, of polyphosphate in marine microbialecosystems remains poorly understood. However, polyphosphatesappear to play an important role in the precipitation ofsedimentary apatite. When sulfide-oxidizing bacteria are exposedto sulfidic and/or anoxic conditions, they hydrolyze their polyphosphatestores, likely deriving energy for metabolism under theanoxic or sulfidic conditions. Laboratory experiments show thatthe hydrolysis of polyphosphate by these bacteria is sufficient toexplain pore water phosphate enrichments observed in sedimentswhere apatite minerals are precipitating in close proximity to matsof sulfide-oxidizing bacteria. Additionally, radioisotope-labelingstudies show a link between the polyphosphate in the cells andapatite precipitation in the surrounding sediments.So, large sulfide-oxidizing bacteria appear to be importantfor the formation of apatite in modern sediments. But interestingly,our research group has recently observed a similar releaseof phosphate in response to sulfide exposure from methane seepsediments collected with the remotely-operated vehicle (ROV)Jason from 5000 meter water depths off of Barbados (Figure 1B).These sediments were found to contain vacuolate sulfide-oxidizingbacteria, but these sediments are not known to host the precipitationof apatite. So perhaps there is more to the story of apatiteprecipitation and phosphorite formation than just the concentrationand pulsed release of phosphate from polyphosphate-accumulatinggiant microbes? To further complicate the story, we alsoobserved a substantial phosphate release from sulfidic sedimentsin the Santa BarbaraBasin off of SouthernCalifornia. Sedimentsfrom the Santa BarbaraBasin often containlarge sulfide-oxidizingbacteria, but the specificsediments we sampleddid not host thesebacteria. So where didthe observed phosphaterelease come from?Smaller, less obviousbacteria are also knownto store polyphosphate,and perhaps theseorganisms are responsiblefor the observedphosphate release. Ourgroup is investigatingthis hypothesis, in part by looking at the expression of genes thatregulate polyphosphate metabolism under various geochemicalconditions via a process known as metatranscriptomics. We hopeto learn more about this process in the near future.Figure 2: Polyphosphates are linear polymers of phosphate (A) that can be found ininclusions (bright spots) in diverse eukaryotes such as in this DAPI-stained diatom (B)(nucleus is diffuse blue spot), and bacteria, such as in this culture of Halomonas spp. (C)Scale bars = 5 mm. Source: (A) Rao et al., 2009; (B-C) Jones and Bailey unpublished.We are also interested in the formation of phosphoritesover geological time scales. We have identified fossilized cells inancient phosphorites that may represent the mineralized remainsof sulfide-oxidizing bacteria. The taxonomic identification of someof these structures remains equivocal because reliance on morphologicfeatures can be misleading. However, in other fossilizedbacteria preserved in phosphorites, we have identified geochemicalsignatures that we suggest are diagnostic of sulfide-oxidizingbacteria. The oldest of these microfossils occur in 600 million yearold phosphorites. This period of geologic time, the Neoproterozoic,is known for an unprecedented explosion in the occurrence ofmineable phosphorites on nearly every continent. Neoproterozoicrocks also show evidence for the widespread oxygenation of theoceans, as opposed to the atmosphere, which is thought to havebecome oxygenated in Paleoproterozoic times. Because sulfideoxidizingbacteria require access to oxygen, or oxygen-dependentchemical species such as nitrate, the correlation between phosphoriteproliferation and the spread of oxygen in the Neoproterozoic,may be explained by the expansion of conditions that harborpolyphosphate-accumulating bacteria. However, the observationsthat we have recently made in modern sediments suggest that theAlumni NewsletterPage 3