Temporal dynamics of Prochlorococcus cells with the potential for nitrate assimilation in the subtropical Atlantic and Pacific oceans

Utilization of nitrate as a nitrogen source is broadly conserved among marine phytoplankton, yet many strains of Prochlorococcus lack this trait. Among cultured strains, nitrate assimilation has only been observed within two clades of Prochlorococcus: the high-light adapted HLII clade and the low-light adapted LLI clade. To better understand the frequency and dynamics of nitrate assimilation potential among wild Prochlorococcus, we measured seasonal changes in the abundance of cells containing the nitrate reductase gene (narB) in the subtropical North Atlantic and North Pacific oceans. At the Atlantic station, the proportion of HLII cells containing narB varied with season, with the highest frequency observed in stratified waters during the late summer, when inorganic nitrogen concentrations were lowest. The Pacific station, with more persistent stratification and lower N : P ratios, supported a perennially stable subpopulation of HLII cells containing narB. Approximately 20–50% of HLII cells possessed narB under stratified conditions at both sites. Since HLII cells dominate the total Prochlorococcus population in both ecosystems, nitrate potentially supports a significant fraction of the Prochlorococcus biomass in these waters. The abundance of LLI cells containing narB was positively correlated with nitrite concentrations at the Atlantic station. These data suggest that Prochlorococcus may contribute to the formation of primary nitrite maxima through incomplete nitrate reduction and highlight the potential for interactions between Prochlorococcus and sympatric nitrifying microorganisms. Further examination of these relationships will help clarify the selection pressures shaping nitrate utilization potential in low-light and high-light adapted Prochlorococcus.Gordon and Betty Moore Foundation (Grant GBMF495)National Science Foundation (U.S.) (OCE-1153588)National Science Foundation (U.S.) (DBI-0424599

Volcanic CO2 output at the Central American subduction zone inferred from melt inclusions in olivine crystals from mafic tephras

The volatile contents of olivine‐hosted (Fo89–71) melt inclusion glasses in rapidly quenched mafic tephras from volcanic front volcanoes of the Central American Volcanic Arc (CAVA) in Guatemala, Nicaragua, and Costa Rica, were analyzed by secondary ion mass spectrometry (SIMS) in order to derive the minimum eruptive output of CO2, along with H2O, Cl, and S. Details of the analytical method are provided that establish melt inclusion CO2 analyses with the Cameca ims6f at the Helmholtz Centre Potsdam. The highest CO2 concentrations (up to 1800 mg/g) are observed in Nicaraguan samples, while melt inclusions from Guatemala and Costa Rica have CO2 contents between 50 and 500 mg/g. CO2 does not positively covary with sediment/slab fluid tracers such as Ba/La, Ba/Th, or U/La. Instead, the highest CO2 concentrations occur in the inclusions with the most depleted incompatible element compositions and low H2O, approaching the composition of mid‐ocean ridge basalts (MORBs), whereas the most H2O‐rich inclusions are relatively CO2‐poor (<800 mg/g). This suggests that CO2 degassing was more extensive in the melts with the highest slab contribution. CO2/Nb ratios in the least degassed CAVA melt inclusions are similar to those of primitive MORBs. These are interpreted here as recording a minimum CO2 output rate from the mantle wedge, which amounts to 2.8 × 104 g/s for the ∼1100 km long CAVA. Previously published estimates from quiescent degassing and numerical modeling, which also encompassed the slab contribution, are 3 times higher. This comparison allows us to estimate the proportion of the total CO2 output derived from the mantle wedge

Genomes of diverse isolates of the marine cyanobacterium Prochlorococcus

The marine cyanobacterium Prochlorococcus is the numerically dominant photosynthetic organism in the oligotrophic oceans, and a model system in marine microbial ecology. Here we report 27 new whole genome sequences (2 complete and closed; 25 of draft quality) of cultured isolates, representing five major phylogenetic clades of Prochlorococcus. The sequenced strains were isolated from diverse regions of the oceans, facilitating studies of the drivers of microbial diversity—both in the lab and in the field. To improve the utility of these genomes for comparative genomics, we also define pre-computed clusters of orthologous groups of proteins (COGs), indicating how genes are distributed among these and other publicly available Prochlorococcus genomes. These data represent a significant expansion of Prochlorococcus reference genomes that are useful for numerous applications in microbial ecology, evolution and oceanography.Gordon and Betty Moore Foundation (Grant GBMR #495.01)National Science Foundation (U.S.) (Grant OCE-1153588)National Science Foundation (U.S.) (Grant OCE-0425602)National Science Foundation (U.S.) (Grant DBI-0424599)Center for Microbial Oceanography: Research and Educatio

Physiology and evolution of nitrate acquisition in Prochlorococcus

Prochlorococcus is the numerically dominant phototroph in the oligotrophic subtropical ocean and carries out a significant fraction of marine primary productivity. Although field studies have provided evidence for nitrate uptake by Prochlorococcus, little is known about this trait because axenic cultures capable of growth on nitrate have not been available. Additionally, all previously sequenced genomes lacked the genes necessary for nitrate assimilation. Here we introduce three Prochlorococcus strains capable of growth on nitrate and analyze their physiology and genome architecture. We show that the growth of high-light (HL) adapted strains on nitrate is ~17% slower than their growth on ammonium. By analyzing 41 Prochlorococcus genomes, we find that genes for nitrate assimilation have been gained multiple times during the evolution of this group, and can be found in at least three lineages. In low-light adapted strains, nitrate assimilation genes are located in the same genomic context as in marine Synechococcus. These genes are located elsewhere in HL adapted strains and may often exist as a stable genetic acquisition as suggested by the striking degree of similarity in the order, phylogeny and location of these genes in one HL adapted strain and a consensus assembly of environmental Prochlorococcus metagenome sequences. In another HL adapted strain, nitrate utilization genes may have been independently acquired as indicated by adjacent phage mobility elements; these genes are also duplicated with each copy detected in separate genomic islands. These results provide direct evidence for nitrate utilization by Prochlorococcus and illuminate the complex evolutionary history of this trait.Gordon and Betty Moore Foundation (Grant GBMF495)National Science Foundation (U.S.) (Grant OCE-1153588)National Science Foundation (U.S.) (Grant DBI-0424599

Crystal and melt inclusion timescales reveal the evolution of magma migration before eruption

Volatile element concentrations measured in melt inclusions are a key tool used to understand magma migration and degassing, although their original values may be affected by different re-equilibration processes. Additionally, the inclusion-bearing crystals can have a wide range of origins and ages, further complicating the interpretation of magmatic processes. To clarify some of these issues, here we combined olivine diffusion chronometry and melt inclusion data from the 2008 eruption of Llaima volcano (Chile). We found that magma intrusion occurred about 4 years before the eruption at a minimum depth of approximately 8 km. Magma migration and reaction became shallower with time, and about 6 months before the eruption magma reached 3–4 km depth. This can be linked to reported seismicity and ash emissions. Although some ambiguities of interpretation still remain, crystal zoning and melt inclusion studies allow a more complete understanding of magma ascent, degassing, and volcano monitoring data.NRF (Natl Research Foundation, S’pore)MOE (Min. of Education, S’pore)Published versio

Insight into volatile behavior at Nyamuragira volcano (D.R. Congo, Africa) through olivine-hosted melt inclusions

Author Posting. © American Geophysical Union, 2011. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geochemistry Geophysics Geosystems 12 (2011): Q0AB11, doi:10.1029/2011GC003699.We present new olivine-hosted melt inclusion volatile (H2O, CO2, S, Cl, F) and major element data from five historic eruptions of Nyamuragira volcano (1912, 1938, 1948, 1986, 2006). Host-olivine Mg#'s range from 71 to 84, with the exception of the 1912 sample (Mg# = 90). Inclusion compositions extend from alkali basalts to basanite-tephrites. Our results indicate inclusion entrapment over depths ranging from 3 to 5 km, which agree with independent estimates of magma storage depths (3–7 km) based on geophysical methods. Melt compositions derived from the 1986 and 2006 Nyamuragira tephra samples best represent pre-eruptive volatile compositions because these samples contain naturally glassy inclusions that underwent less post-entrapment modification than crystallized inclusions. Volatile concentrations of the 1986 and 2006 samples are as follows: H2O ranged from 0.6 to 1.4 wt %, CO2 from 350 to 1900 ppm, S from 1300 to 2400 ppm, Cl from 720 to 990 ppm, and F from 1500 to 2200 ppm. Based on FeOT and S data, we suggest that Nyamuragira magmas have higher fO2 (>NNO) than MORB. We estimate the total amount of sulfur dioxide (SO2) released from the 1986 (0.04 Mt) and 2006 (0.06 Mt) Nyamuragira eruptions using the petrologic method, whereby S contents in melt inclusions are scaled to erupted lava volumes. These amounts are significantly less than satellite-based SO2 emissions for the same eruptions (1986 = ∼1 Mt; 2006 = ∼2 Mt). Potential explanations for this observation are: (1) accumulation of a vapor phase within the magmatic system that is only released during eruptions, and/or (2) syn-eruptive gas release from unerupted magma.Funding for this work was provided by NSF (grant EAR 0910795 (to SAC) and grant EAR 0646694 (to AMS)), as well as the National Geographic Society (grant 7698-04 (to SAC))

Intraplate seismicity and releated mantle hydration at the Nicaraguan trench outer rise

We examine micro-earthquake records from a dense temporary array of ocean bottom seismometers (OBS) and hydrophones that has been installed from September to November 2005 at the trench outer rise offshore Nicaragua. Approximately 1.5 locatable earthquakes per day within the array of 110 × 120 km show the high seismic activity in this region. Seismicity is restricted to the upper ∼15 km of the mantle and hence where temperatures reach 350–400 °C, which is smaller than values observed for large mantle intraplate events (650 °C). Determination of moment tensor solutions suggest a change of the stress region from tensional in the upper layers of the oceanic plate to compressional beneath. The neutral plane between both regimes is located at ∼6–9 km beneath Moho and thus very shallow. Fluids, which are thought to travel through the tensional fault system into the upper mantle, may not be able to penetrate any deeper. The earthquake catalogue, which seems to be complete for magnitudes above Mw = 1.6–1.8, suggests a strong change of the lithospheric rheology when approaching the trench. And b-factors, that is the ratio between small and large earthquakes increase significantly in the closest 20 km to the trench axis, implying that the crust and upper mantle is massively weakened and hence ruptures more frequently but under less release of stress. We explain this with a partly serpentinized upper mantle

Settling-driven gravitational instabilities associated with volcanic clouds: new insights from experimental investigations

Downward propagating instabilities are often observed at the bottom of volcanic plumes and clouds. These instabilities generate fingers that enhance the sedimentation of fine ash. Despite their potential influence on tephra dispersal and deposition, their dynamics is not entirely understood, undermining the accuracy of volcanic ash transport and dispersal models. Here, we present new laboratory experiments that investigate the effects of particle size, composition and concentration on finger generation and dynamics. The experimental set-up consists of a Plexiglas tank equipped with a removable plastic sheet that separates two different layers. The lower layer is a solution of water and sugar, initially denser than the upper layer, which consists of water and particles. Particles in the experiments include glass beads as well as andesitic, rhyolitic and basaltic volcanic ash. During the experiments, we removed the horizontal plastic sheet separating the two fluids. Particles were illuminated with a laser and filmed with a HD camera; particle image velocimetry (PIV) is used to analyse finger dynamics. Results show that both the number and the downward advance speed of fingers increase with particle concentration in the upper layer, while finger speed increases with particle size but is independent of particle composition. An increase in particle concentration and turbulence is estimated to take place inside the fingers, which could promote aggregation in subaerial fallout events. Finally, finger number, finger speed and particle concentration were observed to decrease with time after the formation of fingers. A similar pattern could occur in volcanic clouds when the mass supply from the eruptive vent is reduced. Observed evolution of the experiments through time also indicates that there must be a threshold of fine ash concentration and mass eruption rate below which fingers do not form; this is also confirmed by field observations.Published395V. Dinamica dei processi eruttivi e post-eruttiviJCR Journa