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At least 667 records · Page 37

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NASA NTRS1997

Stereological analysis of gravitropism in protonemata of the moss Ceratodon

Apical cells of dark-grown protonemata of the moss Cerotodon purpureus are negatively gravitropic. Previous light microscopy has shown that reorientation to the horizontal induces amyloplast sedimentation and redistribution of microtubules. To determine whether other components become redistributed laterally or axially, the apical 35 micrometers of both vertical and horizontal apical cells were compared stereologically using transmission electron microscopy. Reorientation to the horizontal changed the longitudinal distributions of tubular ER, Golgi stacks, and vesicles but not cisternal ER, mitochondria, and plastids. Only plastids showed a statistically significant lateral redistribution after horizontal placement. Qualitative examination of the sedimentation zone showed plastids sedimented close to peripherally located ER with vacuoles displaced above plastids. These results argue against a model where differential tip growth results from a redistribution of Golgi stacks or exocytic vesicles.

Non-NASA Center↗

NASA NTRS1989

Geochemical and isotopic evidence for paleoredox conditions during deposition of the Devonian-Mississippian New Albany Shale, southern Indiana

The upper part of the New Albany Shale is divided into three members. In ascending order, these are (1) the Morgan Trail Member, a laminated brownish-black shale; (2) the Camp Run Member, an interbedded brownish-black and greenish-gray shale; and (3) the Clegg Creek Member, also a laminated brownish-black shale. The Morgan Trail and Camp Run Members contain 5% to 6% total organic carbon (TOC) and 2% sulfide sulfur. Isotopic composition of sulfide in these members ranges from -5.0% to -20.0%. C/S plots indicate linear relationships between abundances of these elements, with a zero intercept characteristic of sediments deposited in a non-euxinic marine environment. Formation of diagenetic pyrite was carbon limited in these members. The Clegg Creek Member contains 10% to 15% TOC and 2% to 6% sulfide sulfur. Isotopic compositions of sulfide range from -5.0% to -40%. The most negative values occur in the uppermost Clegg Creek Member and are characteristic of syngenetic pyrite, formed within an anoxic water column. Abundances of carbon and sulfur are greater and uncorrelated in this member, consistent with deposition in as euxinic environment. In addition, DOP (degree of pyritization) values suggest that formation of pyrite was generally iron limited throughout Clegg Creek deposition, but sulfur isotopes indicate that syngenetic (water-column) pyrite becomes an important component in the sediment only in the upper part of the member. At the top of the Clegg Creek Member, a zone of phosphate nodules and trace-metal enrichment coincides with maximal TOC values. During euxinic deposition, phosphate and trace metals accumulated below the chemocline because of limited vertical circulation in the water column. Increased productivity would have resulted in an increased flux of particulate organic matter to the sediment, providing an effective sink for trace metals in the water column. Phosphate and trace metals released from organic matter during early diagenesis resulted in precipitation of metal-rich phosphate nodules.

NASA Discipline Exobiology↗

NASA NTRS1995

Evidence for gammacerane as an indicator of water column stratification

A new route for the formation of gammacerane from tetrahymanol is proposed; in addition to dehydration and hydrogenation, sulphurisation and early C-S cleavage are shown to be important in the pathway of formation, especially in marine sediments. Evidence is twofold. First, relatively large amounts of the gammacerane skeleton are sequestered in S-rich macromolecular aggregates formed by natural sulphurisation of functionalised lipids. Selective cleavage of polysulphide linkages with MeLi/MeI led to formation of 3-methylthiogammacerane, indicating that the gammacerane skeleton is primarily bound via sulphur at position 3, consistent with the idea that tetrahymanol (or the corresponding ketone) is the precursor for gammacerane. Second, upon mild artificial maturation of two sediments using hydrous pyrolysis, gammacerane is released from S-rich macromolecular aggregates by cleavage of the relatively weak C-S bonds. The stable carbon isotopic compositions of gammacerane and lipids derived from primary producers and green sulphur bacteria in both the Miocene Gessoso-solfifera and Upper Jurassic Allgau Formations indicate that gammacerane is derived from bacterivorous ciliates which were partially feeding on green sulphur bacteria. This demonstrates that anaerobic ciliates living at or below the chemocline are important sources for gammacerane, consistent with the fact that ciliates only biosynthesize tetrahymanol if their diet is deprived of sterols. This leads to the conclusion that gammacerane is an indicator for water column stratification, which solves two current enigmas in gammacerane geochemistry. Firstly, it explains why gammacerane is often found in sediments deposited under hypersaline conditions but is not necessarily restricted to this type of deposits. Secondly, it explains why lacustrine deposits may contain abundant gammacerane since most lakes in the temperate climatic zones are stratified during summer.

NASA Discipline Exobiology↗

NASA NTRS1994

A comparison of iron extraction methods for the determination of degree of pyritisation and the recognition of iron-limited pyrite formation

Measurements of degree of pyritisation require an estimate of sediment iron which is capable of reaction with dissolved sulphide to form pyrite, either directly or indirectly via iron monosulphide precursors. Three dissolution techniques (buffered dithionite, cold 1 M HCl, boiling 12 M HCl) were examined for their capacity to extract iron from a variety of iron minerals, and iron-bearing sediments, as a function of different extraction times and different grain sizes. All the iron oxides studied are quantitatively extracted by dithionite and boiling HCl (but not by cold HCl). Both HCl techniques extract more iron from silicates than does dithionite but probably about the same amounts as are potentially capable of sulphidation. Modern sediment studies indicate that most sedimentary pyrite is formed rapidly from iron oxides, with smaller amounts formed more slowly from iron silicates (if sufficient geologic time is available). It is therefore recommended that the degree of pyritisation be defined with respect to the dithionite-extractable (mainly iron oxide) pool and/or the boiling HCl-extractable pool (which includes some silicate iron) for the recognition of iron-limited pyritisation.

NASA Discipline Exobiology↗

NASA NTRS1995

Paleolakes on Mars

Observational evidence such as outflow channels and valley networks suggest that in the past there was flowing water on Mars. The images of fluvial features on Mars logically suggest that there must exist downstream locations in which the water pooled and the sediment load deposited (i.e. lakes). Sediments and morphological features associated with the martian paleolakes are believed to occur in Valles Marineris, and several large basins including Amazonis, Chryse and Elysium planitia. As Mars became progressively colder over geological time, any lakes on its surface would have become seasonally, and eventually perennially ice-covered. We know from polar lakes on Earth that ice-covered lakes can persist even when the mean annual temperature falls below freezing. Thus, the most recent lacustrine sediments on Mars were probably deposited in ice-covered lakes. While life outside of the Earth's atmosphere has yet to be observed, there is a general consensus among exobiologists that the search for extraterrestrial life should be based upon liquid water. The inference that there was liquid water on Mars during an earlier epoch is the primary motivation for considering the possibility of life during this time. It would be of enormous interest from both an exobiological and paleolimnological perspective to discover lakes or the evidence of former lakes on another planet such as Mars. Limnology would then become an interplanetary science.

NASA Center ARC↗

NASA NTRS1989

Gravitropism in a starchless mutant of Arabidopsis: implications for the starch-statolith theory of gravity sensing

The starch-statolith theory of gravity reception has been tested with a mutant of Arabidopsis thaliana (L.) Heynh. which, lacking plastid phosphoglucomutase (EC 2.7.5.1) activity, does not synthesize starch. The hypocotyls and seedling roots of the mutant were examined by light and electron microscopy to confirm that they did not contain starch. In upright wild-type (WT) seedlings, starch-filled plastids in the starch sheath of the hypocotyl and in three of the five columellar layers of the root cap were piled on the cell floors, and sedimented to the ceilings when the plants were inverted. However, starchless plastids of the mutant were not significantly sedimented in these cells in either upright or inverted seedlings. Gravitropism of light-grown seedling roots was vigorous: e.g., 10 degrees curvature developed in mutants rotated on a clinostat following a 5 min induction at 1 g, compared with 14 degrees in the WT. Curvatures induced during intervals from 2.5 to 30 min were 70% as great in the mutant as the WT. Thus under these conditions the presence of starch and the sedimentation of plastids are unnecessary for reception of gravity by Arabidopsis roots. Gravitropism by hypocotyls of light-grown seedlings was less vigorous than that by roots, but the mutant hypocotyls exhibited an average of 70-80% as much curvature as the WT. Roots and hypocotyls of etiolated seedlings and flower stalks of mature plants were also gravitropic, although in these cases the mutant was generally less closely comparable to the WT. Thus, starch is also unnecessary for gravity reception in these tissues.

NASA Program Space Biology↗

NASA NTRS1987

The structure of the stem endodermis in etiolated pea seedlings

Differentiation of the endodermis was examined in third internodes of etiolated Pisum sativum L. cv. Alaska seedlings. The endodermis in young internodes contains large, sedimented amyloplasts; in older internodes, a casparian strip differentiates and the endodermis becomes depleted of starch except for the proximal region of the stem, which retains sedimented amyloplasts and remains graviresponsive. Sedimentation occurs in the hook but does not occur consistently until cells reach the base of the hook, where the axis becomes vertical, rapid cell elongation starts, and amyloplast diameter increases substantially. Contact between endoplasmic reticulum and amyloplasts was observed. Endoplasmic reticulum is not distributed polarly with respect to gravity. No symplastic or apoplastic blockages exist in the endodermis at the level of the stem where lateral gradients may be established during tropic curvature.

Non-NASA Center↗

NASA NTRS1994

Evidence for only minor contributions from bacteria to sedimentary organic carbon

Because their molecular signatures are often prominent in extracts of sediments, bacteria are thought to be important contributors to petroleum source beds. It has been shown recently, however, that abundances of biomarkers do not always reflect relative contributions to sedimentary organic carbon (Corg). The contribution of photosynthetic green sulphur bacteria to sediments can be assessed effectively because the diagenetic products of distinctive carotenoids from these organisms occur widely and their biomass is isotopically labelled, being enriched in 13C. We show here that, although sediments and oils from the Western Canada and Williston basins contain prominent biomarkers of photosynthetic bacteria, the absence of 13C enrichment in the total Corg requires that the bacterial contribution is in fact minimal. Although the importance of bacterial reworking of sedimentary debris cannot be doubted, we argue that our findings, when considered in conjunction with those from other settings, suggest that bacterial biomass may commonly represent only a minor component of total Corg in carbonaceous rocks.

NASA Discipline Exobiology↗

NASA NTRS1993

Stable isotopic biogeochemistry of carbon and nitrogen in a perennially ice-covered Antarctic lake

Lake Hoare (77 degrees 38' S, 162 degrees 53' E) is an amictic, oligotrophic, 34-m-deep, closed-basin lake in Taylor Valley, Antarctica. Its perennial ice cover minimizes wind-generated currents and reduces light penetration, as well as restricts sediment deposition into the lake and the exchange of atmospheric gases between the water column and the atmosphere. The biological community of Lake Hoare consists solely of microorganisms -- both planktonic populations and benthic microbial mats. Lake Hoare is one of several perennially ice-covered lakes in the McMurdo Dry Valleys that represent the end-member conditions of cold desert and saline lakes. The dry valley lakes provide a unique opportunity to examine lacustrine processes that operate at all latitudes, but under an extreme set of environmental conditions. The dry valley lakes may also offer a valuable record of catchment and global changes in the past and present. Furthermore, these lakes are modern-day equivalents of periglacial lakes that are likely to have been common during periods of glacial maxima at temperate latitudes. We have analyzed the dissolved inorganic carbon (DIC) of Lake Hoare for delta 13C and the organic matter of the sediments and sediment-trap material for delta 13C and delta 15N. The delta 13C of the DIC indicates that 12C is differentially removed in the shallow, oxic portions of the lake via photosynthesis. In the anoxic portions of the lake (27-34 m) a net addition of 12C to the DIC pool occurs via organic matter decomposition. The dissolution of CaCO3 at depth also contributes to the DIC pool. Except near the Canada Glacier where a substantial amount of allochthonous organic matter enters the lake, the organic carbon being deposited on the lake bottom at different sites is isotopically similar, suggesting an autochthonous source for the organic carbon. Preliminary inorganic carbon flux calculations suggest that a high percentage of the organic carbon fixed in the water column is remineralized as it falls through the water column. At nearby Lake Fryxell, the substantial (relative to Lake Hoare) glacial meltstream input overprints Fryxell's shallow-water biological delta 13C signal with delta 13C-depleted DIC. In contrast, Lake Hoare is not significantly affected by surface-water input and mixing, and therefore the delta 13C patterns observed arise primarily from biological dynamics within the lake. Organic matter in Lake Hoare is depleted in 15N, which we suggest is partially the result of the addition of relatively light inorganic nitrogen into the lake system from terrestrial sources.

NASA Program Exobiology↗

NASA NTRS1994

Tectonic control of the crustal organic carbon reservoir during the Precambrian

Carbon isotopic trends indicate that the crustal reservoir of reduced, organic carbon increased during the Proterozoic, particularly during periods of widespread continental rifting and orogeny. No long-term trends are apparent in the concentration of organic carbon in shales, cherts and carbonates. The age distribution of 261 sample site localities sampled for well-preserved sedimentary rocks revealed a 500-700-Ma periodicity which coincided with tectonic cycles. It is assumed that the numbers of sites are a proxy for mass of sediments. A substantial increase in the number of sites in the late Archean correlates with the first appearance between 2.9 and 2.5 Ga of extensive continental platforms and their associated sedimentation. It is proposed that the size of the Proterozoic crustal organic carbon reservoir has been modulated by tectonic control of the volume of sediments deposited in environments favorable for the burial and preservation of organic matter. Stepwise increases in this reservoir would have caused the oxidation state of the Proterozoic environment to increase in a stepwise fashion.

NASA Center ARC↗

NASA NTRS1985

Microbial biomass and productivity in seagrass beds

Different methods for measuring the rates of processes mediated by bacteria in sediments and the rates of bacterial cell production have been compared. In addition, net production of the seagrass Zostera capricorni and bacterial production have been compared and some interrelationships with the nitrogen cycle discussed. Seagrass productivity was estimated by measuring the plastochrone interval using a leaf stapling technique. The average productivity over four seasons was 1.28 +/- 0.28 g C m-2 day-1 (mean +/- standard deviation, n = 4). Bacterial productivity was measured five times throughout a year using the rate of tritiated thymidine incorporated into DNA. Average values were 33 +/- 12 mg C m-2 day-1 for sediment and 23 +/- 4 for water column (n = 5). Spatial variability between samples was greater than seasonal variation for both seagrass productivity and bacterial productivity. On one occasion, bacterial productivity was measured using the rate of 32P incorporated into phospholipid. The values were comparable to those obtained with tritiated thymidine. The rate of sulfate reduction was 10 mmol SO4(-2) m-2 day-1. The rate of methanogenesis was low, being 5.6 mg CH4 produced m-2 day-1. A comparison of C flux measured using rates of sulfate reduction and DNA synthesis indicated that anaerobic processes were predominant in these sediments. An analysis of microbial biomass and community structure, using techniques of phospholipid analysis, showed that bacteria were predominant members of the microbial biomass and that of these, strictly anaerobic bacteria were the main components. Ammonia concentration in interstitial water varied from 23 to 71 micromoles. Estimates of the amount of ammonia required by seagrass showed that the ammonia would turn over about once per day. Rapid recycling of nitrogen by bacteria and bacterial grazers is probably important.

NASA Program CELSS↗

NASA NTRS1994

Gravity-regulated formation of the peg in developing cucumber seedlings

It has been proposed that peg formation in the vascular transition region (TR zone) between the hypocotyl and the root in Cucurbitaceae seedlings is a gravimorphogenetic phenomenon. Initiation of the peg became visible 36 h after imbibition when cucumber (Cucumis sativus L. cv. Burpee Hybrid II) seeds were germinated in a horizontal position at 24 degrees C in the dark. Simultaneously, sedimented amyloplasts (putative statoliths) were apparent in the sheath cells surrounding the vascular strands, and in the cortical cells immediately adjacent to them, in the TR zone. In contrast, the other cortical cells, some of which were destined to develop into the peg, contained amyloplasts which were not sedimented. These results suggest that the graviperception mechanism for peg formation may be like that of statoliths in shoot gravitropism. By 48 h following imbibition, the cells of the TR zone still had sedimented amyloplasts but had lost their sensitivity to gravity, possibly because of their maturation.

NASA Program Space Biology↗

NASA NTRS1990

Activity and adaptation of nitrilotriacetate (NTA)-degrading bacteria: field and laboratory studies

Adaptation of bacterial activity for the degradation of nitrilotriacetate (NTA) was studied using natural sediment samples and an NTA-degrading bacterium (strain ATCC 29600). Sediment samples from a river with persistent levels of NTA had much higher NTA-degradative activity than comparable samples from a less contaminated control site. When sediment from the control site was exposed to high levels of NTA a 5 day lag preceded an abrupt increase in NTA degradation while strain 29600 colonized on sand and grown in the absence of NTA became induced within eight hours. The induction of strain 29600 was compared between bacteria in suspension and cells attached to sand. The sand-associated bacteria became induced 4 to 5 h before the planktonic suspension and displayed over threefold greater specific activity. Suspensions of strain 29600 became adapted within 8 h when placed in membrane diffusion chambers that were immersed within a municipal wastewater reactor containing NTA. These findings support the concept that induction is a part of the process of bacterial adaptation to degrade NTA and sand-associated bacteria can adapt more quickly to and have a greater degradative activity for NTA than planktonic cells.

NASA Discipline Number 04-10↗

NASA NTRS2003

Molecular characterization of sulfate-reducing bacteria in the Guaymas Basin

The Guaymas Basin (Gulf of California) is a hydrothermal vent site where thermal alteration of deposited planktonic and terrestrial organic matter forms petroliferous material which supports diverse sulfate-reducing bacteria. We explored the phylogenetic and functional diversity of the sulfate-reducing bacteria by characterizing PCR-amplified dissimilatory sulfite reductase (dsrAB) and 16S rRNA genes from the upper 4 cm of the Guaymas sediment. The dsrAB sequences revealed that there was a major clade closely related to the acetate-oxidizing delta-proteobacterial genus Desulfobacter and a clade of novel, deeply branching dsr sequences related to environmental dsr sequences from marine sediments in Aarhus Bay and Kysing Fjord (Denmark). Other dsr clones were affiliated with gram-positive thermophilic sulfate reducers (genus Desulfotomaculum) and the delta-proteobacterial species Desulforhabdus amnigena and Thermodesulforhabdus norvegica. Phylogenetic analysis of 16S rRNAs from the same environmental samples resulted in identification of four clones affiliated with Desulfobacterium niacini, a member of the acetate-oxidizing, nutritionally versatile genus Desulfobacterium, and one clone related to Desulfobacula toluolica and Desulfotignum balticum. Other bacterial 16S rRNA bacterial phylotypes were represented by non-sulfate reducers and uncultured lineages with unknown physiology, like OP9, OP8, as well as a group with no clear affiliation. In summary, analyses of both 16S rRNA and dsrAB clone libraries resulted in identification of members of the Desulfobacteriales in the Guaymas sediments. In addition, the dsrAB sequencing approach revealed a novel group of sulfate-reducing prokaryotes that could not be identified by 16S rRNA sequencing.

Non-programmatic↗

NASA NTRS1999

Phototrophs in high-iron-concentration microbial mats: physiological ecology of phototrophs in an iron-depositing hot spring

At Chocolate Pots Hot Springs in Yellowstone National Park the source waters have a pH near neutral, contain high concentrations of reduced iron, and lack sulfide. An iron formation that is associated with cyanobacterial mats is actively deposited. The uptake of [(14)C]bicarbonate was used to assess the impact of ferrous iron on photosynthesis in this environment. Photoautotrophy in some of the mats was stimulated by ferrous iron (1.0 mM). Microelectrodes were used to determine the impact of photosynthetic activity on the oxygen content and the pH in the mat and sediment microenvironments. Photosynthesis increased the oxygen concentration to 200% of air saturation levels in the top millimeter of the mats. The oxygen concentration decreased with depth and in the dark. Light-dependent increases in pH were observed. The penetration of light in the mats and in the sediments was determined. Visible radiation was rapidly attenuated in the top 2 mm of the iron-rich mats. Near-infrared radiation penetrated deeper. Iron was totally oxidized in the top few millimeters, but reduced iron was detected at greater depths. By increasing the pH and the oxygen concentration in the surface sediments, the cyanobacteria could potentially increase the rate of iron oxidation in situ. This high-iron-content hot spring provides a suitable model for studying the interactions of microbial photosynthesis and iron deposition and the role of photosynthesis in microbial iron cycling. This model may help clarify the potential role of photosynthesis in the deposition of Precambrian banded iron formations.

NASA Discipline Exobiology↗

NASA NTRS1999

Dispersion of 0.5- to 2-micron aerosol in microG and hypergravity as a probe of convective inhomogeneity in the lung

We used aerosol boluses to study convective gas mixing in the lung of four healthy subjects on the ground (1 G) and during short periods of microgravity (microG) and hypergravity ( approximately 1. 6 G). Boluses of 0.5-, 1-, and 2-micron-diameter particles were inhaled at different points in an inspiration from residual volume to 1 liter above functional residual capacity. The volume of air inhaled after the bolus [the penetration volume (Vp)] ranged from 150 to 1,500 ml. Aerosol concentration and flow rate were continuously measured at the mouth. The dispersion, deposition, and position of the bolus in the expired gas were calculated from these data. For each particle size, both bolus dispersion and deposition increased with Vp and were gravity dependent, with the largest dispersion and deposition occurring for the largest G level. Whereas intrinsic particle motions (diffusion, sedimentation, inertia) did not influence dispersion at shallow depths, we found that sedimentation significantly affected dispersion in the distal part of the lung (Vp >500 ml). For 0.5-micron-diameter particles for which sedimentation velocity is low, the differences between dispersion in microG and 1 G likely reflect the differences in gravitational convective inhomogeneity of ventilation between microG and 1 G.

NASA Discipline Cardiopulmonary↗

NASA NTRS2004

Estimating Coastal Turbidity using MODIS 250 m Band Observations

Terra MODIS 250 m observations are being applied to a Suspended Sediment Concentration (SSC) algorithm that is under development for coastal case 2 waters where reflectance is dominated by sediment entrained in major fluvial outflows. An atmospheric correction based on MODIS observations in the 500 m resolution 1.6 and 2.1 micron bands is used to isolate the remote sensing reflectance in the MODIS 25Om resolution 650 and 865 nanometer bands. SSC estimates from remote sensing reflectance are based on accepted inherent optical properties of sediment types known to be prevalent in the U.S. Gulf of Mexico coastal zone. We present our findings for the Atchafalaya Bay region of the Louisiana Coast, in the form of processed imagery over the annual cycle. We also apply our algorithm to selected sites worldwide with a goal of extending the utility of our approach to the global direct broadcast community.

Davies, James E.↗

NASA NTRS2009

"The Waters of Meridiani" - Further Support for a Fluvial Interpretation of the Ridged, Layered Units

A relatively unknown terrestrial fluvial environment, the mesoscale megafan, provides analogs for various Martian landscapes, including the etched unit (etched unit, Unite E of Arvidson et al., 2003; ridge-forming unit R of Edgett, 2005) of the Sinus Meridiani region on Mars. A global survey of Earth shows that megafans are very large partial cones of dominantly fluvial sediment with radii on the order of hundreds of km, and very low slopes. Responsible fluvial processes are sufficiently different from those of classical arid alluvial fans and deltas that it is useful to class megafans as separate features. The megafan model calls into question two commonly held ideas. 1. Earth examples prove that topographic basins per se are unnecessary for the accumulation of large sedimentary bodies. 2. River channels are by no means restricted to valleys (Meridiani sediments are termed a "valley-ed volume" of Edgett). These perspectives reveal unexpected parallels with features at Meridiani-several channel-like features that are widespread, mostly as ridges inverted by eolian erosion; channel networks covering thousands of sq km, especially on intercrater plains; and regional relationships of sediment bodies situated immediately downstream of highland masses. These all suggest that fluvial explanations are at least part of the Meridiani story.

Wilkinson, Justin↗