Refractory Organic Material Research Articles

Influence of change in land use on the refractory organic macromolecular fraction of a sandy spodosol (Landes de Gascogne, France)

The refractory (insoluble and resistant to drastic laboratory hydrolyses) organic macromolecular material (ROM), isolated from a sandy spodosol from Cestas (Landes de Gascogne, France), was examined via a combination of isotopic ( δ 13C measurements), spectroscopic (FTIR and solid-state 13C NMR) and pyrolytic (conventional pyrolysis and thermally assisted hydrolysis and methylation) methods. This soil was collected from a plot cleared 22 years ago and since then continuously used for intensive maize cropping. The ROM of this cultivated soil was also compared to the ROM from the reference forest soil. It thus appeared that (i) the carbon of the ROM accounts for a substantial part (20%) of the total organic carbon of the cropped soil, (ii) the ROM shows a heterogeneous composition with contributions of altered lignin, polysaccharides, suberans, bacterial and higher plant fatty acids incorporated in the macromolecular structure, melanoidin-type components and suberin, (iii) extensive degradation of the ROM inherited from the forest soil (ca. two thirds) occurred upon cropping, a conspicuous uncoupling is thus observed between the resistance of this material to drastic laboratory hydrolyses (i.e. refractoriness) and its resistance to degradation under natural conditions, (iv) a low input of maize components took place in the ROM of the cropped soil so that, in spite of such a large degradation of the refractory carbon inherited from the forest soil, the latter is still predominant (ca. 85%) in this ROM, (v) differential degradation took place, upon cropping, in the ROM inherited from the forest soil: some components were eliminated (condensed alkyl lipids of higher plants, cellulosic units) or heavily degraded (other polysaccharides), a part of the lignin was retained but underwent side-chain oxidation, while increase in relative abundance was observed for suberin, melanoidins and suberans.

The molecular composition of comets and its interrelation with other small bodies of the Solar System

The present status of our knowledge of the composition of cometary nuclei is reviewed and compared with what we know on the composition of other Solar System minor bodies – interplanetary dust, meteorites, asteroids, trans-Neptunian objects. The current methods of investigations – by both in situ analysis and remote sensing – are described. Comets are active objects pouring their internal material to form a dusty atmosphere which can be investigated by remote sensing. This is not the case for minor planets and trans-Neptunian objects for which only the outer surface is accessible. Collected interplanetary dust particles and meteorites can be analysed at leisure in terrestrial laboratories, but we do not know for certain which are their parent bodies.Considerable progresses have been made from spectroscopic observations of active comets, mainly at infrared and radio wavelengths. We probably know now most of the main components of cometary ices, but we still have a very partial view of the minor ones. The elemental composition of cometary dust particles is known from in situ investigations, but their chemical nature is only known for species like silicates which have observable spectral features. A crucial component, still ill-characterized, is the (semi-)refractory organic material of high molecular mass present in grains. This component is possibly responsible for distributed sources of molecules in the coma. A large diversity of composition from comet to comet is observed, so that no “typical comet” can be defined. No clear correlation between the composition and the region of formation of the comets and their subsequent dynamical history can yet be established.

Study of the composition of the macromolecular refractory fraction from an acidic sandy forest soil (Landes de Gascogne, France) using chemical degradation and electron microscopy

The composition of the insoluble, non-hydrolysable, macromolecular fraction from an acidic sandy forest soil (Cestas, Landes de Gascogne, S.W. France) was examined via chemical degradation and high resolution transmission electron microscopy (HRTEM). Chemical degradation was performed using four reagents (sodium persulfate followed by potassium permanganate, sodium perborate and ruthenium tetroxide). Large differences in the level of degradation of this refractory organic material (ROM) and in the nature and relative abundance of the GC-amenable reaction products were observed between these reagents. Chemical degradation supported the presence of condensed tannins in the ROM, previously suggested from NMR data, and showed the contribution of lipid-derived moieties condensed to the macromolecular refractory material. The contribution of black carbon was revealed with ruthenium tetroxide oxidation and by HRTEM observations that showed the presence of charcoal particles probably related to fire events. All the oxidation treatments point to the lack of a significant contribution of bacteria-derived moieties to the Cestas ROM. The different oxidants revealed different components of the ROM. Thus, the occurrence of suberin was better indicated by the perborate treatment, the contribution of moieties originating from the oxidative cross-linking of unsaturated acids by alkaline permanganate and RuO 4, and the presence of condensed tannins and black carbon was only revealed by the RuO 4 treatment.

Overtones of silicate and aluminate minerals and the 5-8 μm ice bands of deeply embedded objects

The distinct patterns, relatively low intensities and peak positions of overtone-combination bands of silicates and oxides suggest that the 5–8 μm spectral region can provide clues for the dust composition when near optically thick conditions exist for the 10-μm silicate feature. We present 1000–2500 cm−1 room-temperature laboratory spectra obtained from powders of silicate, aluminate and nitride minerals and silicate glasses. The spectra exhibit overtone absorption bands with mass absorption coefficients ∼100 times weaker than the fundamentals. These data are compared with the 5–8 μm spectra of deeply embedded young stellar objects observed with the Short Wavelength Spectrometer on the Infrared Space Observatory. Fits of the laboratory data to the observations, after subtraction of the 6.0-μm H2O ice feature and the 6.0-μm feature identified with organic refractory material, indicate that crystalline melilite (a silicate) or metamict hibonite (a radiation-damaged crystalline aluminate) may be responsible for much of the 6.9-μm absorption feature in the observations, with melilite providing the best match. A weaker 6.2-μm absorption in the young stellar object spectra is well matched by the spectra of hydrous crystalline amphibole silicates (actinolite and tremolite). Relative abundances of Si–O in room-temperature amphiboles to low-temperature H2O ice are in the range 0.46–3.9 and in melilite are in the range 2.5–8.6. No astronomical feature was matched by the overtones of amorphous silicates because these bands are too broad and peak at the wrong wavelength. Hence, this analysis is consistent with the 10-μm features of these objects being due to a mixture of crystalline and amorphous silicates, rather than only amorphous silicates.

Dust Destruction and Ion Formation in the Inner Solar System

We suggest that dust-dust collisions feed ions into the interplanetary medium and produce a significant amount of the heavy inner-source pickup ions detected by the Solar Wind Ion Composition Spectrometer aboard the Ulysses spacecraft. Organic refractory material contained in the cometary dust could explain the existence of carbon among the measured pickup ions. Impact-produced ions may locally influence the plasma parameters of the solar wind. It will be possible to study in situ these dust-plasma interactions and the central region of meteoroid activity through measurements in the inner solar system proposed for the currently discussed Solar Probe and Solar Orbiter missions.

Interstellar Dust Models Consistent with Extinction, Emission, and Abundance Constraints

We present new interstellar dust models which have been derived by simultaneously fitting the far-ultraviolet to near-infrared extinction, the diffuse infrared (IR) emission and, unlike previous models, the elemental abundance constraints on the dust for different interstellar medium abundances, including solar, F and G star, and B star abundances. The fitting problem is a typical ill-posed inversion problem, in which the grain size distribution is the unknown, which we solve by using the method of regularization. The dust model contains various components: PAHs, bare silicate, graphite, and amorphous carbon particles, as well as composite particles containing silicate, organic refractory material, water ice, and voids. The optical properties of these components were calculated using physical optical constants. As a special case, we reproduce the Li & Draine (2001) results, however their model requires an excessive amount of silicon, magnesium, and iron to be locked up in dust: about 50 ppm (atoms per million of H atoms), significantly more than the upper limit imposed by solar abundances of these elements, about 34, 35, and 28 ppm, respectively. A major conclusion of this paper is that there is no unique interstellar dust model that simultaneously fits the observed extinction, diffuse IR emission, and abundances constraints.

Origin of cometary extended sources from degradation of refractory organics on grains: polyoxymethylene as formaldehyde parent molecule

The radial distribution of some molecules (CO, H 2CO, HNC, …) observed in the coma of some comets cannot be explained only by a direct sublimation from the nucleus, or by the photolysis of a detected parent compound. Such molecules present a so-called extended source in comae. We show in this paper that extended sources can be explained by refractory organic material slowly releasing gas from grains ejected from the cometary nucleus, due to solar UV photons or heat. The degradation products are produced throughout the coma and therefore are presenting an extended distribution. To model this multiphase chemistry we derive new equations, which are applied to Comet 1P/Halley for the case of the production of formaldehyde from polyoxymethylene (POM), the polymer of formaldehyde (–CH 2–O–) n . We show that the presence of a few percent of POM on cometary grains (a nominal value of ∼4% in mass of grains is derived from our calculations) is in good agreement with the observed distribution, which so far were not interpreted by the presence of any gaseous parent molecule.

UV-photoprocessing of interstellar ice analogs: New infrared spectroscopic results

We simulate experimentally the physical conditions present in dense clouds by means of a high vacuum experimental setup at low temperature T ≈ 12 K. The accretion and photoprocessing of ices on grain surfaces is simulated in the following way: an ice layer with composition analogous to that of interstellar ices is deposited on a substrate window, while being irradiated by ultraviolet (UV) photons. Subsequently the sample is slowly warmed up to room temperature; a residue remains containing the most refractory products of photo- and thermal processing. In this paper we report on the Fourier transform-infrared (FT-IR) spectroscopy of the refractory organic material formed under a wide variety of initial conditions (ice composition, UV spectrum, UV dose and sample temperature). The refractory products obtained in these experiments are identified and the corresponding efficiencies of formation are given. The first evidence for carboxylic acid salts as part of the refractory products is shown. The features in the IR spectrum of the refractory material are attributed to hexamethylenetetramine (HMT, [ (CH2)6N4] ), ammonium salts of carboxylic acids [ (R–COO-)(NH+_4)] , amides [ H2NC(=O)–R] , esters [ R–C(=O)–O–R'] and species related to polyoxymethylene (POM, [ (–CH2O–)n] ). Furthermore, evidence is presented for the formation of HMT at room temperature, and the important role of H2O ice as a catalyst for the formation of complex organic molecules. These species might also be present in the interstellar medium (ISM) and form part of comets. Ongoing and future cometary missions, such as Stardust and Rosetta, will allow a comparison with the laboratory results, providing new insight into the physico-chemical conditions present during the formation of our solar system.

Alkaline subcritical-water treatment and alkaline heat treatment for the increase in biodegradability of newsprint waste

This work describes two alkaline semicontinuous processes for the conversion of refractory organic materials into biodegradable substances. Newsprint was used as a lignocellulosic waste. Methane conversion efficiencies and cellulose removals were investigated for the two following processes: alkaline subcritical-water treatment (ASWT) coupled with methane fermentation and alkaline heat treatment (newsprint heated with steam in an autoclave; AHT) coupled with methane fermentation with a neutral subcritical-water treatment (NSWT) recycle. Results showed that for ASWT coupled with methane fermentation higher methane conversion efficiencies and higher cellulose removals were achieved as HRT increased. At HRT = 20 days, average CH4 conversion efficiency and average cellulose removal reached 26% and 44%, respectively. After a final HRT of 40 days, average CH4 conversion efficiency and average cellulose removal reached 50% and 60%, respectively. On the other hand, for AHT coupled with methane fermentation, methane conversion efficiencies did not show a greater improvement using this pretreatment process. Average conversion reached 9% with an average cellulose removal of 20%. In order to improve the yield of the reactor, approximately one-third of the effluent was recycled using NSWT (150 degrees C; neutral pH). Methane conversion efficiency of this process increased as more recycles were performed. For the fifth operation, the total average methane conversion efficiency was 44% with a total average cellulose removal of 55%.

Phosphorus dynamics during decomposition of mangrove ( Rhizophora apiculata) leaves in sediments

Rhizophora apiculata leaf litter decomposition and the influence of this process on phosphorus (P) dynamics were studied in mangrove and sand flat sediments at the Bangrong mangrove forest, Phuket, Thailand. The remaining P in the mangrove leaf litter increased with time of decomposition to 174% and 220% of the initial amount in the litter in sand flat and mangrove sediment, respectively, although about 50% of the dry weight had been lost. The incorporation of P into the litter was probably associated with humic acids and metal bridging, especially caused by iron (Fe), which also accumulated in considerable amounts in the litter (5–10 times initial concentration). The addition of leaves to the sediment caused increased concentrations of dissolved reactive phosphate (DRP) in the porewater, especially in sand flat sediment. The DRP probably originated from Fe-bound P in the sediment, because decomposition of buried leaf litter caused increased respiration and reduced the redox potential ( E h) in the sediments. Binding of P to refractory organic material and oxidized Fe at the sediment–water interface explains the low release of DRP from the sediment. This mechanism also explains the generally low DRP concentration in the mangrove porewater, the low nutrient content of the R. apiculata leaves, but also the higher total sediment P concentration of the mangrove sediment as compared to sediments outside the mangrove. Both the low release rates for DRP from the sediment and the accumulation of P associated with leaf litter decomposition tend to preserve P in the sediments.

Benthic sulfate reduction along the Chesapeake Bay central channel. II. Temporal controls

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 260:55-70 (2003) - doi:10.3354/meps260055 Benthic sulfate reduction along the Chesapeake Bay central channel. II. Temporal controls M. C. Marvin-DiPasquale1,*, W. R. Boynton2, D. G. Capone3 1Water Resource Division, US Geological Survey, MS 480, 345 Middlefield Road, Menlo Park, California 94025, USA 2Chesapeake Biological Laboratory, Center for Environmental Science, University of Maryland, 1 Williams St., Solomons, Maryland 20688, USA 3Department of Biological Sciences, University of Southern California, AHF107, Los Angeles, California 90089-0371, USA *Email: mmarvin@usgs.gov ABSTRACT: Seasonal and interannual controls of benthic sulfate reduction (SR) were examined at 3 sites (upper [UB], mid- [MB] and lower [LB] bay) along the Chesapeake Bay central channel, from early spring through fall, for 6 yr (1989 to 1994). The combined influences of temperature, sulfate, organic loading and bioturbation affected seasonal SR rates differently in the 3 regions. Consistently low SR rates at UB resulted from low overlying-water sulfate concentrations and the dominance of refractory organic terrestrial material. Combined seasonal variation in temperature and sulfate accounted for 50% of the annual variability in 0 to 2 cm depth interval SR rates, while sediment organic content had no significant seasonal influence. In contrast, MB and LB sites had high rates of SR fostered by high levels of overlying water SO42- and organic input dominated by labile phytoplankton detritus. New organic loading (measured as chl a) stimulated 0 to 2 cm SR during spring at both sites. Combined organic quantity (as particulate C and/or N) and temperature accounted for >75% of the variability in 0 to 2 cm SR at MB during spring and fall. Molecular diffusion supplied 25 to 45% of the SO42- needed to fuel 0 to 12 cm depth interval SR at MB, with the balance presumably supplied by S-recycling. Interannual differences in summertime SR rates were linked to the extent of freshwater flow during spring, with high-flow years associated with high SR rates at UB and MB, and low rates at LB. The negative trend between benthic SR and river flow at LB may result from the up-estuary transport of senescing organic matter in bottom water, which increases in the lower reach of the estuary with increasing freshwater inflow. KEY WORDS: Sulfate reduction · Benthic metabolism · Sediment · Estuary · Chesapeake Bay Full text in pdf format PreviousNextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 260. Online publication date: September 30, 2003 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2003 Inter-Research.

Assessment of terrigenous organic carbon input to the total organic carbon in sediments from Scottish transitional waters (sea lochs): methodology and preliminary results

Abstract. This paper addresses the assessment of terrestrially derived organic carbon in sediments from two Scottish sea lochs. The results illustrate a smooth decrease in area-specific sediment oxygen uptake rates along a transect of six stations from the head of Loch Creran to the sea, from 18.7 mmol O2m-2d-1 to 6.6 mmol O2m-2d-1. Measurement of the losses on ignition at two temperatures (250°C and 500°C) of the sediment fraction from 1–2 cm depth at the same stations enabled the proportion of weight loss that occurred over the high temperature range to be calculated. These show a smooth increase from 0.33 to 0.62. These observations indicate that (a) the amount of easily biodegradable organic material in the sediment decreases by two-thirds along the transect and (b) the proportion of refractory organic material in the sediment increases along the same transect. This suggests strongly that terrigenous organic material, brought down by the River Creran is a very important fuel for sediment diagenetic processes in this system. Preliminary analyses of the lignin composition of the same sediments indicate the predominance of non-woody gymnosperm tissue. Lignin is used as a proxy for terrigenous allochthonous material. Comparative data for Loch Etive are also presented. Keywords: sedimentary organic carbon, Loch Creran, Loch Etive, oxygen uptake, lignin analysis

Water‐soluble organic compounds in biomass burning aerosols over Amazonia 2. Apportionment of the chemical composition and importance of the polyacidic fraction

Chemical characterization was performed on carbonaceous aerosols from Rondônia in the Brazilian Amazon region as part of the European contribution to the Large‐Scale Biosphere‐Atmosphere Experiment in Amazonia (LBA‐EUSTACH). The sampling period (October 1999) included the peak of the burning season as well as the dry‐to‐wet season transition. Characterization of the carbonaceous material was performed by using a thermal combustion method. This enabled determination of aerosol total carbon (TC), black carbon (BC), and organic carbon (OC). A significant fraction of the BC material (on average about 50%) seemed to be highly refractory organic material soluble in water. A more detailed analysis of the water‐soluble organic carbon (WSOC) fraction of the TC was undertaken, involving measurements of WSOC content, high‐performance liquid chromatography (HPLC) separation (with UV detection) of the water‐soluble components, and characterization of individual components by gas chromatography/mass spectrometry (GC/MS). The WSOC fraction accounted for 45−75% of the OC. This high WSOC fraction suggests an aerosol derived mainly from smoldering combustion. Using GC/MS, many different compounds, containing hydroxy, carboxylate, and carbonyl groups, were detected. The fraction of the WSOC identified by GC/MS was about 10%. Three classes of compounds were separated by HPLC/UV: neutral compounds (N), monocarboxylic and dicarboxylic acids (MDA), and polycarboxylic acids (PA). The sum of these three groups accounted for about 70% of the WSOC, with MDA and PA being most abundant (about 50%). Good correlations (r2between 0.84 and 0.99) of BCwater(BC after water extraction) and levoglucosan (both indicators of biomass combustion) with the water‐soluble species (i.e., WSOC, N, MDA, and PA), and their increase in concentrations during the burning period provided strong evidence that biomass burning is a major source of the WSOC. Particularly interesting is that PA and therefore, probably, humic‐like substances (due to their polyacidic nature) are generated in significant amounts during biomass burning. These substances, due to their water solubility and surface tension‐lowering effects, may play an important role in determining the overall cloud condensation nuclei activity of biomass burning aerosols and, consequently, could be important in cloud processes and climate forcing.

The Organic Refractory Material in the Diffuse Interstellar Medium: Mid‐Infrared Spectroscopic Constraints

This is an analysis of the 4000¨1000 cm~1 (2.5¨10 km) region of the spectrum of diUuse interstellar medium (DISM) dust compared with the spectra of 13 materials produced in the laboratory which serve as analogs to the interstellar material. The organic signatures of extragalactic dust, carbonaceous chondritic material, and E. coli bacteria are also presented because these have been discussed in the literature as relevant to the diUuse interstellar medium. Spectral analysis of the DISM allows us to place signi—cant constraints on the applicability of proposed candidate materials. The spectra of candidate materials are evaluated using four spectral characteristics based on the interstellar data: (i) comparisons of the pro—le and subpeak positions of the 2940 cm~1 (3.4 km) aliphatic CH stretching-mode band, (ii) the ratio of the optical depth (O.D.) of the aliphatic CH stretch to the O.D. of the OH stretch near 3200 cm~1 (3.1 km), (iii) the ratio of the O.D. of the aliphatic CH stretch to the O.D. of the carbonyl band near 1700 cm~1 (5.9 km), and (iv) the ratio of the O.D. of the aliphatic CH stretch feature to the O.D. of the CH deformation modes near 1470 cm~1 (6.8 km) and 1370 cm~1 (7.25 km). We conclude that the organic refractory material in the diUuse interstellar medium is predominantly hydrocarbon in nature, possessing little nitrogen or oxygen, with the carbon distributed between the aromatic and aliphatic forms. Long alkane chains with n much greater than 4 or 5 are not major constituents H 3 Cw(CH 2 ) n w of this material. Comparisons to laboratory analogs indicate the DISM organic material resembles plasma processed pure hydrocarbon residues much more so than energetically processed ice residues. This result is consistent with a birth site for the carrier of the 3.4 km band in the out—ow region of evolved carbon stars. The organic material extracted from the Murchison carbonaceous meteorite and the spectrum of E. coli bacteria reveal spectral features in the 5¨10 km region that are absent in the DISM. Although the presence of unaltered circumstellar components in the Murchison meteorite has been established through several lines of evidence, it is unclear whether or not the aliphatic component which gives rise to the 3.4 km band is in that category. Considering the complete 2¨10 km wavelength region, there is no spectral evidence for a biological origin of the 3.4 km interstellar absorption band. The similarity of the aliphatic CH stretch region of dust from our own Galaxy compared with that of distant galaxies suggests that the organic component of the ISM is widespread and may be an important universal reservoir of prebiotic organic carbon. )

Effect of addition of Rhodobacter sp. to activated-sludge reactors treating piggery wastewater

Under aerobic conditions, the decay rates of purple nonsulfur bacteria ( Rhodobacter sp.) in the light and dark follow first-order kinetics with rate constants of 0.22 and 0.32 day −1, respectively. The performance of the conventional activated-sludge reactor (CASR) treating anaerobically pretreated piggery wastewater (656–1,110 mg chemical oxygen demand, COD/L) can be enhanced by the addition of Rhodobacter sp. By performing regressive and statistical analyses using the proposed model and experimental data, the kinetic constants k and Y T , and the fraction of refractory organic materials ( f) of the Rhodobacter sp.-supplemented activated-sludge reactor (RASR) are 40% larger, 21% less, and 34% less than those of the CASR, respectively. From parametric sensitivity analyses, the substrate removal efficiencies of the CASR and RASR are most sensitive to the parameters k and the food to microorganisms ratio (F/M) but least sensitive to the parameter f; the specific oxygen utilization rates of the CASR and RASR are most sensitive to the parameters a and k but least sensitive to the parameter b.

Evaluation of refractory organic removal in combined biological powdered activated carbon - microfiltration for advanced wastewater treatment

Biological powdered activated carbon (BPAC) was incorporated with a microfiltration (MF, 0.2 micron pore size) system to remove the refractory organic matter contained in secondary sewage effluent. A synthetic secondary sewage effluent was used as influent in this study, containing both non-biodegradable organic substances (such as humic acid, lignin sulfonate, tannic acid and arabic gum powder) and biodegradable ones. These refractory organic materials were possibly degraded in contact with microorganisms for 20-27 days. Although humic acid and arabic gum were weakly adsorbed on the activated carbon, they could be effectively removed in the BPAC reactor. The TOC removal at a powdered activated carbon (PAC) concentration of 20 g/L was higher than at 0.5-2 g PAC/L (83% and 66-68%, respectively). The higher removal efficiency was due to the increased rejection at the membrane module in which most of the PAC was accumulated. More than 90% of non-biodegradable compounds removal (detected as E280, UV absorption at 280 nm) occurred in the BPAC reactor. The biological growth parameter b/Y, used in system design, was estimated to be 0.017 d-1. Relatively high permeate flux of 1.88 m/d could be obtained even at higher PAC concentration of 20 g/L.

Polarimetry of young stellar objects -- III. Circular polarimetry of OMC-1

We present the first imaging circular polarimetry of the Orion Molecular Cloud, OMC-1. The observations, taken in the J, H, Kn and nbL bands, reveal a complex pattern of circular polarization. Globally, there is a background circular polarization of the order of ±2 per cent in the Kn band, conforming to the typical quadrupolar patterns that have been observed in other outflow sources. Overlying this pattern are regions of relatively high degrees of circular polarization to the east and west of the source IRc2, with degrees as high as +17 per cent in the Kn band, the highest circular polarization yet measured for any young stellar object. No circular polarization is seen in the J band, indicating that the circular polarization detected at longer wavelengths originates from within OMC-1 and not from scattering off the foreground ionization front associated with the M42 nebula. We demonstrate a correlation between these patches of high circular polarization and regions of enhanced linear polarization, and argue that these observations are best explained using a model that incorporates scattering of radiation off oblate grains, which have been aligned by the local magnetic field. Modelling of the ellipticity (the ratio of circular to linear polarization) suggests that the grains are composed of silicate and/or organic refractory material, and that grains larger than are typically found in the interstellar medium are needed. The lower, background, circular polarization is produced by scattering off randomly oriented grains in the outflow cavities, the grain alignment being destroyed by the passage of shocks. We put forward a morphological model for OMC-1 which has the regions of high polarization separate from, but near to, the main outflow region. Those regions exhibiting high polarization must somehow have a direct view of the illuminating source of the nebula. Implications of this work to the origins of life are briefly discussed.

Deflection of the local interstellar dust flow by solar radiation pressure.

Interstellar dust grains intercepted by the dust detectors on the Ulysses and Galileo spacecrafts at heliocentric distances from 2 to 4 astronomical units show a deficit of grains with masses from 1 x 10(-17) to 3 x 10(-16) kilograms relative to grains intercepted outside 4 astronomical units. To divert grains out of the 2- to 4-astronomical unit region, the solar radiation pressure must be 1.4 to 1.8 times the force of solar gravity. These figures are consistent with the optical properties of spherical or elongated grains that consist of astronomical silicates or organic refractory material. Pure graphite grains with diameters of 0.2 to 0.4 micrometer experience a solar radiation pressure force as much as twice the force of solar gravity.

Effects of chronic metal exposure and sediment organic matter on digestive absorption efficiency of cadmium by the deposit‐feeding polychaete Capitella species I

AbstractOrganic matter such as humic acid and bacterial slime exopolymer are common in estuarine and coastal sediments, where they are ingested by animals that process particulate detritus. Both humic acid (HA; refractory) and exopolymer (EPS; easily digestible) bind metals and therefore might represent a source of particulate‐bound metals to deposit‐feeding organisms. This study examined how cadmium preexposure (3 and 30 μg Cd/g dry wt. sed.), gut passage time (GPT), and quality and quantity of the organic coating on sediment particles interact to determine cadmium absorption efficiency (Cd‐AE) in Capitella sp. I. Pulse‐chase experiments using 109Cd and 51Cr were used to determine Cd‐AE in individual worms. Worms were given a pulse of carbon‐cleaned, HA‐coated or EPS‐coated sediment particles. The third treatment was divided into three EPS concentrations (high, medium, and low). A 5‐d preexposure to cadmium did not affect the egestion rates during either the preexposure period or the chase phase. Worms given a pulse of carbon‐cleaned particles exhibited higher egestion rates during the chase phase than worms given a pulse of organic‐coated particles, and no differences were seen in egestion rate between worms exposed to HA‐ and high‐EPS‐coated particles. Egestion rates decreased with increasing EPS concentration. The presence of refractory organic material decreased the absorption efficiency of cadmium from sediment relative to Cd‐AE from carbon‐cleaned sediment but not relative to Cd‐AE from sediment coated with a high concentration of EPS. The Cd‐AE increased linearly with increasing exopolymer coating on sediment particles. Overall, Cd‐AE increased with increasing gut passage time in worms that were not preexposed, although Cd‐AE from HA‐coated particles was independent of gut passage time. Preexposure to cadmium reversed the relationship between gut passage time and cadmium absorption efficiency. Thus, the implications are that sedimentary organic matter and worm physiology might be especially important in controlling metal bioavailability in deposit‐feeding organisms and should be considered in sediment quality approaches.

EFFECTS OF CHRONIC METAL EXPOSURE AND SEDIMENT ORGANIC MATTER ON DIGESTIVE ABSORPTION EFFICIENCY OF CADMIUM BY THE DEPOSIT-FEEDING POLYCHAETE CAPITELLA SPECIES I

Organic matter such as humic acid and bacterial slime exopolymer are common in estuarine and coastal sediments, where they are ingested by animals that process particulate detritus. Both humic acid (HA; refractory) and exopolymer (EPS; easily digestible) bind metals and therefore might represent a source of particulate-bound metals to deposit-feeding organisms. This study examined how cadmium preexposure (3 and 30 μg Cd/g dry wt. sed.), gut passage time (GPT), and quality and quantity of the organic coating on sediment particles interact to determine cadmium absorption efficiency (Cd-AE) in Capitella sp. I. Pulse-chase experiments using 109Cd and 51Cr were used to determine Cd-AE in individual worms. Worms were given a pulse of carbon-cleaned, HA-coated or EPS-coated sediment particles. The third treatment was divided into three EPS concentrations (high, medium, and low). A 5-d preexposure to cadmium did not affect the egestion rates during either the preexposure period or the chase phase. Worms given a pulse of carbon-cleaned particles exhibited higher egestion rates during the chase phase than worms given a pulse of organic-coated particles, and no differences were seen in egestion rate between worms exposed to HA- and high-EPS-coated particles. Egestion rates decreased with increasing EPS concentration. The presence of refractory organic material decreased the absorption efficiency of cadmium from sediment relative to Cd-AE from carbon-cleaned sediment but not relative to Cd-AE from sediment coated with a high concentration of EPS. The Cd-AE increased linearly with increasing exopolymer coating on sediment particles. Overall, Cd-AE increased with increasing gut passage time in worms that were not preexposed, although Cd-AE from HA-coated particles was independent of gut passage time. Preexposure to cadmium reversed the relationship between gut passage time and cadmium absorption efficiency. Thus, the implications are that sedimentary organic matter and worm physiology might be especially important in controlling metal bioavailability in deposit-feeding organisms and should be considered in sediment quality approaches.