Roy J. Baerwald

TEM study of in situ organic matter on continental margins: occurrence and the “monolayer” hypothesis

Abstract TEM photomicrographs show that organic matter in the continental margin sediments that we have examined is primarily patchy in distribution and occurs as: (1) discrete, discontinuous blebs and micro-blebs of differentiated and undifferentiated protoplasm; (2) bacterial cells and associated muco-polysaccharide networks and (3) localized smears generally associated with clay minerals and domain junctions in clay-rich flocs, not as thin uniform grain coatings or as infillings of nicks or etch pits on sediment grains with high degrees of surface roughness. This occurrence contradicts recent assertions that most organic matter in continental margins occurs as monolayer or “monolayer equivalent” coatings on mineral grains and inorganic bioclastic particles. Our results are based on TEM photomicrographs of samples from the northern California continental slope that fall within the “monolayer equivalent” envelope defined by Mayer (1994), marine snow from the overlying nepheloid layer, and sediments from Eckernforde Bay in the Baltic Sea. These samples were prepared specifically to preserve and image in situ sediment fabric and organic constituents. Textural evidence suggests that low permeabilities caused by the interaction between the organic, inorganic and biologic components of the clay size fraction play a major role in determining organic matter preservation on continental margins.

Comparison of pelagic and nepheloid layer marine snow: implications for carbon cycling

Abstract Marine snow from upper and mid-water (i.e., pelagic) depths on the California margin is texturally and compositionally different from that traveling in the nepheloid layer. Transmission electron microscopy shows that pelagic marine snow consists primarily of bioclasts (e.g., diatom frustules, foram tests), organic matter, and microbes. These components are entrained as discrete particles or small aggregates (≤10 μm in diameter) in a loose network of exocellular, muco-polysaccharide material. Clays are infrequent but, when present, are constituents of comparatively compact organic-rich microaggregates. Microbes are abundant and appear to decrease in number with increasing water depth. In contrast, marine snow aggregates collected from just above the sea floor in the nepheloid layer are assemblages of clay particles, clay flocs, and relatively dense clay–organic-rich microaggregates in an exocellular organic matrix. Bioclasts and microorganisms occur only rarely. The prevalence of clay–organic-rich aggregates in the nepheloid layer suggests that, prior to final deposition and burial, marine snow from the pelagic zone is subject to disaggregation and recombination with terrigenous detrital material near or at the sea floor. Results have significant implications for the accumulation and burial rates of organic carbon on continental margins and the aging and bioavailability of sedimentary organic matter. Samples examined were collected offshore of northern and central California.

In situ conditions and interactions between microbes and minerals in fine-grained marine sediments; a TEM microfabric perspective

Abstract Microbes, their exocellular secretions, and their impact on the mineralogy and microfabric of fine-grained continental margin sediments were investigated by transmission electron microscopy. Techniques were used that retained the in situ spatial relations of both bio-organic and mineralogical constituents. Photomicrographs were taken of characteristic mineral-microbe associations in the first meter of burial at conditions ranging from aerobic to anaerobic. Single-celled prokaryotes, prokaryotic colonies, and eukaryotic organisms were observed as were motile, sessile, and predatory species. Bacterial cells dominate the assemblage. The most commonly observed mineral-biological interaction was the surrounding, or close association, of isolated heterotrophic bacterial cells by clay minerals. Almost without exception, the external surfaces of the bacteria were covered with secreted exocellular slimes composed of cross-linked polysaccharide fibrils. These fibrils act to bind sediment grains into relatively robust microaggregates, roughly ≤ 25 μm in diameter. These exocellular polymers can significantly impact the interaction between microbes and minerals, as well as the chemical and physical transport of fluids and dissolved aqueous species through the sediment. Although pore water chemical profiles from the field sites studied have dissolved Fe and Mn, no close association was found between the microbes imaged and precipitated metal oxyhydroxides or other authigenic minerals, such as is commonly reported from laboratory cultures.

Techniques for the Preparation of Submarine Sediments for Electron Microscopy

The collection of high-quality sediment samples recovered from the sea floor is prerequisite to initiating laboratory investigations of sedimentological, microstructural, or geotechnical properties. These types of studies require virtually undisturbed material. Likewise, microfabric studies require the utmost care in the handling of samples and in the preparation of subsamples for electron microscopic observations. The literature is replete with discussions and techniques concerning submarine sediment coring and sample recovery in general. Richards (1962) presented a thorough review of the various types of coring devices and the significance of sample disturbance. In general, large- diameter thin-wall sampling devices provide high-quality sediment cores (Richards and Keller, 1961; Lambert and Merrill, 1979). High-quality sediment core samples now can be recovered from deep core drilling using specially designed sampling tools (DSDP, Vols. 64 and 68). Analyses and evaluation of sample quality were discussed by Bennett (1976) and Bennett et al. (1977). Discussion of the numerous sampling techniques is beyond the scope of this chapter and the interested reader is referred to the above references for details.

Double membrane-bounded intestinal microvilli in Oncopeltus fasciatus

SummaryA double plasma membrane (DPM) surrounding intestinal microvilli of the migratory milkweed bug, Oncopeltus fasciatus, is described. Mutant and wild types of the phytophagous insect have been studied by conventional SEM and TEM procedures with the use of membrane-enhancing staining methods. Longitudinal and transverse sections revealed a DPM surrounding microvilli and continuing over the apical portions of the intestinal cell. The outer membrane of the DPM contributes to an intestinal lining or peritrophic membrane (PTM), which apparently accumulates in layers. SEM studies reveal a rugose intestinal surface and complete PTM in both starved and fed insects. Only rarely are exposed microvilli seen by SEM. SEM examinations also enable the observation of numerous blebs on the luminal side of the PTM apparently held in position by a neck-like attachment and apparently derived from the outer membrane of the DPM. Preliminary TEM studies of microvilli revealed unique microvesicle-like structures, lying just inside the inner membrane of the DPM, which may be of membrane origin based on their typical trilaminar appearance after en bloc staining with uranyl acetate. Highly ordered microfilaments were observed to occupy the most central aspect of the microvilli.

Observations of the sediment-water interface: Marine and fresh water environments

The sediment at the interface immediately beneath the water column is distinct from deeper-lying sediments in its properties and, at least quantitatively, in the processes driving diagenesis. Progress in understanding the sediment-water interface can be based on consideration of fundamentals of biogeochemical particle / fluid interactions and on application of certain biological techniques especially suited to this challenging portion of the sediment column. This article reports results achieved by combining theoretical fundamentals and specialized experimental techniques in the study of the interface from selected depositional environments. For fine-grained and sandy deposits from fresh-water to coastal marine environments, the interface is characterized by exaggerated microrelief, great porosity, and significant biological alteration. Additional application of this research approach is poised to further our understanding of engineering, and acoustic and xenochemical responses of sedimentary materials, with special emphasis on the influence of the bio-organic phase of the interface upon its fabric and physical properties.

In situ porosity and permeability of selected carbonate sediment: Great Bahama bank Part 2: Microfabric

Abstract Selected oolitic sediments from the Great Bahama Bank were studied to assess (1) the role of microfabric in determining porosity and permeability, and (2) particle packing relationships, i. e., grain support versus matrix support. Scanning electron microscopy and light microscopy revealed that the sediment consists of ooids, which are the major constituents of the sand‐size fraction, supported by a matrix composed predominantly of aragonite needles. The supporting matrix of aragonite needle clusters, which constitutes only about 10–20% of the total sediment dry weight, is the microstructural characteristic that increases the porosity and lowers the wet bulk density compared to a grain‐supported microfabric characteristic of clean sands. The presence of a fine‐grained matrix reduces the permeability of these sediments relative to clean sands. The influence of the microfabric is clearly reflected in the mass physical and depositional (particle packing) properties of the sediment. Laboratory values ...

Ultrastructure studies of the extracellular filaments in the ventral nerve of Panulirus argus

The ventral nerve cord of the spiny lobster, Panulirus argus was examined by transmission and scanning electron microscopy. Tannic acid mordant stain was used to enhance extracellular filaments. The ventral nerve cord is surrounded by an unusual perineurial sheath composed primarily of interwoven extracellular filaments. Gap junctions were found associated with the glial cells making up the perineurium. The axo-glial wrappings also contained extracellular filaments associated in bundles rather than uniformly around the axons. The extracellular filaments of the perineurium and axo-glial wrappings appeared to be morphologically identical with diameters ranging from 10-15 nm.

Multiple SR-T tubule junctions in a single insect flight muscle fiber

The membranes of mesothoracic dorsoventral flight muscle of Libellua needhami and Erythemis simplicicolis (Odonata: Libellulidae) were studied by transmission electron microscopy using uranyl acetate en bloc staining. Intact fibers were studied by conventional scanning electron microscopy procedures. Dyadic, triadic, and tetradic transverse tubule (TT)—sarcoplasmic reticulum (SR) junctions as well as plasma membrane—SR junctions are described for the first time in the same insect muscle fiber. Triadic (SR—TT—SR) and, rarely tetradic (TT—SR—TT—SR) junctions were limited to regions between adjacent myofibrils and near the ends of fibers. Dyadic (TT—SR) junctions are found between myofibrils or within indentations of slablike mitochondria. Interspecific variation in SR—TT junctional assemblies from previous similar studies is reviewed and summarized and a three-dimensional model based on SEM and TEM observations is presented.