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  • 1
    ISSN: 1615-6102
    Keywords: Microtubule ; Plasma membrane ; Cellular morphogenesis ; Allogromia ; Foraminifera
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology
    Notes: Summary Video-enhanced microscopy was used to study the behavior of cytoplasmic microtubules in flattened reticulopodia of the marine protistAllogromia. Linear microtubule bundles were observed bending to various degrees and then straightening. When microtubules bent sufficiently to contact the plasma membrane, protuberances extended from the pseudopodial margins. These protuberances withdrew as the bent microtubules straightened. In extreme cases, microtubules formed c-shaped loops which moved laterally through the cytoplasm and contacted fenestrae formed within the flattened pseudopodia. A given fenestra first deformed at the site of microtubule contact and then closed as the loop continued its motion; reversal of the microtubule motion reopened the fenestra. By electron microscopy, microtubules are consistently seen within 20 nm of the plasma membrane and are often connected to the membrane by detergent-resistant crosslinks. Together, these observations indicate that microtubule movements can deform the plasma membrane and thus mediate certain aspects of cellular morphogenesis.
    Type of Medium: Electronic Resource
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  • 2
    ISSN: 1615-6102
    Keywords: Allogromia ; Helical filaments ; Microtubules ; Paracrystals ; Reticulopods
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology
    Notes: Summary Allogromia sp. is a benthic foraminiferan protozoan which extends and withdraws a dynamic network of branching and anastomosing pseudopodia,i.e., reticulopods. Each reticulopod contains an elongate cytoskeleton composed primarily of microtubules (MT). When withdrawal was induced with artificial seawater supplemented with MgCl2, we found a time-dependent decrease in the number of reticulopodial MTs and a concomitant increase in 5-nm-diameter helical filaments. During the initial stages of withdrawal these helical filaments associated laterally to form loose aggregates. Later they formed dense paracrystalline aggregates, which appeared similar to those seen in the cell bodies of untreatedAllogromia juveniles prior to network extension. Similar results were obtained when withdrawal was induced by using seawater supplemented with other salts (NaCl, KCl). Treatment with an isotonic seawater substitute with an altered Na+:K+ ratio induced a momentary withdrawal, after which the organism recovered and reextended a network. During the withdrawal phase of this response, MTs became less abundant and aggregates of helical filaments more conspicuous. Together with earlier observations these findings suggest that helical filaments and paracrystalline material are an alternative or intermediate assembly form of MT proteins.
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  • 3
    Publication Date: 2003-09-22
    Print ISSN: 0027-8424
    Electronic ISSN: 1091-6490
    Topics: Biology , Medicine , Natural Sciences in General
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  • 4
    Publication Date: 2012-11-13
    Description: Shallow water Antarctic cibicidids are traditionally identified as Cibicides refulgens or Cibicidoides lobatulus . However, a recent phylogenetic study based on SSU rDNA sequence data has demonstrated that these Antarctic cibicidids form a well-supported clade that branches sister to Mediterranean C. refulgens and is only distantly related to C. lobatulus . Based on these DNA sequences, and a detailed scanning electron microscopic examination of material from Explorers Cove (McMurdo Sound), we place Antarctic cibicidids in a separate species Cibicides antarcticus ( Saidova, 1975 ), which is emended here with molecular, morphological, and ecological features.
    Print ISSN: 0096-1191
    Topics: Geosciences
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  • 5
    Publication Date: 2015-03-14
    Description: A bstract :  Little is known about land-to-sea sediment transport and deposition in polar regimes with multi-year sea ice such as at the mouth of the Taylor Valley, one of the Dry Valleys in Antarctica. This study integrates textural analysis of 574 coastal subaerial and nearshore submarine sediment samples with data from sediment traps and with published grain-size analyses of offshore sea-ice surface and submarine sediments to reconstruct transport and depositional processes from the shoreline to ~ 8 km offshore. Sediment, primarily sand sized, is transferred into the marine environment via two processes: 1) erosion, transport, and deposition of till by ephemeral meltwater streams that deliver sediment to the subaqueous part of the delta built over the last ~ 7,500 years, and 2) onshore to sea-ice sediment transport by wind during rare but powerful foehn wind events. Because the distributaries flow into quiet water beneath the sea ice, most of their load is dropped close to shore, but a small amount of fine-grained sediment, mostly silt, is carried kilometers offshore, probably by low-density, low-salinity plumes. Wind deposits sediment on the sea ice with median grain size decreasing from medium to fine sand within 1.5 km of the shoreline; even offshore sea-ice sediment contains very little silt and almost no clay. Nearshore the sea-ice sediment is coarser than that on the underlying seafloor, but in a few kilometers offshore the median grain sizes of the seafloor and sea-ice sediments are similar; this reflects a mixture of sea-ice-rafted eolian sediment and fine-grained delta-derived sediment transported beneath the sea ice. This system is unusual in the relative effectiveness of the transport systems. The feeble distributaries deposit their sediment load immediately upon entering the quiet water, and only a minor amount of silt and clay is carried seaward under the ice. In contrast, winds deposit coarser sediment on the nearshore sea ice and carry fine sand 〉 7 km offshore, although the amount and grain size of sediment is attenuated within 1.5 km of shore. Melting of the sea ice drops sediment onto the seafloor, where it mixes with the delta-derived silt and clay. Similar combinations of eolian and fluvial transport processes may have operated in polar regimes dominated by multi-year sea ice in the past. Understanding of the modern processes in Explorers Cove may aid recognition of ancient sediments deposited under multi-year sea ice, as well as help predict future changes in processes around Antarctica as the distribution of multi-year sea ice shifts in response to climate change.
    Print ISSN: 1527-1404
    Topics: Geosciences
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  • 6
    Publication Date: 2014-07-12
    Description: Benthic foraminifera are ubiquitous marine organisms in Antarctica, but little is known about their ecology despite their importance as climate-change proxies. A fundamental question remains unanswered: is foraminiferal diversity highly localized and similar to reports for Antarctic benthic invertebrates? If so, high endemism would result driven by dispersal limitation with little connectivity among sites. To answer this question, the composition, abundance, spatial distribution and alpha (α) and beta (β) diversity of foraminiferal species that encrust living shells of the Antarctic scallop Adamussium colbecki were compared within and between two sites located 30-km apart in McMurdo Sound, Ross Sea, Antarctica: Bay of Sails (BOS), an open coastal region with annual sea-ice cover, and Explorers Cove (EC), a protected embayment with multiannual sea-ice cover. Foraminiferal community structure differed between the sites, generated by variations in species abundance, turnover of rare species, and heterogeneity in species composition between top and bottom valves. Foraminifera were significantly more common on the scallop’s auricles, perhaps driven by nutrients generated by the scallop. Calcareous taxa were significantly more abundant at EC despite lower seawater pH, while agglutinated taxa were significantly more abundant at BOS. Alpha diversity (species richness, Shannon H , e H ) was higher at BOS than at EC and is similar to deep-sea diversity reported for Caribbean encrusting foraminifera. Based on Shannon H , foraminifera at EC may represent a stressed community dominated by Cibicides antarcticus , while those at BOS may represent a transitional community. Within-site β-diversity was high, spurred by the turnover of rare species between top and bottom valves, which acted as separate microhabitats. Higher β-diversity at BOS suggests that foraminiferal dispersal and recruitment were more localized than at EC. Additive partitioning revealed that encrusting diversity was mostly held at the local level, and that BOS and EC have high connectivity, similar to deep-sea communities. The Antarctic endemicity of these protists is questionable, and is in stark contrast to Antarctic invertebrates with high within- and between-site diversity. Therefore, Antarctic encrusting foraminifera appear to be influenced primarily by niche-based processes, rather than dispersal limitation. Indeed, habitat islands, such as carbonate shells, are essential for generating Antarctic diversity, and are integral for the conservation of Antarctica’s unique polar ecosystem.
    Print ISSN: 0096-1191
    Topics: Geosciences
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  • 7
    Publication Date: 2016-10-20
    Description: Homotrema rubrum is a (sub)tropical, sessile foraminiferan that reinforces the coral-reef framework by calcifying in cracks, crevices and on the cryptic undersides of coral colonies and other reef substrata. It secretes a pigmented carbonate skeleton that can be well preserved in the fossil record and, thus, has the potential to influence paleoecological analyses. In an effort to gain insights about the ecology and laboratory maintenance of this species, we studied its distribution in the field as well as its survival in the laboratory under different environmental conditions. The distribution of H. rubrum along transects across Tennessee Reef (Florida Keys, USA) revealed an abundance of encrusting, knobby and hemispherical morphologies on the reef flat, and all five of the morphotypes delineated by others were identified. The highest abundances of H. rubrum occurred on the reef flat, shoreward of the living reef, attached to non-living, detrital coral, most of which was covered by a veneer of coarse carbonate sediment. Direct observation of living individuals and epifluorescence microscopy of specimens incubated with calcein demonstrate that H. rubrum can calcify under controlled laboratory conditions. Calcification is not limited to chamber formation, but rather occurs differentially over the surface. Of the five distinct designs tested for laboratory maintenance of this species, the quasi-static aquarium in indirect natural light proved the most effective. Together, these findings help establish H. rubrum as a useful model for studying natural environmental changes and anthropogenic impacts on marine ecosystems.
    Print ISSN: 0096-1191
    Topics: Geosciences
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  • 8
    Publication Date: 2017-03-08
    Description: We review the ecology and fossil record of parasitic and suspected parasitic foraminifera. Nine species of foraminifera are known to be parasitic, obtaining nutrients from their host, and 13 are suspected parasites that require taxonomic and ecologic work to document their trophic relationships. Roughly 0.22% and 0.32% of all benthic foraminifera are known or suspected parasites, respectively. Endo- and ectoparasites are most common, followed by kleptoparasites and putatively hermit endoparasites. While most diverse in the shallow-water tropics, the best-known foraminiferal parasites live in colder North Atlantic and Antarctic waters. Body size comparisons reveal patterns similar to parasitic metazoans: 1) a few parasitic foraminifera are larger than most benthic foraminifera and most are larger than their free-living relatives; 2) most are smaller than their metazoan hosts, but are roughly similar in size to protozoan hosts with one exception; and 3) larger parasitic foraminifera infest larger hosts, consistent with Harrison’s rule. Ectoparasitic foraminifera also follow Harrison’s rule, but endoparasites do not because they are spatially constrained by living within the host. Suspected foraminiferal parasites are first recorded from the Late Jurassic and parasitic foraminifera from the Early Cretaceous, while the majority evolved in the Cenozoic. Cretaceous Talpinella cunicularia has one of the longest known host-parasitic relationships of 18 Myr. Sixteen species bioerode their hosts, but only two trace fossils from parasitic foraminifera are known. Their abundance, broad geographic distribution and excellent fossil record make parasitic foraminifera and their hosts an excellent model to study how climate and environmental change affects intimately-associated species.
    Print ISSN: 0096-1191
    Topics: Geosciences
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  • 9
    Publication Date: 2013-06-25
    Description: The brittlestar, Ophionotus victoriae , is abundant in Explorers Cove, offshore Taylor Valley. However its ossicles, composed of high-Mg calcite, have not been reported from Cenozoic cores taken from McMurdo Sound. To identify taphonomic processes we analyzed (1) ossicle dissolution and silhouette area loss during a 2-year in situ experiment in which whole dead brittlestars were suspended above or placed on the sediment-water interface at water depths of 7–25 m; (2) ossicle dissolution in a 27-day, in situ experiment using ossicles freed of soft tissue; (3) porosities of experimental and pristine ossicles; and (4) abundance of ossicles in short cores taken at shallow depths in Explorers Cove. SEM analysis demonstrates significantly higher levels of dissolution in ossicles submerged for two years than in pristine ossicles. Submerged ossicles also had significant breakage reflected in silhouette area loss. During the 27-day experiment, submerged ossicles lost between 0.07 wt% and 1.31 wt%. At the observed rate of dissolution it would take between 6 and 105 years for vertebral ossicles to dissolve completely. Ossicles submerged for two years had a slightly higher mean porosity than pristine ossicles; porosity is controlled by variability in the porous stereom structure as well as dissolution. Results demonstrate that ossicle dissolution starts soon after death and that the stratigraphic record does not accurately reflect the presence and abundance of ophiuroids, thus complicating their use in paleoenvironmental, paleoclimatic, and paleoecologic reconstructions. These results also provide baseline information about CaCO 3 skeletal dissolution needed to monitor the ocean acidification that is predicted to affect high-latitude benthic ecosystems within decades.
    Print ISSN: 0883-1351
    Electronic ISSN: 0883-1351
    Topics: Geosciences
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  • 10
    Publication Date: 2013-10-29
    Description: Foraminifera and radiolarians are closely related amoeboid protists (i.e., retarians) often characterized by their shells and pseudopodia. Previous studies hypothesized that the unusual "Type 2" β-tubulin (β2) is critically involved in forming helical filaments (HFs), a unique microtubule (MT) assembly/disassembly intermediate found in foraminiferan reticulopodia. Such noncanonical β-tubulin sequences have also been found in two radiolarian species and appear to be closely related to the foraminiferan β2. In this study, we report 119 new β-tubulin transcript sequences from six foraminiferans, four radiolarians, and a related non-retarian species. We found that foraminiferan and radiolarian β2-tubulins share some of the unusual substitutions in the structurally essential and usually conserved domains. In the β-tubulin phylogeny, retarian β2-tubulin forms a monophyletic clade, well separated from the canonical β-tubulin (β1) ubiquitous in eukaryotes. Furthermore, we found that foraminiferan and radiolarian β2-tubulin lineages were under positive selection, and used homology models for foraminiferan α- and β-tubulin hexamers to understand the structural effect of the positively selected substitutions. We suggest that the positively selected substitutions play key roles in the transition of MT to HF by altering the lateral and longitudinal interactions between α- and β-tubulin heterodimers. Our results indicate that the unusual β2-tubulin is a molecular synapomorphy of retarians, and the β-tubulin gene duplication occurred before the divergence of Foraminifera and radiolarians. The duplicates have likely been subjected to neofunctionalization responsible for the unique MT to HF assembly/disassembly dynamics, and/or other unknown physiological processes in retarian protists.
    Print ISSN: 0737-4038
    Electronic ISSN: 1537-1719
    Topics: Biology
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