Evolution Of Diatoms Research Articles

Opal sedimentation shifts in the World Ocean over the last 15 Myr

Biogenic silica (opal) accumulation records were used to trace mechanisms, consequence, and geographic pattern of shifts in the main locus of opal deposition of the World Ocean over the last 15 Myr. Over this time interval, the main opal “sink” seems to have moved from the North Atlantic, to the Pacific, equatorial Pacific, eastern equatorial Pacific, eastern boundary current upwelling systems (California, Namibia, Peru), and finally to the Southern Ocean. The interplay between opal deposition and a series of climatic, tectonic, oceanographic, and biologic events has been analyzed and discussed. These events include the Cenozoic global cooling trend, intensified glaciation in Antarctica, Late Miocene–Early Pliocene biogenic bloom, development of Northern Hemisphere Glaciation (NHG), closing of the Panama Seaway, transition of the climate system from a monopolar- to a bipolar-glaciated world, Mid-Pleistocene Revolution (MPR), nutrient availability, evolution of diatoms and C4 plants, and changes in continental weathering rates. While the observed shifts are mostly traceable to oceanic reorganizations and global climatic evolution, conditions favorable to opal deposition involve the above-mentioned complex mix of processes. For this reason, the interpretation of opal deposition records might not always be straightforward. We, however, believe that it can still provide clear indications of large-scale oceanographic reorganizations in the geological past.

RESULTATS PRELIMINAIRES SUR LES DIATOMEES BENTHIQUES DE SUBSTRATS MEUBLES DANS LA LAGUNE DE VENISE, ITALIE (MARS 1994–JANVIER 1995)

Research on samples from the Venice lagoon was carried out to determine a suitable method for the quali-quantitative evolution of diatoms living within the sediments. From March 1994 to March 1995, surface sediment samples were collected monthly from a station located in the southern basin of the lagoon. Samples were observed using both light and scanning electron microscope. A list of the taxa found during this research is reported; counts of diatoms present in a volume of sediment were also carried out. Values for cell numbers ranged from 0.2 × 106 to 1.8 × 106 cells/cm3; higher values were recorded in spring/summer and lower values in autumn/winter. Many species belonged to the nannophytobenthos (< 30 μm), which better adapted to move among the grains of sediment than larger species. Benthic taxa, belonging to the centric group of diatoms, were also found.

Clusters, species concepts, and Morphological Evolution of Diatoms

Empirical comparison of species concepts can be made by taking advantage of a common operational exigency: the use of phenotypically irreducible clusters of individuals as first order estimates of species. A phylogenetic analysis of diatoms was undertaken using such clusters as terminal taxa. Cladistic analysis and biogeographic distributions suggest that different populations of the widespread, plesiomorphic Stephanodiscus niagarae (at least 2 million years old) are ancestral to three autapomorphic taxa that emerged during the Holocene in North America. This phylogenetic hypothesis is supported in one case by a time-transgressive morphological continuity between fossils of the plesiomorphic species (S. niagarae) and of one of the derived endemics(S. yellowstonensis) found only within the Yellowstone Lake basin. Given the phylogenetic hypothesis, the clusters diagnosed by autapomorphy each evolved as a unit, but the cluster identified solely by a unique combination of plesiomorphic characters did not (S. niagarae). Each of the autapomorphic species is found in close proximity to lakes containing S. niagarae, whereas lakes containingS. niagarae populations are widely dispersed. Thus, it would be difficult to simultaneously explain evolution of autapomorphic endemics by breeding isolation and maintenance of the plesiomorphic similarity of S. niagarae populations by interbreeding. Autapomorphic taxa were found on the ecological periphery of the species complex but S. niagarae.10/1/2008occurs both in the ecological center and periphery of the entire complex; therefore, the plesiomorphic cluster is not an ecological unit. The results caution against the acceptance of smallest phenetically recognizable clusters of organisms as either units or products of evolution. Cladistic analysis and the autapomorphic species concept should be a routine part of studies of speciation. Stratigraphy and parsimony analysis agreed on the ancestral morphology of S.yellowstonensis.

Clusters, Species Concepts, and Morphological Evolution of Diatoms

Clusters, Species Concepts, and Morphological Evolution of Diatoms

Radiolarian Preservation in Present and Past Oceans: ABSTRACT

Three methods were used to collect living and recently dead radiolarians and fecal pellets containing radiolarians--plankton tows for those in the water column, and gravity cores and box cores for recovery from surficial sediments. Deep Sea Drilling Project cores were used to study radiolarians from fossil sediments. Our techniques differ from previous studies by the investigation of individual skeletons and suggest that skeletons are removed from the water column primarily by dissolution and, secondarily, by settling as individuals or via fecal pellets. Laboratory experiments suggest that metallic coatings help to protect the skeletons from dissolution in the water column and in sediments. Other factors of extreme importance are differential rates of settling thickness of the radiolarian dissolution zone, productivity of overlying waters, amounts and ratios of terrigenous and authigenic sediment, presence of different water masses in the water column and at the sediment-water interface, degree of bioturbation, and chemical conditions at and below the sediment-water interface. Paleobiologic developments such as the evolution of diatoms appear to be related to conservation. The use of silica by diatoms may be partly responsible for the preferential dissolution of some Neogene radiolarians. Major changes in oceanic circulation appear to be related to such changes as the cessation of preservation in the middle and tropical Atlantic during the middle Miocene. End_of_Article - Last_Page 483------------

The amino acid and sugar composition of diatom cell-walls

The cell walls of diatoms consist of a silica frustule encased in an organic coating. Biochemical characterization of this coating should allow insight into: (1) the mechanism of silicification; (2) taxonomy and evolution of diatoms; (3) preservation of fossil frustules. The amino acid and sugar composition of cell walls from 6 diatom species have been elucidated. When compared to cellular protein, cell-wall protein is enriched in serine plus threonine and glycine, and depleted in acidic, sulfur-containing and aromatic amino acids. The sugars of the cell-wall carbohydrates are quite variable and fucose tends to replace glucose in estuarine species. Condensation of silicic acid, in epitaxial order, on a protein template enriched in serine and threonine, is suggested as the Si-depositing mechanism in diatoms. The nature of this template and the polysaccharides in the cell wall may determine the solubility of diatom frustules in various environments. There is sufficient variability in cell-wall amino acids to warrant further investigation of their taxonomic utility. The sugars appear to be related to environmental factors, but they may also serve in biosystematic studies.