87Sr/86Sr Chronostratigraphy and dolomitization history of the Seroe Domi Formation,Curaçao (Netherlands Antilles)

Summary The Seroe Domi Formation is a 350 m-thick sequence of Neogene marine limestones and silicilastic sandstones cropping out on the leeward coast of Cura?ao, Netherlands Antilles. Integrated analyses of lithofacies, biostratigraphy, geochemistry and Sr isotope model age analyses indicate that Seroe Domi Formation has experienced three major episodes of limestone diagenesis and dolomitization (Dolomites I, I′, and II) that have taken place after successive Mio-Plio-Pleistocene depositional and subaerial exposure events (Subunits 1, 2, and 3). Subunit 1, the lowermost 30 to 100 m of the Seroe Domi Formation, is composed of interbedded coralgal grainstone gravity flows, pelagic wackestones, and allochthonous blocks deposited in Middle Miocene deep-water (>500 m) fore-reef and carbonate slope environments. Subunit 2, the uppermost 250 m of the Seroe Domi Formation, consists of coralgal packstones with basement-derived siliciclastic sands that were deposted in shallowing fore-reef to reef-front environments during the Late Miocene to Pliocene. Subunit 3 siliciclastic sandstones were deposited during the Early Pleistocene within erosional cavities in the Subunit 2 limestones, and are overlain by Late Pleistocene Quaternary Limestone Terraces. The petrography, distribution and geochemistry of Dolomites I, I′ and II indicate that they were precipitated from seawater-freshwater mixing zone fluid environments. Dolomite rhombs and meteoric calcite cements within biomolds illustrate that the host Seroe Domi Formation limestones were subaerially exposed prior to each dolomitization event. Dolomite I (δ¹⁸O = +1.04 to +2.46% PDB; δ¹³C = −2.55 to −6.79 PDB;⁸⁷Sr/⁸⁶Sr=0.708866 to 0.708915; Zn=0 ppm; Cu=0 ppm) was precipitated from mixtures of seawater with isotopically-depleted freshwater during the late Middle Miocene. Dolomite I′ (δ¹⁸O = +2.08 to +3.55 PDB, δ¹³C = −1.53 to 1.69 PDB,⁸⁷Sr/⁸⁶Sr=0.708981−0.709030; Zn=0 ppm; Cu=0 ppm) was also precipitated from mixtures of seawater with isotopically-depleted freshwater, but during late Late Miocene. In contrast, Dolomite II (δ¹⁸O = +2.69 to +3.51 PDB; δ¹³C = −0.34 to +1.53 PDB;⁸⁷Sr/⁸⁶Sr=0.708954 to 0.709088; Zn=20 ppm; Cu=20 ppm) precipitated from late Early Pliocene mixtures of seawater with isotopically-depleted freshwater that had derived Zn, Cu, and less-radiogenic Sr from basalts comprising the Cura?ao basement.     

The fate of 15N-labelled nitrate additions to a northern hardwood forest in eastern Maine,USA

We followed the movements of ¹⁵N-labelled nitrate additions into biomass and soil pools of experimental plots (15×15 m each) in a mid-successional beech-maple-birch-spruce forest in order to identify sinks for nitrate inputs to a forest ecosystem. Replicate plots (n=3) were spray-irrigated with either 28 or 56 kg N ha^–1 year^–1 using ¹⁵N-labelled nitric acid solutions (¹⁵N = 344 ) during four successive growing seasons (April–October). The ¹⁵N contents of foliage, bolewood, forests floor and mineral soil (0–5 cm) increased during the course of treatments. Mass balance calculations showed that one-fourth to one-third of the nitrate applied to forest plots was assimilated into and retained by above ground plant tissues and surface soil horizons at both rates of nitrate application. Plant and microbial assimilation were of approximately equal importance in retaining nitrate additions to this forest. Nitrate use among tree species varied, however, with red spruce showing lower rates of nitrate assimilation into foliage and bolewood than American beech and other deciduous species.     

Net nitrogen mineralization and net nitrification rates in soils following deforestation for pasture across the southwestern Brazilian Amazon Basin landscape

Previous studies of the effect of tropical forest conversion to cattle pasture on soil N dynamics showed that rates of net N mineralization and net nitrification were lower in pastures compared with the original forest. In this study, we sought to determine the generality of these patterns by examining soil inorganic N concentrations, net mineralization and nitrification rates in 6 forests and 11 pastures 3 years old or older on ultisols and oxisols that encompassed a wide variety of soil textures and spanned a 700-km geographical range in the southwestern Brazilian Amazon Basin state of Rondônia. We sampled each site during October-November and April-May. Forest soils had higher extractable NO₃ ^?-N and total inorganic N concentrations than pasture soils, but substantial NO₃ ^?-N occurred in both forest and pasture soils. Rates of net N mineralization and net nitrification were higher in forest soils. Greater concentrations of soil organic matter in finer textured soils were associated with greater rates of net N mineralization and net nitrification, but this relationship was true only under native forest vegetation; rates were uniformly low in pastures, regardless of soil type or texture. Net N mineralization and net nitrification rates per unit of total soil organic matter showed no pattern across the different forest sites, suggesting that controls of net N mineralization may be broadly similar across a wide range of soil types. Similar reductions in rates of net N transformations in pastures 3 years old or older across a range of textures on these soils suggest that changes to soil N cycling caused by deforestation for pasture may be Basin-wide in extent. Lower net N mineralization and net nitrification rates in established pastures suggest that annual N losses from largely deforested landscapes may be lower than losses from the original forest. Total ecosystem N losses since deforestation are likely to depend on the balance between lower N loss rates from established pastures and the magnitude and duration of N losses that occur in the years immediately following forest clearing.     

Evolution of eutherian cytochrome c oxidase subunit II: heterogeneous rates of protein evolution and altered interaction with cytochrome c

Cytochrome c oxidase subunit II (COII), encoded by the mitochondrial genome, exhibits one of the most heterogeneous rates of amino acid replacement among placental mammals. Moreover, it has been demonstrated that cytochrome c oxidase has undergone a structural change in higher primates which has altered its physical interaction with cytochrome c. We collected a large data set of COII sequences from several orders of mammals with emphasis on primates, rodents, and artiodactyls. Using phylogenetic hypotheses based on data independent of the COII gene, we demonstrated that an increased number of amino acid replacements are concentrated among higher primates. Incorporating approximate divergence dates derived from the fossil record, we find that most of the change occurred independently along the New World monkey lineage and in a rapid burst before apes and Old World monkeys diverged. There is some evidence that Old World monkeys have undergone a faster rate of nonsynonymous substitution than have apes. Rates of substitution at four-fold degenerate sites in primates are relatively homogeneous, indicating that the rate heterogeneity is restricted to nondegenerate sites. Excluding the rate acceleration mentioned above, primates, rodents, and artiodactyls have remarkably similar nonsynonymous replacement rates. A different pattern is observed for transversions at four-fold degenerate sites, for which rodents exhibit a higher rate of replacement than do primates and artiodactyls. Finally, we hypothesize specific amino acid replacements which may account for much of the structural difference in cytochrome c oxidase between higher primates and other mammals.      

Relationships among msx gene structure and function in zebrafish and other vertebrates

The zebrafish genome contains at least five msx homeobox genes, msxA, msxB, msxC, msxD, and the newly isolated msxE. Although these genes share structural features common to all Msx genes, phylogenetic analyses of protein sequences indicate that the msx genes from zebrafish are not orthologous to the Msx1 and Msx2 genes of mammals, birds, and amphibians. The zebrafish msxB and msxC are more closely related to each other and to the mouse Msx3. Similarly, although the combinatorial expression of the zebrafish msx genes in the embryonic dorsal neuroectoderm, visceral arches, fins, and sensory organs suggests functional similarities with the Msx genes of other vertebrates, differences in the expression patterns preclude precise assignment of orthological relationships. Distinct duplication events may have given rise to the msx genes of modern fish and other vertebrate lineages whereas many aspects of msx gene functions during embryonic development have been preserved.      

Surface structure changes of rat adipocytes during lypolysis stimulated by various lypolysis stimulated by various lypolytic agents

A qualitative and quantitative electron microscopic study was performed on rat adipocytes during stimulation of lipolysis by various agents. Scanning electron microscopy of control cells revealed a spherical cell with a textured glycocalyx surface exhibiting small irregular projections. Globular surface evaginations or protrusions measuring 8-18 μM in diameter were seen on cell hemispheres, and there was an average of one protrusion for every two hemispheres examined. Distribution analysis showed that 60 percent of the hemispheres had no protrusions, and 25, 10, and 5 percent of the hemispheres had one, two or three protrusions, respectively. Thin-section and freeze- fracture electron microscopy of the protrusions showed a small triglyceride droplet surrounded by a thin cytoplasmic rim that was continuous with the main cytoplasmic matrix. The glycocalyx coating and plasma membrane extended from the cell surface onto, and over, the protrusion. Scanning microscopy of cells stimulated by lipolytic agents, including epinephrine, adrenocorticotropic hormone, theophylline, and dibutyryl cyclic AMP, revealed a dose-dependent increase in the number of protrusions per cell hemisphere. Maximal concentrations of lipolytic hormones cuase an average 2.5-fold increase in the number of protrusions per hemisphere without changing the average size of the protrusions. Only 40 percent of the stimulated cell hemispheres exhibited no protrusions; over 15 percent of the cells contained three or more; and a number of the protrusions were multilobulate. Insulin prevented the increase in the number of protrusions and the change in distribution caused by the lipolytic hormones but did not prevent the increase caused by theophylline and dibutryl cyclic AMP. The data suggest that the protrusions are a structural feature of the cell and may be related to the lypolytic pathway. These observations may help explain some of the discrepant biochemical data relating to hormonal stimulation of lipolysis.     

On the structure of the Z pseudo-organ in Xiphinema diversicaudatum

The structure of the Z pseudo-organ in Xiphinema diversicaudatum consists of a muscular mass of about 80 cells arranged in rings of four cells each. These cells have peripheral nuclei and are connected with a glandular monolayered epithelium, which lines the lumen of the Z pseudo-organ. Globular bodies originate from the membranes of the epithelial tissue. They consist of a series of closed vesicles containing secretions, and of tubules through which the secretions flow into the lumen. The globular bodies have a helicoidal distribution within the lumen of the Z pseudo-organ which presumably slows the passage of the egg. Enzymatic digestion, in vivo, indicates that the secretion consists of acid and basic proteins involved in egg shell formation.