Oxygen concentration profiles and exchange in sediment cores with circulated overlying water

SUMMARY. 1. The overlying water of intact sediment cores was constantly stirred with an impeller at a rate sufficient to mix turbulently the water column and maintain the diffusive boundary layer at a determined thickness. The system allowed standardization of water circulation in laboratory sediment core experiments.
2. Both oxygen concentration and oxygen penetration depth in the sediments decreased, the former by 70% and the latter from 4.2 mm to 2.0 mm, when the overlying water was not stirred for 24 h, as measured with oxygen microelectrodes in a lake sediment core.
3. Oxygen profiles measured in sediment cores in the laboratory were similar to those measured in situ when the overlying water was stirred with an impeller at such a rate that a similar thickness of the diffusive boundary layer at the sediment-water interface developed in the laboratory as that in situ.
4. Sediment oxygen consumption was calculated from: (1) measured oxygen profiles in the diffusive boundary layer and the molecular diffusion coefficient for oxygen in water; (2) the measured oxygen decrease in the top of the sediments and the estimated diffusion coefficient in the sediment; and (3) by oxygen differences in the overlying water after incubation of sediment cores.     

Living Vessel Elements in the Late Metaxylem of Sheathed Maize Roots

The two types of nodal roots of field-grown maize, sheathedand bare, were found to have such different water conductivitiesthat an investigation of the anatomy of their large metaxylemvessels was made. While the vessels of the bare roots were openfor scores of centimetres, those of the sheathed roots werefound to be not vessels but developing vessel elements, withcross walls at 1 mm intervals, and protoplasts. The cross wallsbetween the elements had several unique histochemical properties.Previous investigators have often failed to find the cross wallsbecause they are very easily dislodged during the usual methodsof tissue preparation. They are best identified by microdissectionof fresh xylem. The living elements persist in the late metaxylemup to 20 – 30 cm from the tip. As the roots become longerthan this both the cross walls and the soil sheaths disappearand there is a transition to a bare root with open vessels inthe proximal region. The soil sheath persists a little longerthan the cross walls. The two types are thus stages in a developmentalsequence through which all nodal roots pass. A fundamental differencebetween the two types is in their water status, since the estimatedconductive capacity of a bare root is about 100 times greaterthan that of a sheathed root. These observations point to theneed for a reassessment of the published work on transport ofions into the xylem of grass roots through a reinvestigationof the ‘maturity’ of their xylem vessels. Grass roots, dimorphic roots, ion secretion to xylem, soil sheaths, xylem vessels, xylem differentiation, water conduction, Zea mays L     

Reconstitution of a GTPase Activity a 50S Ribosomal Protein from <Emphasis Type="Italic">E. coli</Emphasis>

DURING each step of peptide chain elongation the ribosome shifts up one triplet along the messenger RNA with concomitant movement of the peptidyl-transfer RNA from the donor to the acceptor site. This process, commonly known as translocation, is triggered by a supernatant protein, factor G, which in association with the ribosome cleaves GTP into GDP and inorganic phosphate^1,2 and it has been argued that the energy liberated in this reaction is used “to carry the complex one triplet forward”³.     

The rhodopsin system of the squid

Squid rhodopsin (λ_max 493 mµ)—like vertebrate rhodopsins—contains a retinene chromophore linked to a protein, opsin. Light transforms rhodopsin to lumi- and metarhodopsin. However, whereas vertebrate metarhodopsin at physiological temperatures decomposes into retinene and opsin, squid metarhodopsin is stable. Light also converts squid metarhodopsin to rhodopsin. Rhodopsin is therefore regenerated from metarhodopsin in the light. Irradiation of rhodopsin or metarhodopsin produces a steady state by promoting the reactions, See PDF for Equation Squid rhodopsin contains neo-b (11-cis) retinene; metarhodopsin all-trans retinene. The interconversion of rhodopsin and metarhodopsin involves only the stereoisomerization of their chromophores. Squid metarhodopsin is a pH indicator, red (λ_max 500 mµ) near neutrality, yellow (λ_max 380 mµ) in alkaline solution. The two forms—acid and alkaline metarhodopsin—are interconverted according to the equation, Alkaline metarhodopsin + H⁺ acid metarhodopsin, with pK 7.7. In both forms, retinene is attached to opsin at the same site as in rhodopsin. However, metarhodopsin decomposes more readily than rhodopsin into retinene and opsin. The opsins apparently fit the shape of the neo-b chromophore. When light isomerizes the chromophore to the all-trans configuration, squid opsin accepts the all-trans chromophore, while vertebrate opsins do not and hence release all-trans retinene. Light triggers vision by affecting directly the shape of the retinene chromophore. This changes its relationship with opsin, so initiating a train of chemical reactions.     

Incubation in Mice Provides a Signal for the Differentiation of Trypanosoma cruzi Epimastigotes to Trypomastigotes1

The differentiation of Trypanosoma cruzi epimastigotes into trypomastigotes was studied in diffusion chambers sub-cutaneously implanted in mice. Using epimastigotes of the Tulahuén strain, transformation was first evident at 16 h after implantation and reached its maximum (92% trypomastigotes) by 24 h. Shortly before their differentiation into trypomastigotes, epimastigotes were found to develop resistance to lysis by the alternative pathway of complement. Furthermore, implantation of stationary-phase (as opposed to log-phase) parasites resulted in the accumulation of large numbers of complement-resistant epimastigotes in the chambers. These observations suggest that epimastigotes pass through a complement-resistant transitional stage before differentiating into trypomastigotes and that transformation may require cell division. In a further series of experiments, epimastigotes recovered 7 h after implantation in mice were found to differentiate into trypomastigotes when cultured in vitro for an additional 17 h at 37°C. This observation indicates that the events which trigger the morphologic transformation of epimastigotes into trypomastigotes can be dissociated operationally from the differentiation process itself.