Grazing Rates in Euplotes mutabilis: Relationship between Particle Size and Concentration

Abstract Grazing behavior of both individual cells and populations of the marine hypotrich Euplotes mutabilis, a largely benthic ciliate that feeds on suspended particles, was studied using fluorescent latex microspheres. Microspheres of sizes 0.57-, 1.90-, 3.06-, 5.66-, and 10.0-μm diam were offered at concentrations from 10² to 10⁶ ml⁻¹. Their uptake in a ten-min period was determined. Food particles within such ranges of size and concentration are found under natural conditions. The ciliates ingested particles of all sizes offered. Uptake rates at all concentrations were dependent upon particle size, with 1.90- and 3.06-μm diam microspheres having the highest uptake rate in all cases. For all sizes, there was an increase in the percentage of feeding cells and in the uptake rate (the number of particles ingested cell⁻¹ h⁻¹), with increasing particle concentration. When uptake was expressed as the volume ingested, maximum values were obtained for 5.85-μm diam microspheres at a concentration of 10⁶ ml⁻¹. By taking a small number of large particles, present at a low concentration in the medium, a ciliate can ingest a greater biovolume than by taking a high number of small particles which are abundant in the medium. These results demonstrate that some ciliates can graze particles of a wide range of sizes. Also, its nutrition can be affected by changing ambient concentrations of different prey, both through effects on the proportion of feeding cells and through the relative food content of the particles. The data can also add to the understanding of food niche partitioning between species. At low particle concentrations, particularly, it is important to consider the behavior of individual ciliates as well as of the whole population. Received: 11 February 1997; Accepted: 21 October 1997     

Interplay between inflammation and cancer

Tumor-promoting inflammation is one of the hallmarks of cancer. It has been shown that cancer development is strongly influenced by both chronic and acute inflammation process. Progress in research on inflammation revealed a connection between inflammatory processes and neoplastic transformation, the progression of tumour, and the development of metastases and recurrences. Moreover, the tumour invasive procedures (both surgery and biopsy) affect the remaining tumour cells by increasing their survival, proliferation and migration. One of the concepts explaining this phenomena is an induction of a wound healing response. While in normal tissue it is necessary for tissue repair, in tumour tissue, induction of adaptive and innate immune response related to wound healing, stimulates tumour cell survival, angiogenesis and extravasation of circulating tumour cells. It has become evident that certain types of immune response and immune cells can promote tumour progression more than others. In this review, we focus on current knowledge on carcinogenesis and promotion of cancer growth induced by inflammatory processes.     

The Interplay between Myc and CTP Synthase in Drosophila

CTP synthase (CTPsyn) is essential for the biosynthesis of pyrimidine nucleotides. It has been shown that CTPsyn is incorporated into a novel cytoplasmic structure which has been termed the cytoophidium. Here, we report that Myc regulates cytoophidium formation during Drosophila oogenesis. We have found that Myc protein levels correlate with cytoophidium abundance in follicle epithelia. Reducing Myc levels results in cytoophidium loss and small nuclear size in follicle cells, while overexpression of Myc increases the length of cytoophidia and the nuclear size of follicle cells. Ectopic expression of Myc induces cytoophidium formation in late stage follicle cells. Furthermore, knock-down of CTPsyn is sufficient to suppress the overgrowth phenotype induced by Myc overexpression, suggesting CTPsyn acts downstream of Myc and is required for Myc-mediated cell size control. Taken together, our data suggest a functional link between Myc, a renowned oncogene, and the essential nucleotide biosynthetic enzyme CTPsyn.     

The Interplay of Light and Oxygen in the Reactive Oxygen Stress Response of Chlamydomonas reinhardtii Dissected by Quantitative Mass Spectrometry

Light and oxygen are factors that are very much entangled in the reactive oxygen species (ROS) stress response network in plants, algae and cyanobacteria. The first obligatory step in understanding the ROS network is to separate these responses. In this study, a LC-MS/MS based quantitative proteomic approach was used to dissect the responses of Chlamydomonas reinhardtii to ROS, light and oxygen employing an interlinked experimental setup. Application of novel bioinformatics tools allow high quality retention time alignment to be performed on all LC-MS/MS runs increasing confidence in protein quantification, overall sequence coverage and coverage of all treatments measured. Finally advanced hierarchical clustering yielded 30 communities of co-regulated proteins permitting separation of ROS related effects from pure light effects (induction and repression). A community termed redox^II was identified that shows additive effects of light and oxygen with light as the first obligatory step. Another community termed 4-down was identified that shows repression as an effect of light but only in the absence of oxygen indicating ROS regulation, for example, possibly via product feedback inhibition because no ROS damage is occurring. In summary the data demonstrate the importance of separating light, O₂ and ROS responses to define marker genes for ROS responses. As revealed in this study, an excellent candidate is DHAR with strong ROS dependent induction profiles.Life originated in an environment in which the atmosphere was reducing. More than 2.2 Gyr ago, photosynthetic bacteria managed to extract electrons from water, thereby releasing oxygen (O₂) as a side product (1). Although molecular O₂ is a triplet state (³O₂), and is thus kinetically inhibited, its related reactive oxygen species (ROS)¹, i.e., superoxide (O₂^•−), peroxides (ROOR), singlet oxygen (¹O₂), and hydroxyl radicals (HO^•) are not. Nevertheless, molecular O₂ itself oxidizes biomolecules, for example, thiol groups, albeit at a much slower rate. The fundamental change in environment and the appearance of O₂ and ROS triggered the biggest mass extinction ever seen on Earth (2, 3). Soon after, the much more efficient O₂ based metabolism (compared with fermentation) lead to an evolutionary explosion (4). Today, cells obtain energy from reduced organic molecules through O₂ based respiration.In the past ROS were associated with cellular stress but strong evidence points toward a cellular ROS network that keeps ROS production and ROS scavenging in tight balance to ensure the maintenance of the cellular redox homeostasis and protection against ROS stress (5, 6). An imbalance in this network has been associated with a wide array of human diseases such as cancer (7), neurodegeneration (8), Keshan disease (9), and many others (see also review (6)), although arguments have been brought forward that the origin of some diseases is not directly linked to ROS and that ROS are more likely to be the result of deteriorating cells (10). In any case, the cellular ROS network response to ROS stress is implicated in the progress of these diseases and understanding the network dynamics will have a significant impact in medicine.Equally important, reduced ROS capacity or imbalance in the ROS network results in decreased crop yields and simple attempts to increase production yields by increasing ROS scavenging capacities in plants failed because those plants lost their ability to mount a defense against pathogens efficiently by the hypersensitive reaction (11), which implicates intended localized high yield ROS production. On the other hand Chang et al. could show that the knock-out of glutathione peroxidase 7 (gpx7), i.e., reducing ROS scavenging capacity, leads to an increased pathogen resistance but, unfortunately, to an increased photosensitivity as well (12), thus resulting in reduced crop production. The quintessence is that plants require the ability to produce sufficient amounts of ROS as part of their defense mechanism yet require some ROS scavenging capacity because photosynthetic growth inevitably produces damaging ROS. In order to effectively mount a hypersensitivity defense reaction, the ROS scavenging capacities have to be suppressed. Thus understanding the ROS network is an important global issue in the light of hunger in some parts of the world and the need for biofuels. Elucidating the key players of the ROS network will allow high production crop plants to be designed.It seems clear that the ROS network, its dynamics and homeostasis are poorly understood. Understanding how to evaluate the ROS balance and how to restore ROS balance within a cell would have a strong impact on a medical and agricultural level. To put it in the words of Barry Halliwell: “the likely clinical value of ‘antioxidant therapy’ will depend on how well the exact role of reactive oxygen species,” i.e., the ROS network, “is known” (13).ROS can be divided into two classes, i.e., H₂O₂ and ¹O₂ based ones. Especially in plants, algae, and cyanobacteria, it is now widely accepted that the signaling pathways of H₂O₂ (14) and ¹O₂ (15) are complex and entangled (16, 17) simply through the nature of their production, i.e., via an active photosynthetic electron transport chain. However, there have been reports that clearly show the independence of H₂O₂ and ¹O₂ mediated responses (see e.g. (18, 19)). In Arabidopsis thaliana the ROS network, in particular the ¹O₂ aspect has been widely studied, but comprehensive proteomic studies are still required. The A. thaliana flu mutant was used to reveal ¹O₂ related retrograde signaling. The flu mutant accumulates protochlorophyllide when grown in the dark, and seedlings bleach and die whereas mature plants stop growing when transferred into light (20). ¹O₂ production yielded an induction of distinct genes and these differed significantly from genes induced by H₂O₂ (15). Apel and co-workers identified the chloroplast localized EXECUTER1/2 proteins as key players in ¹O₂ retrograde signaling (18, 21), highlighting that specific ¹O₂ induced signals trigger programmed cell death (PCD) rather than ROS induced damage. A flu-like gene (flp) was identified in Chlamydomonas reinhardtii, and its gene product FLP in its two splicing variants was shown to be involved in the chlorophyll biosynthesis (22). Regulation of FLPs were suggested to occur via light and retrograde plastid signals (22). The specific ¹O₂ signaling mechanism in A. thaliana was further extended by Ramel et al. (23). The authors could show that ¹O₂ induced damage to β-carotene, a major component in a ROS defense strategy, yields β-cyclocitral, which when produced and applied exogenously triggers a selective ¹O₂ response, similar to the one reported by Apel and co-workers when describing the effects of the flu mutant (15, 18, 21). However, the signaling pathways involving EXECUTER and β-cyclocitral show more and more independent features (see e.g. Lundquist et al. (24)).ROS production is an inevitable part of the oxygenic photosynthesis and thus can be controlled noninvasively by light intensities. This is why plants, algae, and cyanobacteria offer a unique opportunity to investigate the ROS network. However, in plants the majority of ROS is produced in the chloroplast requiring O₂ as educt and the presence of light. Therefore, careful separation of the light, O₂, and ROS responses is required. As a consequence, simple high light/low light comparisons are overshadowed by additional ROS production, and vice versa. A classical example is HSP70A in C. reinhardtii, which was originally reported to be light regulated (25) and later proven to be regulated by ROS (19), via two promoters that react specifically on H₂O₂ and ¹O₂, to be precise.We have devised an experimental setup, which allows the ROS, high light/low light (HL/LL) and aerobic/anaerobic (AR/AN) responses to be dissected on a proteome level using metabolic labeling and quantitative proteomics. We used an interlinked experimental setup that connects all four possible treatments in such a way that each treatment is compared with two other treatments. This offers a strong internal control because the changes in protein levels comparing two not directly connected treatments can be measured by two independent estimates. MS data was analyzed employing high quality retention time alignment to increase overall confidence in protein quantification, increase protein sequence coverage and increase coverage of all conditions. PyGCluster, a novel hierarchical clustering approach (26) was used to identify communities of proteins that are coregulated. Five communities/expression profiles are discussed: a) light and O₂ dependent induction, i.e., potential ROS related regulations, b) a novel regulation type, which shows induction of protein expression influenced additively by light and O₂, but with light as the obligatory first step, c) light related induction (O₂ independent), d) light dependent repression (O₂ independent), and e) light dependent repression in the absence of O₂, which might be a regulation linked to feedback inhibition by for example, molecules that are normally damaged by ROS.     

Strong Interplay between Sodium and Oxygen in Kesterite Absorbers: Complex Formation,Incorporation, and Tailoring Depth Distributions

Sodium and oxygen are prevalent impurities in kesterite solar cells. Both elements are known to strongly impact performance of the kesterite devices and can be connected to efficiency improvements seen after heat treatments. The sodium distribution in the kesterite absorber is commonly reported, whereas the oxygen distribution has received less attention. Here, a direct relationship between sodium and oxygen in kesterite absorbers is established using secondary ion mass spectrometry and explained by defect analyses within the density functional theory. The calculations reveal a binding energy of 0.76 eV between the substitutional defects Na_Cu and O_S in the nearest neighbor configuration, indicating an abundance of Na? O complexes in kesterite absorbers at relevant temperatures. Oxygen incorporation is studied by introducing isotopic ¹⁸O at different stages of the Cu₂ZnSnS₄/Mo/soda‐lime glass baseline processing. It is observed that oxygen from the Mo back contact and contaminations during the sulfurization are primary contributors to the oxygen distribution. Indeed, unintentional oxygen incorporation leads to immobilization of sodium. This results in a strong correlation between sodium and oxygen, in excellent agreement with the theoretical calculations. Consequently, oxygen availability should be monitored to optimize postdeposition heat treatments to control impurities in kesterite absorbers and ultimately, the solar cell efficiency.     

Egg-Mass Size and Cell Size: Effects of Temperature on Oxygen Distribution

Two processes strongly influence the distribution of oxygenwithin egg masses and cells: the supply of oxygen by diffusionand the consumption of oxygen by embryos and mitochondria. Theseprocesses are differentially sensitive to temperature. The diffusioncoefficient of oxygen depends only weakly on temperature, havinga Q₁₀ of approximately 1.4. In contrast, the consumption ofoxygen depends strongly on temperature, having a Q₁₀ between1.5 and 4.0. Thus, at higher temperatures, the ratio of oxygensupply to demand decreases. I show, by extending a model ofoxygen distribution within metabolizing spheres, that maximalegg-mass sizes and cell sizes are predicted to be smaller athigher temperatures. For egg masses, definitive data are notyet available. For ectothermic cells, this prediction appearsto be supported; cells from a variety of ectothermic organisms,unicellular and multicellular, are smaller when the cells areproduced at warmer temperatures. Establishing a specific connectionbetween this pattern and oxygen distributions requires demonstrationof (1) oxygen concentration gradients within metabolizing spheresand (2) central oxygen concentrations low enough to affect function.Egg masses from a variety of taxa show steep oxygen concentrationgradients and often are severely hypoxic or anoxic in centrallocations. Severe hypoxia appears capable of retarding developmentor killing embryos. Similar kinds of data for ectothermic cellshave not yet been collected, but the literature on oxygen gradientswithin mammalian cells suggests that intracellular gradientsmay be important.     

The Effect of Oxygen Concentration on Photosynthesis in Higher Plants

The influence of oxygen concentration in the range 0–21% on photosynthesis in intact leaves of a number of higher plants has been investigated. Photosynthetic Co₂ fixation of higher plants is markedly inhibited by oxygen in concentrations down to less than 2%. The inhibition increases with oxygen concentration and is about 30% in an atmosphere of 21% O₂ and 0.03% Co.₂. Undoubtedly, therefore, oxygen in normal air exerts a strong inhibitory effect on photosynthetic Co₂ fixation of land plants under natural conditions. The inhibitory effect of oxygen is rapidly produced and fully reversible. The degree of inhibition is independent of light intensity. The quantum yield for Co₂ fixation, i.e. the slope of the linear part of the curve for Co₂ uptake versus absorbed quanta, is inhibited to the same degree as the light saturated rate at all oxygen concentrations studied. Diverse species of higher plants, varying greatly in photosynthetic response to light intensity and Co₂ concentration, and with light saturated roles of Co₂ fixation differing by a factor of more than 10 times, show a remarkable similarity in their response to oxygen concentration. By contrast, when studied under the same conditions as the higher plants, the green algae Chlorella and Ulva did not show-any measurable inhibition of photosynthetic Co₂ fixation. Similarity, the increase in fluorescence intensity with increasing oxygen concentrations found in higher plants also was not seen in Chlorella. The present results, together with previous data on the photosynthetic response of algae to oxygen concentration, indicate that the photosynthetic apparatus of higher plants differs considerably from that of algae in its sensitivity to oxygen. The inhibitory effect of oxygen on photosynthetic Co₂ fixation in higher plants is somewhat higher at wavelengths which excite preferentially photosystem I. Also, the Emerson enhancement of Co₂ fixation measured when a far red beam of low intensity is imposed on a background of red light is greater under low oxygen concontrution than under air. Measurements of reversible light-induced absorbance changes reveal that the change at 591 nm, probably caused by pla.stocyanin, is affected by oxygen concentration only if photosystem II is excited. the reducing effect on plastocyanin, caused by excitation of this system, decreases with increasing oxygen concentration. From these results it is suggested that a possible site of the inhibition by oxygen is in the electron carrier chain between the two photosystems. Oxygen might act as an electron acceptor at this site, causing reducing power to react back with molecular oxygen. However, this hypothesis does not account for equal inhibitions of the quantum yield and the light saturated rate of photosynthetic CO₂ uptake. Through the photosynthetic process plants take up carbon dioxide and evolve oxygen. The present high concentration of molecular oxygen in the atmosphere is generally considered to have arisen from the activity of photo-synthetic organisms. The effect of oxygen concentration would seem, therefore, to he a problem of great interest, not only in the field of the biophysics and biochemistry of photosynthesis, but in ecology and other branches of biology as well. It was discovered by Warburg (1920) that high concentrations of oxygen inhibit the rate of photosynthetic oxygen evolution in the unicellular alga Chlorella. Since then, it has been confirmed by various authors that oxygen cconcentrations in the range 21–100 per cent have a marked inhibitory effect on photosynthesis, particularly at saturating light intensities. There is some evidence that under conditions when carbon dioxide concentration limits photosynthesis, the inhibition may become obvious even in 21 per cent oxygen. The inhibition has not been considered to operate at low light intensities. A review on the subject has been given by Turner and Brittain (1962). Various hypotheses have been put forward to explain the inhibitory effect of oxygen, commonly referred to as the Warhurg effect. Some authors favor the idea of enzyme inhibition; Turner et al. (1958) that one or more enzymes of the carbon reduction cycle are inactivated by oxygen: lirianlals (1962) that enzymes of the oxygen-evolving complex are inhihited. Other hypotheses concern back-reactions in which molecular oxygen is taken up, thus reversing the photosynthetic process. These reactions include photo-oxidation, photorespiration, and the Mehler reaction (Tamiya et al., 1957). At present, there is no generally accepted hypothesis explaining the effect. The often conflicting results on which these hypotheses were based have been obtained mostly on algae. The first observation of an inhibitory effect on photosynthesis in a higher plant was made hy McAlister and Myers (1940) in wheat leaves. They found that the photosyntlietic CO₂ uptake was markedly lower in air than in an atmosphere of about 0.5 per cent oxygen. At the CO₂ concentration used (0.03%) the inhibition was present both at high and moderate light intensities. No data were obtained at low light intensities. Although the study of the effect of oxygen concentration on photosynthesis in higher plants would seem to be of great interest, particularily since the natural environment of most land plants is an atmosphere with an oxygen content of 21 per cent, it has attracted very little attention. To the author's knowledge no thorough investigation on the subject has been published. The present investigalion is directed toward elucidatirng the photosynthetic response of higher plants to oxygen concentrations up to that of normal air. Data are presented showing that the photosynthetic CO₂ fixation in intact leaves of higher plants, regardless of light intensity, is strongly inhibited by oxygen in normal air, and that the pholosynthetic response to oxygen differs considerably from that of green algae. The present investigalion is directed toward elucidatirng the photosynthetic response of higher plants to oxygen concentrations up to that of normal air. Data are presented showing that the photosynthetic CO₂ fixation in intact leaves of higher plants, regardless of light intensity, is strongly inhibited by oxygen in normal air, and that the pholosynthetic response to oxygen differs considerably from that of green algae.     

Interplay between low-temperature pathways and light reduction

Low temperature is one of the major factors that adversely affect crop yields by causing restraints on plant growth and productivity. However, most temperate plants have the ability to acclimate to cooler temperatures. Cold acclimation is a process which increases the freezing tolerance of an organism after exposure to low, non-freezing temperatures. The main trigger is a decrease in temperature levels, but light reduction has also been shown to have an important impact on acquired tolerance. Since the lowest temperatures are commonly reached during the night hours in winter time and is an annually recurring event, a favorable trait for plants is the possibility of sensing an imminent cold period. Consequently, extensive crosstalk between light- and temperature signaling pathways has been demonstrated and in this review interesting interaction points that have been previously reported in the literature are highlighted.Key words: cold acclimation, light-reduction, signaling pathways, photoperiodism, circadian clock, light quality     

Interplay between viruses and host mRNA degradation

Messenger RNA degradation is a fundamental cellular process that plays a critical role in regulating gene expression by controlling both the quality and the abundance of mRNAs in cells. Naturally, viruses must successfully interface with the robust cellular RNA degradation machinery to achieve an optimal balance between viral and cellular gene expression and establish a productive infection in the host. In the past several years, studies have discovered many elegant strategies that viruses have evolved to circumvent the cellular RNA degradation machinery, ranging from disarming the RNA decay pathways and co-opting the factors governing cellular mRNA stability to promoting host mRNA degradation that facilitates selective viral gene expression and alters the dynamics of host–pathogen interaction. This review summarizes the current knowledge of the multifaceted interaction between viruses and cellular mRNA degradation machinery to provide an insight into the regulatory mechanisms that influence gene expression in viral infections. This article is part of a Special Issue entitled: RNA Decay mechanisms.     

Interplay between endogenous and exogenous human retroviruses

In humans, retroviruses thrive more as symbionts than as parasites. Apart from the only two modern exogenous human retroviruses (human T-cell lymphotropic and immunodeficiency viruses; HTLV and HIV, respectively), ~8% of the human genome is occupied by ancient retroviral DNA [human endogenous retroviruses (HERVs)]. Here, we review the recent discoveries about the interactions between the two groups, the impact of infection by exogenous retroviruses on the expression of HERVs, the effect of HERVs on the pathogenicity of HIV and HTLV and on the severity of the diseases caused by them, and the antiviral protection that HERVs can allegedly provide to the host. Tracing the crosstalk between contemporary retroviruses and their endogenized ancestors will provide better understanding of the retroviral world.     

Interplay between MAMP-triggered and SA-mediated defense responses

Plants respond to pathogen infection using an innate immune system with at least two distinct recognition mechanisms. One mechanism recognizes microbe-associated molecular patterns (MAMPs). The other is based on resistance (R) genes and specifically recognizes certain pathogen virulence factors, including those delivered through the type III secretion system (TTSS) of bacteria. Salicylic acid (SA)-mediated responses are an important part of the R gene-mediated defense. Substantial overlaps between MAMP-triggered and SA-mediated responses have been reported. However, interactions between MAMP-triggered and SA-mediated signaling mechanisms have not been well documented. Here we report intimate interactions between MAMP-triggered and SA-mediated signaling. We found that SA accumulated at a higher level 6 h after treatment with a MAMP, flg22 or inoculation with Pseudomonas syringae pv. tomato DC3000 ( Pst DC3000) hrcC mutant, which is deficient in TTSS function. Disruptions of SA signaling components, such as SID2 and PAD4 , strongly affected MAMP-triggered responses monitored by expression profiling. We found two groups of genes that were induced by Pst DC3000 hrcC in an SA-dependent manner. One group was SID2 -dependent at all time points, whereas the other was SID2 -independent at early time points and SID2 -dependent at later time points. Thus, the expression of the latter genes responds to MAMPs through both SA-independent and SA-dependent signaling mechanisms. Strong resistance to Pst DC3000 hrcC was dependent on SA signaling. These results indicate that the SA increase triggered by MAMPs is a major component of the MAMP-triggered signaling mechanism, explaining the overlapping spectra of MAMP-triggered and SA-mediated responses.