Coexistence properties of some predator-prey systems under constant rate harvesting and stocking

The global behaviour of a class of predator-prey systems, modelled by a pair of non-linear ordinary differential equations, under constant rate harvesting and/or stocking of both species, is presented. Theoretically possible structures and transitions are developed and validated by computer simulations. The results are presented as transition loci in the F-G (prey harvest rate-predator harvest rate) plane.Sponsored by the United States Army under Contract No. DAAG29-80-C-0041 and in part by NSERC of Canada, Grant No. 67-3138The authors wish to thank Mr. Al MacKenzie of the Department of Electrical Engineering, University of British Columbia, for preparing the figures in this paper.     

Quantitative study of state 1-state 2 transitions in broken chloroplasts—comparison to in-vivo properties

A detailed quantitative study was conducted on state 1-state 2 transition and its reversal in broken chloroplasts by modulated fluorimetry. The characteristics of the transition obtained supported other previous in-vitro findings. More importantly, a very close quantitative similarity was obtained under suitable conditions to previous in-vivo studies, particularly in approaching a constancy of F_m/F₀ during the transition and the equality of the fractional change of these fluorescence parameters with the calculated light distribution fraction to PS II. This confirms that in broken chloroplasts too, the state transitions involve reciprocal changes in the absorption cross-sections of PS II and PS I.Abbreviations AMP-PNP adenylylimidodiphosphate - LHC II light harvesting chlorophyll a/b-protein complex - MeV methylviologen     

Phycobilisome diffusion is required for light-state transitions in cyanobacteria

Phycobilisomes are the major accessory light-harvesting complexes of cyanobacteria and red algae. Studies using fluorescence recovery after photobleaching on cyanobacteria in vivo have shown that the phycobilisomes are mobile complexes that rapidly diffuse on the thylakoid membrane surface. By contrast, the PSII core complexes are completely immobile. This indicates that the association of phycobilisomes with reaction centers must be transient and unstable. Here, we show that when cells of the cyanobacterium Synechococcus sp. PCC7942 are immersed in buffers of high osmotic strength, the diffusion coefficient for the phycobilisomes is greatly decreased. This suggests that the interaction between phycobilisomes and reaction centers becomes much less transient under these conditions. We discuss the possible reasons for this. State transitions are a rapid physiological adaptation mechanism that regulates the way in which absorbed light energy is distributed between PSI and PSII. Immersing cells in high osmotic strength buffers inhibits state transitions by locking cells into whichever state they were in prior to addition of the buffer. The effect on state transitions is induced at the same buffer concentrations as the effect on phycobilisome diffusion. This implies that phycobilisome diffusion is required for state transitions. The main physiological role for phycobilisome mobility may be to allow such flexibility in light harvesting.     

Kinetics and mechanism of transitions involving the lamellar, cubic, inverted hexagonal, and fluid isotropic phases of hydrated monoacylglycerides monitored by time-resolved X-ray diffraction

A study of the dynamics and mechanism of the various thermotropic phase transitions undergone by the hydrated monoacylglycerides monoolein and monoelaidin, in the temperature range of 20-120 degrees C and from 0 to 5 M NaCl, has been undertaken. Measurements were made by using time-resolved X-ray diffraction at the Cornell High-Energy Synchrotron Source. The lamellar chain order/disorder, lamellar/cubic (body centered, space group No. 8), cubic (body centered, No. 8)/cubic (primitive, No. 4), cubic (body centered, No. 12)/cubic (primitive, No. 4), cubic (primitive, No. 4)/fluid isotropic, cubic (body centered, No. 12)/inverted hexagonal, cubic (primitive, No. 4)/inverted hexagonal, and hexagonal/fluid isotropic transitions were examined under active heating and passive cooling by using a jump in temperature to effect phase transformation. All of the transitions with the exception of the cubic (body centered, No. 8)/cubic (primitive, No. 4) and the cubic (body centered, No. 12)/cubic (primitive, No. 4) cooling transitions were found (1) to be repeatable, (2) to be reversible, and (3) to have an upper bound on the transit time (time required to complete the transition) of less than or equal to 3 s. The shortest transit times recorded for the various phase changes in the heating direction were less than or equal to 1.9 (lamellar chain melting), less than or equal to 1.7 [lamellar liquid crystal/cubic (body (body centered, No. 8)], less than or equal to 0.5 [cubic (body centered, No. 8)/cubic (primitive, No. 4)], less than or equal to 0.9 [cubic (primitive, No. 4)/hexagonal], less than or equal to 1.3 [cubic (body centered, No. 12)/cubic (primitive, No. 4) and cubic (body centered, No. 12)/hexagonal], and less than or equal to 0.6 s (hexagonal/fluid isotropic). For the exceptions noted above, the transitions were slow with transit times ranging from 0.5 to 30 min and displayed pronounced hysteresis and/or undercooling. Regardless of the direction of the transitions, all but one appear to be two state to within the sensitivity limits of the time-resolved method. In the case of the lamellar liquid crystal/cubic (body centered, No. 8) transition a stable intermediate of unknown identity was apparent. In addition to the time-resolved measurements, data were obtained on the stability of the various phases in the temperature range of 20-120 degrees C and from 0 to 5 M NaCl. In the case of fully hydrated monoolein, high salt strongly favors the hexagonal over the cubic (body centered, No. 8) phase and slightly elevates the hexagonal/fluid isotropic transition temperature.(ABSTRACT TRUNCATED AT 400 WORDS)     

Balance of power: a view of the mechanism of photosynthetic state transitions.

Photosynthesis in plants involves photosystem I and photosystem II, both of which use light energy to drive redox processes. Plants can balance the distribution of absorbed light energy between the two photosystems. When photosystem II is favoured, a mobile pool of light harvesting complex II moves from photosystem II to photosystem I. This short-term and reversible redistribution is known as a state transition. It is associated with changes in the phosphorylation of light harvesting complex II but the regulation is complex. Redistribution of energy during state transitions depends on an altered binding equilibrium between the light harvesting complex II-photosystem II and light harvesting complex II-photosystem I complexes.     

Stability regions and transition phenomena for harvested predator-prey systems

Summary We analyze the global behaviour of a predator-prey system under constant-rate predator harvesting, showing how to classify the possibilities and determine the region of asymptotic stability by a combination of relatively elementary theoretical methods and computer simulations.Research sponsored in part by the National Research Council of Canada. Grant No. A-3138     

Elucidation of an a and L system for amino acid transport in the human lymphoblast using a membrane filtration technique

Summary Optimum conditions have been established for the measurement of amino acid transport by human lymphoblastoid cell lines using a membrane-filtration technique. The parameters we found to be important for the reproducibility of the method are: the types and combination of filters, the strength of the vacuum applied to the filters and the density of the cultures at the time of harvesting and during uptake and filtration. We found that bovine serum albumin added to phosphate buffered saline (PBS) glucose in which the cells are washed, resuspended and assayed is essential for the maintenance of viability, the prevention of clumping and the retention of the accumulated amino acid. Using this procedure we have characterized two transport systems for the neutral amino acids; an A and an L system, which are similar but not identical to the A and L systems characterized in rodent cell lines. These A and L systems have characteristically lower Km's and Vm's for alanine and phenylalanine, when compared to rodent cell lines. In addition, we find α-AIB to be a poor competitor of alanine and phenylalanine uptake. This work was supported by Grant No. CA18644, awarded by the National Cancer Institute, Department of Health, Education and Welfare, and from a grant from the National Science Foundation under Grant No. PCM 76-24328.     

The Light-Harvesting Chlorophyll a/b Binding Proteins Lhcb1 and Lhcb2 Play Complementary Roles during State Transitions in Arabidopsis

Photosynthetic light harvesting in plants is regulated by phosphorylation-driven state transitions: functional redistributions of the major trimeric light-harvesting complex II (LHCII) to balance the relative excitation of photosystem I and photosystem II. State transitions are driven by reversible LHCII phosphorylation by the STN7 kinase and PPH1/TAP38 phosphatase. LHCII trimers are composed of Lhcb1, Lhcb2, and Lhcb3 proteins in various trimeric configurations. Here, we show that despite their nearly identical amino acid composition, the functional roles of Lhcb1 and Lhcb2 are different but complementary. Arabidopsis thaliana plants lacking only Lhcb2 contain thylakoid protein complexes similar to wild-type plants, where Lhcb2 has been replaced by Lhcb1. However, these do not perform state transitions, so phosphorylation of Lhcb2 seems to be a critical step. In contrast, plants lacking Lhcb1 had a more profound antenna remodeling due to a decrease in the amount of LHCII trimers influencing thylakoid membrane structure and, more indirectly, state transitions. Although state transitions are also found in green algae, the detailed architecture of the extant seed plant light-harvesting antenna can now be dated back to a time after the divergence of the bryophyte and spermatophyte lineages, but before the split of the angiosperm and gymnosperm lineages more than 300 million years ago.     

Absence of the Lhcb1 and Lhcb2 proteins of the light-harvesting complex of photosystem II - effects on photosynthesis,grana stacking and fitness

We have constructed Arabidopsis thaliana plants that are virtually devoid of the major light-harvesting complex, LHC II. This was accomplished by introducing the Lhcb2.1 coding region in the antisense orientation into the genome by Agrobacterium-mediated transformation. Lhcb1 and Lhcb2 were absent, while Lhcb3, a protein present in LHC II associated with photosystem (PS) II, was retained. Plants had a pale green appearance and showed reduced chlorophyll content and an elevated chlorophyll a/b ratio. The content of PS II reaction centres was unchanged on a leaf area basis, but there was evidence for increases in the relative levels of other light harvesting proteins, notably CP26, associated with PS II, and Lhca4, associated with PS I. Electron microscopy showed the presence of grana. Photosynthetic rates at saturating irradiance were the same in wild-type and antisense plants, but there was a 10-15% reduction in quantum yield that reflected the decrease in light absorption by the leaf. The antisense plants were not able to perform state transitions, and their capacity for non-photochemical quenching was reduced. There was no difference in growth between wild-type and antisense plants under controlled climate conditions, but the antisense plants performed worse compared to the wild type in the field, with decreases in seed production of up to 70%.     

The Beverton–Holt model with periodic and conditional harvesting

In this theoretical study, we investigate the effect of different harvesting strategies on the discrete Beverton–Holt model in a deterministic environment. In particular, we make a comparison between the constant, periodic and conditional harvesting strategies. We find that for large initial populations, constant harvest is more beneficial to both the population and the maximum sustainable yield. However, periodic harvest has a short-term advantage when the initial population is low, and conditional harvest has the advantage of lowering the risk of depletion or extinction. Also, we investigate the periodic character under each strategy and show that periodic harvesting drives population cycles to be multiples (period-wise) of the harvesting period.     

The Beverton-Holt model with periodic and conditional harvesting

In this theoretical study, we investigate the effect of different harvesting strategies on the discrete Beverton-Holt model in a deterministic environment. In particular, we make a comparison between the constant, periodic and conditional harvesting strategies. We find that for large initial populations, constant harvest is more beneficial to both the population and the maximum sustainable yield. However, periodic harvest has a short-term advantage when the initial population is low, and conditional harvest has the advantage of lowering the risk of depletion or extinction. Also, we investigate the periodic character under each strategy and show that periodic harvesting drives population cycles to be multiples (period-wise) of the harvesting period.     

Moderate heat stress induces state transitions in Arabidopsis thaliana

The effect of temperature on the photosynthetic machinery is crucial for the fundamental understanding of plant physiology and the bioengineering of heat-tolerant varieties. In our study, Arabidopsis thaliana was exposed to mild (40°C), short-term heat stress in the dark to evaluate the heat-triggered phosphorylation and migration of light harvesting complex (LHC) II in both wild-type (wt) and mutant lacking STN7 kinase. The 77K emission spectra revealed an increase in PSI relative to PSII emission similar to increases observed in light-induced state I to state II transitions in wt but not in stn7 mutant. Immunoblotting results indicated that the major LHCII was phosphorylated at threonine sites under heat stress in wt plants but not in the mutant. These results support the proposition that mild heat stress triggers state transitions in the dark similar to light-induced state transitions, which involve phosphorylation of LHCII by STN7 kinase. Pre-treatment of Arabidopsis leaves with inhibitor DBMIB, altered the extent of LHCII phosphorylation and PSI fluorescence emission suggests that activation of STN7 kinase may be dependent on Cyt b(6)/f under elevated temperatures in dark. Furthermore, fast Chl a transient of temperature-exposed leaves of wt showed a decrease in the F(v)/F(m) ratio due to both an increase in F(o) and a decrease in F(m). In summary, our findings indicate that a mild heat treatment (40°C) induces state transitions in the dark resulting in the migration of phosphorylated LHCII from the grana to the stroma region.     

Environmentally modulated phosphorylation and dynamics of proteins in photosynthetic membranes

Recent advances in vectorial proteomics of protein domains exposed to the surface of photosynthetic thylakoid membranes of plants and the green alga Chlamydomonas reinhardtii allowed mapping of in vivo phosphorylation sites in integral and peripheral membrane proteins. In plants, significant changes of thylakoid protein phosphorylation are observed in response to stress, particularly in photosystem II under high light or high temperature stress. Thylakoid protein phosphorylation in the algae is much more responsive to the ambient redox and light conditions, as well as to CO(2) availability. The light-dependent multiple and differential phosphorylation of CP29 linker protein in the green algae is suggested to control photosynthetic state transitions and uncoupling of light harvesting proteins from photosystem II under high light. The similar role for regulation of the dynamic distribution of light harvesting proteins in plants is proposed for the TSP9 protein, which together with other recently discovered peripheral proteins undergoes specific environment- and redox-dependent phosphorylation at the thylakoid surface. This review focuses on the environmentally modulated reversible phosphorylation of thylakoid proteins related to their membrane dynamics and affinity towards particular photosynthetic protein complexes.     

科尔沁草甸草地放牧和割草条件下土壤种子库研究

研究了科尔沁沙地草甸草地在放牧、割草两种利用方式下的土壤种子库的大小、组成及其与地上植被的关系．结果表明,在自由放牧下,土壤种子库密度为6158±1647粒·m^-2,在割草利用下,土壤种子库密度为8312±2540粒·m^-2．放牧干扰下,种子库组成以矮小、短命的一年生植物为主,其中一二年生植物占81．66％,占比例最大的前4种植物分别为虎尾草(38．55％)、灰绿藜(15．42％)、毛马唐(14．95％)和狗尾草(9．83％),多年生植物种子密度仅为1129±302粒·m^-2;割草干扰下,一二年生植物占68．08％,其中狗尾草占52．7％;而割草地多年生植物种子密度为2653±811粒·m^-2．放牧地土壤种子库密度与地上植物相关不显著;割草地土壤种子库密度与地上植物多度显著相关(r=0．76,P<0．01)．放牧地种子库的Shannon—Wiener指数、丰富度指数分别为2．96和2．98,明显小于割草地的3．10和5．09,表明自由放牧更易使物种多样性下降．     

Harvesting and culture of epithelial cells from hollow organs of the female reproductive system

Summary The technique of trypsinization of cells lining the hollow organs applied to the female reproductive tract has proven successful in harvesting living cells which produced luxuriant cultures in vitro.Elements from the oviduct, endometrial cavity and vagina were obtained from 6 rabbits, 2 dogs and 6 surgical specimens by means of bathing the lumen of the organs with the use of a 0.5% solution of trypsin in calcium and magnesium free Gey's balanced salt solution for 30 to 40 min, employing continued gentle agitation provided by a syringe.Epithelial cells so harvested were grown in Rose chambers and in T-30 flasks for as long as 30 days, when they were fixed for study with special stains.No fibrocytic contamination of the cultures was observed.Further applications of the method are under investigation and are briefly outlined in the discussion.This work was performed in the Department of Cellular Biology, Pasadena Foundation for Medical Research, Pasadena, California, and aided in part by a grant from the U. S. Public Health Service, No. 2 G 279 and from the U. S. Army Medical Research and Development Command, Department of the Army, under Research Grant No. DA-MD-49-193-63-G80 administered by C. M. Pomerat.     

Increasing GPI-anchored protein harvest concentrations from suspension and porous microcarrier CHO cell cultures

Chinese hamster ovary (CHO) cells expressing the human melanoma tumour antigen, p97, were used to develop a controlled release process for the production of recombinant glycosyl-phosphatidylinositol (GPI) anchored proteins. The cells were cultured either in suspension or immobilized on porous microcarriers and p97 was selectively cleaved from the cell surface by the bacterial enzyme, phosphatidylinositol-phospholipase C (PI-PLC). The kinetics of p97 cleavage from the cell surface by PI-PLC was shown to be approximated by Michaelis-Menten kinetics. The recovered p97 concentrations were increased by reusing the PI-PLC enzyme solution to harvest multiple batches of cells. A convenient PI-PLC assay was developed to monitor the harvesting process and to determine the stability of PI-PLC under harvesting conditions. Although the Pl-PLC was stable under harvesting conditions, it rapidly adsorbed to the cell surface and was depleted from the reused enzyme solution. In order to maintain PI-PLC activity, it was necessary to add fresh PI-PLC to the reused enzyme solution before harvesting a fresh batch of cells. The maximum p97 concentration that could be obtained from harvesting CHO cells cultured on porous microcarriers was limited by the dilution effects of sample removal, adding fresh PI-PLC and liquid associated with settled microcarriers. A model was developed that adequately predicted the p97 concentration after each harvest and the maximum p97 concentration that could be achieved by this harvesting method. The dilution effects were minimized by harvesting from centrifuged suspension culture cells and the harvested p97 concentration was increased by over sixfold to 0.64 mg/mL. (c) 1997 John Wiley & Sons, Inc. Biotechnol Bioeng 55: 136-147, 1997.     

Environmentally modulated phosphorylation and dynamics of proteins in photosynthetic membranes

Recent advances in vectorial proteomics of protein domains exposed to the surface of photosynthetic thylakoid membranes of plants and the green alga Chlamydomonas reinhardtii allowed mapping of in vivo phosphorylation sites in integral and peripheral membrane proteins. In plants, significant changes of thylakoid protein phosphorylation are observed in response to stress, particularly in photosystem II under high light or high temperature stress. Thylakoid protein phosphorylation in the algae is much more responsive to the ambient redox and light conditions, as well as to CO₂ availability. The light-dependent multiple and differential phosphorylation of CP29 linker protein in the green algae is suggested to control photosynthetic state transitions and uncoupling of light harvesting proteins from photosystem II under high light. The similar role for regulation of the dynamic distribution of light harvesting proteins in plants is proposed for the TSP9 protein, which together with other recently discovered peripheral proteins undergoes specific environment- and redox-dependent phosphorylation at the thylakoid surface. This review focuses on the environmentally modulated reversible phosphorylation of thylakoid proteins related to their membrane dynamics and affinity towards particular photosynthetic protein complexes.     

On the option interpretation of rational harvesting planning

We consider the determination of the harvesting strategy maximizing the present expected value of the cumulative yield from the present up to extinction. By relying on a combination of stochastic calculus, ordinary nonlinear programming, and the classical theory of diffusions, we show that if the underlying population evolves according to a logistic diffusion subject to a general diffusion coefficient, then there is a single threshold density at which harvesting should be initiated in a singular fashion. We derive the condition which uniquely determines the threshold and show that harvesting should be initiated only when the option value of further preserving another individual falls below its opportunity cost. In this way, we present a real option interpretation of rational harvesting planning. We also consider the comparative static properties of the value of the harvesting opportunity and state a set of usually satisfied conditions under which increased stochastic fluctuations (demographic or environmental) decrease the expected cumulative yield from harvesting and increase the optimal harvesting threshold, thus postponing the rational exercise of the irreversible harvesting decision. Received: 19 January 1999 / Revised version: 2 July 1999 / Published online: 16 February 2000     

Supramolecular organization and dual function of the IsiA chlorophyll-binding protein in cyanobacteria

A significant part of global primary productivity is provided by cyanobacteria, which are abundant in most marine and freshwater habitats. In many oceanographic regions, however, the concentration of iron can be so low that it limits growth. Cyanobacteria respond to this condition by expressing a number of iron stress inducible genes, of which the isiA gene encodes a chlorophyll-binding protein known as IsiA or CP43'. It was recently shown that 18 IsiA proteins encircle trimeric photosystem I (PSI) under iron-deficient growth conditions. We report here that after prolonged growth of Synechocystis PCC 6803 in an iron-deficient medium, the number of bound IsiA proteins can be much higher than previously known. The largest complexes bind 12-14 units in an inner ring and 19-21 units in an outer ring around a PSI monomer. Fluorescence excitation spectra indicate an efficient light harvesting function for all PSI-bound chlorophylls. We also find that IsiA accumulates in cyanobacteria in excess of what is needed for functional light harvesting by PSI, and that a significant part of IsiA builds supercomplexes without PSI. Because the further decline of PSI makes photosystem II (PSII) increasingly vulnerable to photooxidation, we postulate that the surplus synthesis of IsiA shields PSII from excess light. We suggest that IsiA plays a surprisingly versatile role in cyanobacteria, by significantly enhancing the light harvesting ability of PSI and providing photoprotection for PSII.     

How do disturbances and environmental heterogeneity affect the pace of forest distribution shifts under climate change?

Although it is widely predicted that the geographic distributions of tree species and forest types will undergo substantial shifts in future, modelling approaches used to date are largely unable to project the pace at which forest distributions will respond to environmental change. The expansion and contraction of forest distributions act against considerable demographic inertia in the present composition and size‐structure of forest stands as climate‐induced changes in growth, mortality, and recruitment alter population dynamics through time. We aimed to better understand how shifts in forest distributions reflect long‐term changes in tree demographic rates and population dynamics, and how such shifts are influenced by 1) disturbance from forest harvesting and 2) local environmental heterogeneity. Using a simple, data‐constrained gap model, we simulated regional forest dynamics in the eastern United States over the next 500 yr. We then compared the geographic distributions of five different forest types through time under present and altered climatic conditions, in scenarios that variously included and excluded forest harvesting and environmental heterogeneity. Although we held climate fixed after 100 yr, it took another 160 yr after this for these forest types to collectively experience 90% of their eventual climate‐related distribution gains and losses. Competition strongly affected the nature of responses to climate change. Harvesting accelerated and amplified gains by an early‐successional forest type at the expense of a late‐successional one, but these gains did not occur faster than those for other forest types. Environmental heterogeneity had little effect on distribution gains or losses through time. These findings indicate that forest distributions should respond quite slowly to climate change, with the leading and trailing edges of different forest types shifting over a span of centuries. Disturbances can expedite some transitions, but are unlikely to lead to wholesale changes in forest types in the coming decades.