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The “paradox of enrichment” predicts that increasing the growth rate of the resource in a resource-consumer dynamic system,
by nutrient enrichment, for example, can lead to local instability of the system—that is, to a Hopf bifurcation. The approach
to the Hopf bifurcation is accompanied by a decrease in resilience (rate of return to equilibrium). On the other hand, studies
of nutrient cycling in food webs indicate that an increase in the nutrient input rate usually results in increased resilience.
Here these two apparently conflicting theoretical results are reconciled with a model of a nutrient-limited resource-consumer
system in which the tightly recycled limiting nutrient is explicitly modelled. It is shown that increasing nutrient input
may at first lead to increased resilience and that resilience decreases sharply only immediately before the Hopf bifurcation
is reached. 相似文献
3.
Analysis of the membrane-interacting domain of the sea urchin sperm adhesive protein bindin 总被引:2,自引:0,他引:2
We have investigated the domain of the bindin polypeptide that selectively associates with gel-phase phospholipid vesicles. We found that small trypsin fragments of bindin retain the ability to selectively associate with gel-phase vesicles. The primary amino acid sequence of bindin suggests that these peptides are derived from the central portion of the polypeptide between residues 77 and 126, which is the most hydrophobic region of bindin. We have also employed 3-(trifluoromethyl)-3-(m-[125I]iodophenyl)diazirine (TID) and novel, radioiodinated, photoactivatable derivatives of the polar head group of phosphatidylethanolamine (ASD-PE and ASA-PE) to identify membrane-associated polypeptide segments after the transfer of radiolabel from the probe to the bindin polypeptide. After photolysis, bindin was selectively labeled only from probes incorporated in gel-phase vesicles. The labeling of bindin was much more efficient from the head group probes ASA-PE and ASD-PE (8 and 2% of the total label, respectively) in comparison to the hydrophobic probe TID (less than 0.02% of the total label), suggesting that bindin is localized within the polar part of the bilayer. Protease mapping experiments with V8 protease, trypsin, and endoprotease Lys-C suggest that some of the probe label is distributed along the amino-terminal portion of bindin between residues 1 and 76 and the rest of the label is restricted to the segments between residues 77 and 126 which also selectively bind to gel-phase vesicles. The carboxyl-terminal portion of bindin between residues 127 and 236 is not labeled. 相似文献
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GC Wood 《New Zealand journal of zoology.》2013,40(3):186-195
Abstract The Westland petrel (Procellaria westlandica) is an endemic New Zealand species and one of the very few burrowing seabird species still breeding on mainland New Zealand. It nests only on a series of coastal ridgelines near to Punakaiki on the West Coast of the South Island. Between 2002 and 2005, surveys were undertaken at 28 of the 29 known colonies. The area occupied by the colonies was 73 ha; most colonies had fewer than 50 burrows, but six colonies had 201–500 burrows and four colonies had more than 1000 burrows. We find that the current breeding range of Westland petrel and the location of individual colonies are similar to those reported in both the 1950s and 1970s. Based on total burrow counts at 28 colonies and burrow occupancy rates determined by annual monitoring, the annual breeding population is estimated to be between 2954 and 5137 breeding pairs. 相似文献
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Mark E. Lauer Tibor T. Glant Katalin Mikecz Paul L. DeAngelis F. Michael Haller M. Elaine Husni Vincent C. Hascall Anthony Calabro 《The Journal of biological chemistry》2013,288(1):205-214
The covalent transfer of heavy chains (HCs) from inter-α-inhibitor (IαI) to hyaluronan (HA) via the protein product of tumor necrosis factor-stimulated gene-6 (TSG-6) forms the HC-HA complex, a pathological form of HA that promotes the adhesion of leukocytes to HA matrices. The transfer of HCs to high molecular weight (HMW) HA is a reversible event whereby TSG-6 can shuffle HCs from one HA molecule to another. Therefore, HMW HA can serve as both an HC acceptor and an HC donor. In the present study, we show that transfer of HCs to low molecular weight HA oligosaccharides is an irreversible event where subsequent shuffling does not occur, i.e. HA oligosaccharides from 8 to 21 monosaccharide units in length can serve as HC acceptors, but are unable to function as HC donors. We show that the HC-HA complex is present in the synovial fluid of mice subjected to systemic and monoarticular mouse models of rheumatoid arthritis. Furthermore, we demonstrate that HA oligosaccharides can be used, with TSG-6, to irreversibly shuffle HCs from pathological, HMW HC-HA to HA oligosaccharides, thereby restoring HC-HA matrices from the inflamed joint to their normal state, unmodified with HCs. This process was also effective for HC-HA in the synovial fluid of human rheumatoid arthritis patients (in vitro). 相似文献
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Acceptor specificity of the Pasteurella hyaluronan and chondroitin synthases and production of chimeric glycosaminoglycans 总被引:1,自引:0,他引:1
The hyaluronan (HA) synthase, PmHAS, and the chondroitin synthase, PmCS, from the Gram-negative bacterium Pasteurella multocida polymerize the glycosaminoglycan (GAG) sugar chains HA or chondroitin, respectively. The recombinant Escherichia coli-derived enzymes were shown previously to elongate exogenously supplied oligosaccharides of their cognate GAG (e.g. HA elongated by PmHAS). Here we show that oligosaccharides and polysaccharides of certain noncognate GAGs (including sulfated and iduronic acid-containing forms) are elongated by PmHAS (e.g. chondroitin elongated by PmHAS) or PmCS. Various acceptors were tested in assays where the synthase extended the molecule with either a single monosaccharide or a long chain (approximately 10(2-4) sugars). Certain GAGs were very poor acceptors in comparison to the cognate molecules, but elongated products were detected nonetheless. Overall, these findings suggest that for the interaction between the acceptor and the enzyme (a) the orientation of the hydroxyl at the C-4 position of the hexosamine is not critical, (b) the conformation of C-5 of the hexuronic acid (glucuronic versus iduronic) is not crucial, and (c) additional negative sulfate groups are well tolerated in certain cases, such as on C-6 of the hexosamine, but others, including C-4 sulfates, were not or were poorly tolerated. In vivo, the bacterial enzymes only process unsulfated polymers; thus it is not expected that the PmCS and PmHAS catalysts would exhibit such relative relaxed sugar specificity by acting on a variety of animal-derived sulfated or epimerized GAGs. However, this feature allows the chemoenzymatic synthesis of a variety of chimeric GAG polymers, including mimics of proteoglycan complexes. 相似文献
10.