Annual and Seasonal Variation in the Phyllosphere Bacterial Community Associated with Leaves of the Southern Magnolia (Magnolia grandiflora)

The phyllosphere contains a diverse bacterial community that can be intimately associated with the host plant; however, few studies have examined how the phyllosphere community changes over time. We sampled replicate leaves from a single magnolia (Magnolia grandiflora) tree in the winter of three consecutive years (2007?C2009) as well as during four seasons of 1?year (2008) and used molecular techniques to examine seasonal and year-to-year variation in bacterial community structure. Multivariate analysis of denaturing gradient gel electrophoresis profiles of 16S rRNA gene fragments revealed minimal leaf to leaf variation and much greater temporal changes, with the summer (August 2008) leaf community being most distinct from the other seasons. This was confirmed by sequencing and analysis of 16S rRNA gene clone libraries generated for each sample date. All phyllosphere communities were dominated by Alphaproteobacteria, with a reduction in the representation of certain Beijerinckiaceae during the summer and a concurrent increase in the Methylobacteriaceae being the most significant seasonal change. Other important components of the magnolia phyllosphere included members of the Bacteroidetes, Acidobacteria, and Actinobacteria, with the latter two lineages also showing differences in their representation in samples collected at different times. While the leaf-associated bacterial community sampled at the same time of year in three separate years showed some similarities, generally these communities were distinct, suggesting that while there are seasonal patterns, these may not be predictable from year to year. These results suggest that seasonal differences do occur in phyllosphere communities and that broad-leafed evergreen trees such as M. grandiflora may present interesting systems to study these changes in the context of changing environmental conditions.     

Scent-Marking of Giant Otter in the Southern Pantanal, Brazil

Giant otters live in social groups, consisting of a mating pair and one or two litters. Groups are territorial and mark their territories often with scent-marks. Our objectives were to evaluate the frequencies of marking and over-marking according to the social status of the individuals and to define the different postures used during the marking. We observed four groups, totaling 25 individuals (five alpha males, four alpha females, seven adult females, one adult male and eight juveniles) with group size ranging between four and 13 individuals. The study was conducted between July 2006 and July 2007 in the Vermelho River and in a stretch of the Miranda River, in the Southern Pantanal. We observed the groups for a total of 2006 min and recorded 95 events of marking totaling 84.9 min. Time spent marking varied between groups and ranged from 4.3 to 44.7 min. The alpha males marked more frequently (62% of marking events, 55 min) than the alpha females (17% of marking events, 13.6 min). Of the 59 events of scent-marking by the alpha males, 32 over-marked the marks of other individuals from the group. Of the 16 events of scent-marking of the alpha females, five over-marked that of other females from the same group. When scent-marking, alpha males used the 'stepping' posture most frequently (63%), then 'fore-paw rubbing' (24%), 'latrine use' (7%), and 'body rubbing' (6%). Alpha females used the 'stepping' posture most frequently (65%), then 'latrine use' (19%) and 'fore-paw rubbing' (12%), with only one event of 'body rubbing' observed during marking. Subordinate females used the 'stepping' posture (76%) and 'latrine use' (24%) during marking. Scent-marking can play many roles in mammals and for giant otters, and the main roles appear to be communication of social and sexual status and territorial defense.     

黄猄蚁对大蜜蜂的捕食行为及其潜在影响

捕食作用会对访花昆虫的种群、行为以及植物适合度产生影响,是植物与传粉者相互关系研究中常被忽视的因素.本文报道了黄猄蚁对大蜜蜂的捕食行为,并模拟捕食的关键环节研究了捕食过程对重要访花昆虫行为的影响.结果表明,黄猄蚁能够利用局部环境主动攻击猎物,利用群体合作捕获采集过程中的体型较大的大蜜蜂,捕食威胁是其影响植物-访花者关系的重要机制.大蜜蜂具有感知花上危险的能力,模拟处理的个体会逃离危险的花或植株并在花上留下标记,将危险信息传递给其它个体.其它拜访者对具有危险信号花的采集频次明显减少,采集时间缩短;模拟处理的影响会随时间推移而较快地消失.此外,该实验没有发现大蜜蜂在花上停留采集过程中具有明显的防御行为.     

Stability and Function of Mammalian Lethal Giant Larvae-1 Oncoprotein Are Regulated by the Scaffolding Protein RanBPM

The evolutionarily conserved lethal giant larvae (Lgl) tumor suppressor gene has an essential role in establishing apical-basal cell polarity, cell proliferation, differentiation, and tissue organization. However, the precise molecular mechanism by which the Lgl carries out its function remains obscure. In the current study, we have identified Ran-binding protein M (RanBPM) as a novel binding partner of Mgl-1, a mammalian homolog of Drosophila tumor suppressor protein lethal (2) giant larvae (L(2)gl) by yeast two-hybrid screening. RanBPM seems to act as a scaffolding protein with a modulatory function with respect to Mgl-1. The Mgl-1 and RanBPM association was confirmed by co-immunoprecipitation and GST pull-down experiments. Additionally, expression of RanBPM resulted in inhibition of Mgl-1 degradation, and thereby extended the half-life of Mgl-1. Furthermore, the ability of Mgl-1 activity in cell migration and colony formation assay was enhanced by RanBPM. Taken together, our findings reveal that RanBPM plays a novel role in regulating Mgl-1 stability and contributes to its biological function as a tumor suppressor.     

Xylem Transport and the Negative Pressures Sustainable by Water

Experimental measurements of water's ability to sustain negativepressures are reviewed with special emphasis on the relevanceof the results to xylem transport. Results vary over severalorders of magnitude and depend on the conditions of testing.The greatest negative pressures are measured using heated, pressurizedwater or purified, degassed water. When conditions more closelyapproximating those found in biological systems are used, wateris considerably weaker. From these measurements, one can predictthe most likely range of negative pressures that can be sustainedin the xylem. This range is between -0·1 and -0·6MPa (absolute pressure). Negative pressures near -1 MPa arepossible, but require stringent conditions. Negative pressuresgreater than -2 MPa are also possible, but unlikely, based onthe experimental evidence.Copyright 1994, 1999 Academic Press Cavitation, embolism, negative pressure, xylem transport, Z-tube     

Xylem colonization of the legume Sesbania rostrata by Azorhizobium caulinodans

A novel pathway of invasion of the legume Sesbania rostrata by Azorhizobium caulinodans is described that involves colonization of the root xylem, possibly following entry into the natural fissures created during emergence of lateral roots. Azorhizobia were detected microscopically, and their presence confirmed by the expression of a lacZ reporter gene. We have shown that rhizobial Nod factors are not required for either xylem colonization or for crack-entry of lateral roots. We discuss the extent to which this discovery of xylem colonization by azorhizobia is likely to improve our understanding of both symbiosis and of pathogenicity in plant–bacterial interactions.     

Diversity of Essential Oil-Secretory Cells and Oil Composition in Flowers and Buds of Magnolia sirindhorniae and Its Biological Activities

Magnolia sirindhorniae Noot. & Chalermglin produces fragrant flowers. The volatile oil secretary cells, quantity and quality as well as antioxidant and antimicrobial activities of the oils extracted from buds and flowers, have been investigated. The distribution of essential oil secretory cell in bud and flower revealed that the density and size of the oil cells were significantly higher in flowers compared to buds. In different floral parts, carpel has a higher oil cell density followed by gynophore and tepal. The histochemical analysis revealed the essential oil is synthesized in oil secretory cells. The volatile oil yield was 0.25 % in the buds and 0.50 % in flowers. GC/FID and GC/MS analysis identified 33 compounds contributing 83.2–83.5 % of the total essential oil composition. Linalool is the main constituent contributing 58.9 % and 51.0 % in the buds and flowers oils, respectively. The essential oil extracted from the flowers showed higher antimicrobial efficacy against Klebsiella pneumoniae and Staphylococcus aureus. Similarly, the essential oil isolated from the flowers depicts higher free radical scavenging, and antioxidant activity compared to buds’ oil.     

Trophic Cascades Induced by Lobster Fishing Are Not Ubiquitous in Southern California Kelp Forests

Fishing can trigger trophic cascades that alter community structure and dynamics and thus modify ecosystem attributes. We combined ecological data of sea urchin and macroalgal abundance with fishery data of spiny lobster (Panulirus interruptus) landings to evaluate whether: (1) patterns in the abundance and biomass among lobster (predator), sea urchins (grazer), and macroalgae (primary producer) in giant kelp forest communities indicated the presence of top-down control on urchins and macroalgae, and (2) lobster fishing triggers a trophic cascade leading to increased sea urchin densities and decreased macroalgal biomass. Eight years of data from eight rocky subtidal reefs known to support giant kelp forests near Santa Barbara, CA, USA, were analyzed in three-tiered least-squares regression models to evaluate the relationships between: (1) lobster abundance and sea urchin density, and (2) sea urchin density and macroalgal biomass. The models included reef physical structure and water depth. Results revealed a trend towards decreasing urchin density with increasing lobster abundance but little evidence that urchins control the biomass of macroalgae. Urchin density was highly correlated with habitat structure, although not water depth. To evaluate whether fishing triggered a trophic cascade we pooled data across all treatments to examine the extent to which sea urchin density and macroalgal biomass were related to the intensity of lobster fishing (as indicated by the density of traps pulled). We found that, with one exception, sea urchins remained more abundant at heavily fished sites, supporting the idea that fishing for lobsters releases top-down control on urchin grazers. Macroalgal biomass, however, was positively correlated with lobster fishing intensity, which contradicts the trophic cascade model. Collectively, our results suggest that factors other than urchin grazing play a major role in controlling macroalgal biomass in southern California kelp forests, and that lobster fishing does not always catalyze a top-down trophic cascade.     

Scandianthus gen.nov., Angiosperm Flowers of Gaxifragalean Affinity from the Upper Cretaceous of Southern Sweden

Structurally preserved angiosperm flowers are described fromthe Upper Cretaceous of southern Sweden. They are found in fluviatiledeposits dated, on palynological evidence, as Upper Santonianor Lower Campanian. The material studied includes two speciesof Scandianthus gen.nov. and one unnamed, related taxon. Thefossil flowers represent the best-preserved Cretaceous floralstructures available and the preservation permits detailed studyof organization and arrangement of parts. Comparison with flowersof extant angiosperms indicates a close relationship with membersof the Saxifragales. Scandianthus gen.nov. fossil angiosperm, Upper Cretaceous     

Spatio-temporal overlap between the at-sea distribution of Southern Giant Petrels and fisheries at the Patagonian Shelf

The interactions between seabirds and fisheries pose significant threats for the seabird species such as incidental capture. In contrast, several species of seabirds meet part of their energetic requirements through the use of fisheries discards. Knowledge about the relationship between at-sea distribution of Procellariiformes and fisheries is a key tool in marine ecosystem management. We analysed the spatio-temporal relationship between the areas used by 16 satellite-tracked breeding adults of the Southern Giant Petrel and fisheries distribution and catch at the Patagonian Shelf. We also determined the time spent by adults in different marine jurisdictions. Results indicated a marked spatio-temporal association between birds and fisheries, mainly trawlers. The Southern Giant Petrels concentrated their foraging effort over Argentinean waters. The use of an abundant and predictable food source provided by the fisheries discards may be one of the factors affecting the dynamics of the Southern Giant Petrel populations in Patagonia, Argentina.     

In situ Inactivation of the Oxidase Activity of Xylem Peroxidases by H2O2 in the H2O2-Producing Xylem of Zinnia elegans

Zinnia elegans stems with 3,3′, 5, 5′-tetramethylbenzidine (TMB) in the presence and in the absence of catalase reveals the presence of xylem oxidase activities in the H₂O₂-producing lignifying xylem cells. This staining of lignifying xylem cells with TMB is the result of two independent mechanisms: one is the catalase-sensitive (H₂O₂-dependent) peroxidase-mediated oxidation of TMB, and the other the catalase-insensitive (H₂O₂-independent) oxidation of TMB, probably due to the oxidase activity of xylem peroxidases. The response of this TMB-oxidase activity of xylem peroxidases to different exogenous H₂O₂ concentrations was studied, and the results showed that H₂O₂ at high concentrations (100–1,000 mM) clearly acted as an inactivator of this xylem TMB-oxidase activity, although some inhibitory effect could still be appreciated at 10 mM H₂O₂. This xylem TMB-oxidase activity resided in a strongly basic cell wall-bound peroxidase (pl about 10.5). Given such a scenario, it may be concluded that this TMB-oxidase activity of peroxidase is located in tissues capable of sustaining H₂O₂ production, and that the in situ oxidase activity shown by this enzyme is inactivated by high H₂O₂ concentrations. Received 20 April 1999/ Accepted in revised form 16 August 1999     

Farming of the Giant Kelp Macrocystis Pyrifera in Southern Chile for Development of Novel Food Products

This study explores the potential cultivation of the giant kelp Macrocystis pyrifera (L.) C.A. Agardh in southern Chile, for the development of novel food products. The study demonstrates the importance of considering the collection site of the parent sporophytes for successful cultivation. This study also shows that the ropes must be seeded with 10,000 to 40,000 spores ml⁻¹, depending on the culture method used. We also demonstrated that under environmental conditions in southern Chile, the seeded ropes must be put at sea at the latest during autumn (April) in order to reach the harvesting season in December. However, several other management aspects must be considered to improve the quality of the product. Our final estimation indicates that over 14.4 kg m⁻¹of rope (fresh weight) can be produced and from this total production, over 70% can reach the quality to produce different food products that are already being introduced in oriental countries. The remaining 30% can be used for abalone feeding and is also available for the organic fertilizer industry located in Chile.     

The Characteristics of the Adjustment in the pH of the Xylem Exudate of Segments of Vigna Hypocotyls during Xylem Perfusion

The xylem vessels of segments of Vigna hypocotyl were perfusedwith solutions of various pH values. With both acid and alkalineperfusion solutions, the pH of the xylem exudate tended to equilibrateat approximately 6 during the course of xylem perfusion. Thisphenomenon was observed both in air and under anoxia, and alsoin segments pretreated with methanol or preheated in a microwaveoven. Therefore, the adjustment in pH does not depend on therespiratory metabolism but originates in the buffering actionof the xylem wall apoplast. High concentrations of protons inducedthe release of other cations into the xylem exudate under anoxia,and high concentrations of K⁺ or Ca²⁺ ions induced the releaseof protons into the xylem under anoxia. These results indicatethat a cation-exchange reaction on the xylem cell wall was responsiblefor the buffering effect. The physiological role of the highbuffering capacity of the xylem wall apoplast is discussed inlight of the role of the proton pump of the plasmalemma in elongationgrowth. (Received September 25, 1990; Accepted January 21, 1991)     

On the Conductivity of the Xylem in Trees

The xylem of three tree species, the ring porous Fraxinus excelsiorL., and the diffuse porous Acer pseudoplatanus L., and Salixatrocinerea Brot., has been examined to determine how closelyflow through it obeys the Poiseuille equation. The equationwas most closly obeyed in the relationship between flow-rateand pressure difference between the ends of the xylem cylinder,though there were differences between the ring and diffuse poroustypes. The relationship of flow-rate and specific conductivityto the radius of the xylem cylinder was complex.Fraxinus showeda rise with increase of radius up to a given size (0.54cm),followed by a steep decline. This decline was shown to be causedby a progressive increase in the number of vessels which wereunable to conduct water. Small increases in the viscosity ofthe xylem liquid produced a disproportionately large fall inthe rate of flow, and the specific conductivity of the xylemdecreased. With viscosities greater than 2.5 centipoise, thespecific conductivity remained constant. An effect of alternatelyraising and lowering the hydrostatic pressure of the xylem wateron the specific conductivity of the wood is described. These results are discussed in relation to the structure ofthe xylem, and particularly with regard to the possibility thatthe conduction channels in the wood consist of a system of microcapillariessurrounding the vessel cavities.     

Step by Step: Deciphering Ion Transport in the Root Xylem Parenchyma

Proton pumps produce electrical potential differences and differences in pH across the plasma membrane of cells which drive secondary ion transport through sym- and antiporters. We used the patch-clamp technique to characterize an H⁺-pump in the xylem parenchyma of barley roots. This cell type is of special interest with respect to xylem loading. Since it has been an ongoing debate whether xylem loading is a passive or an active process, the functional characterization of the H⁺-pump is of major interest in the context of previous work on ion channels through which passive salt efflux into the xylem vessels could occur. Cell-type specific features like its Ca²⁺ dependence were determined, that are important to interpret its physiological role and eventually to model xylem loading. We conclude that the electrogenic pump in the xylem parenchyma does not participate directly in the transfer of KCl and KNO₃ to the xylem but, in combination with short-circuiting conductances, plays a crucial role in controlling xylem unloading and loading through modulation of the voltage difference across the plasma membrane. Here, our recent results on the H⁺ pump are put in a larger context and open questions are highlighted.Key Words: plant nutrition, H⁺-ATPase, anion conductance, K⁺ channel, electrophysiology, signaling networkThe root xylem parenchyma is of major interest with respect to nutrient (and signal) traffic between root and shoot. One of its main functions appears to be xylem loading. However, the cell walls of the vascular tissue provide apoplastic paths between xylem and phloem that represent the upward and downward traffic lanes, allowing nutrient circulation¹ (Fig. 1). Therefore mechanisms for ion uptake and for ion release must exist side by side. In the last 15 years major progress has been made in the investigation of transport properties of xylem-parenchyma cells, and both uptake and release channels and transporters were identified. Today, we have good knowledge on the role of K⁺ and anion conductances in xylem loading with salts.² Note, that from the functionally well characterized conductances only the molecular structure of K⁺ channels is known. In contrast, many transporters are identified on the molecular level, but functional data are scarce.Open in a separate windowFigure 1Distribution of tissues in the periphery of the stele. The stippled area marks the region from which early metaxylem protoplasts originated. E, Endodermis with Casparian strip; eMX, ‘early’ metaxylem vessel; IMX, ‘late’ metaxylem vessel; Mph, metaphloem (sieve tube); Pph, protophloem (sieve tube); P, pericycle; Cx, cortex. Symplasmic and apoplasmic transport routes are indicated in red and black, respectively. The Casparian strip prevents apoplastic transport into the stele. Plasmodesmata are shown exemplarily for the indicated symplastic pathway. All cells of the symplast are connected via plasmodesmata. Sites of active uptake into the root symplast and of release into the stelar apoplast are indicated by a black and an orange arrow. Modified from Wegner and Raschke, 1994.³A challenging question to deal with was the dispute about xylem loading with ions being a passive or active process. While it is clear that energy through electrogenic H⁺ efflux is needed to take up nutrient ions from the soil against their electrochemical gradient into the cortical symplast, it has been a matter of debate if ion release into xylem vessels also is energy-linked or if the electrochemical potentials of ions are raised high enough to allow a thermodynamically passive flux.^2,3 The Casparian strip prohibits apoplastic transport of nutrients into the stele and electrically insulates the stelar from the cortical apoplast. Therefore the electrical potential difference of the cells in the xylem parenchyma could be independent from the cortical potential difference but be subject to control, for instance, from the shoot.⁴ Indeed, evidence points to xylem loading as a second control point in nutrient transfer to the shoot.^5,6 The identification and characterization of K⁺ and anion conductances clearly showed that release of KCl and KNO₃ into the xylem can be passive through voltage-dependent ion channels.^2,3,7–9 No need appeared for a pump energizing the transfer of salts to the xylem.However, H⁺ pumps are ubiquitous. H⁺-ATPases are encoded by a multigene family and heterologous expression in yeast showed that isoforms have distinct enzymatic properties.^10,11 As the example of the amino acid transporter AAP6 from the xylem parenchyma shows, a cell-type specific functional characterization of transporters is essential to draw conclusions on their physiological role. AAP6 is the only member of a multigene family with an affinity for aspartate in the physiologically relevant range. The actual apoplastic concentration of amino acids and the pH will determine what is transported in vivo.^12,13 Xylem-parenchyma cells of barley roots were strongly labelled by antibodies against the plasma membrane H⁺-ATPase.¹⁴ In a recent publication in Physiologia Plantarum we report the functional analysis of the electrogenic pump from the plasma membrane of xylem parenchyma from barley roots that was done with the patch-clamp technique after specific isolation of protoplasts from this cell type. It displayed characteristics of an H⁺-ATPase: current-voltage relationships were characteristic for a ‘rheogenic’ pump¹⁵ and currents were stimulated by fusicoccin or by an enlarged transmembrane pH gradient and inhibited by dicyclohexylcarbodiimide (DCCD). Importantly, it also showed distinct characteristics. Neither intracellular pH nor the intracellular Ca²⁺ concentration affected its activity. Noteworthy, K⁺ and anion conductances from the same cell type are controlled by intracellular [Ca²⁺]^7,9 (Fig. 2). It was proposed that the effect of abscisic acid (ABA) on anion conductances is mediated via an increase in the cytosolic Ca²⁺ concentration.¹⁶ Very likely stelar H⁺ pumps are stimulated by ABA.¹⁷ Thus, a Ca²⁺ independent control has to be hypothesized in this case.Open in a separate windowFigure 2Control of ion conductances in the plasma membrane of xylem-parenchyma cells. Arrowheads indicate stimulation and bars indicate inhibition by an increase in cytosolic [Ca²⁺],^7,9,16 by ABA,^16,17,21 by cytosolic and apoplastic acidification,^4,22 by G-proteins²³ and by an increase in apoplastic [K⁺]⁷ and [NO₃⁻].²⁴ Apoplastic [K⁺] and [NO₃⁻] modify the voltage dependence exerting negative feedback on K⁺ efflux and a positive feedback on NO₃⁻ efflux. Abscisic acid has an immediate effect on ion channel activity, most likely via [Ca²⁺], and causes a change in gene expression as indicated by circles (up) and bars (down). ABA perception is not clear. A Ca²⁺ influx could occur through a hyperpolarization activated cation conductance (HACC).^16,25 Cation transporters are NORC, nonselective cation conductance, KORC, K⁺-selective outwardly rectifying conductance (=SKOR⁸), and KIRC, K⁺-selective inwardly rectifying conductance, and anion conductances with different voltage-dependencies and gating characteristics are X-QUAC, quickly activating anion conductance, X-SLAC, slowly activating anion conductance, and X-IRAC, inwardly rectifying anion channel.^2,3,9,16,26 Transported ions and direction of flux are plotted.To date, we know that besides Ca²⁺ and abscisic acid also the pH, nonhydrolyzable GTP analogs and extracellular NO₃⁻ and K⁺ affect membrane transport capacities of root xylem-parenchyma cells (Fig. 2). Other control mechanisms by metabolites, the redox potential and phytohormones have to be included, especially if they represent signals in xylem loading or root-shoot communication. The composition of the xylem sap changes during the course of a day, depending on nutrient supply and various stresses, and the apoplastic ion concentration is considered to be an important factor in ion circulation.^6,18,19 ABA is such a signal. It is known to increase solute accumulation within the root by inhibiting release of ions into the xylem.¹⁷ Any change in transport activity has an impact on the membrane potential. This again determines whether salt release or uptake takes place. Passive salt release is restricted to a limited range of membrane potentials in which conductances for anions and cations are active simultaneously, that is with depolarization. Negative membrane voltages will be required for reabsorption of NO₃⁻ by a putative NO₃⁻/H⁺-symporter and for the uptake of K⁺ and amino acids.^3,13 As shown in our recent paper, the balance between the activities of the H⁺-pump and the anion conductances could affect the position between a depolarized and a hyperpolarized state of the parenchymal membrane. Thus, H⁺ pump activity is crucial in membrane voltage control. Furthermore, the simultaneous activities of H⁺ pumps and anion conductances make the generation of a high pH gradient possible, whilst maintaining electroneutrality. The proton gradient could be used for ion transport through cotransporters and antiporters as suggested for the loading of borate into the xylem through the boron transporter BOR1.²⁰ So we are on the way to decipher xylem loading in roots and this exciting field will also provide information about small-scale nutrient cycling and root-shoot communication. To determine how the activities of pumps, channels and transporters are adjusted among each other is the next challenge. Further insight has to be obtained by experimentation as well as by biophysical modeling.     

Effects of Local Deforestation on the Diversity and Structure of Southern California Giant Kelp Forest Food Webs

It has been hypothesized that the high diversity of giant kelp forests is due primarily to the provision of energy and habitat by the giant kelp (Macrocystis pyrifera). In this article, I use a 19-year-long kelp forest-monitoring data set from the Channel Islands National Park (a) to identify associations between subtidal species and forested or deforested habitats, (b) to generate an idealized food web for Southern California giant kelp forests in order to identify the primary conduits of energy flow through the system, and (c) to determine changes in the diversity and complexity of this food web due to localized giant kelp deforestation. A total of 275 common species were observed in the park between 1982 and 2000, of which 36% occurred significantly more often in kelp-forested areas than in deforested areas (that is, sea urchin barrens); 25 species were found exclusively in forested areas. Most of these associations were clearly identified as trophic and/or structural associations with giant kelp itself. The producer level of the food web was diverse, although giant kelp apparently represents the greatest single source of fixed carbon through either direct grazing or the production of phytodetritus. Primary, secondary and tertiary consumer levels were also represented by numerous species, and generalist consumers were common. With deforestation, the source of primary production shifts from primarily kelps to ephemeral microalgae, macroalgae, and phytoplankton. These results support the reliance of giant kelp forest food-web structure and diversity on the presence of the forest itself.     

Tuning the Elastic Modulus of Hydrated Collagen Fibrils

Systematic variation of solution conditions reveals that the elastic modulus (E) of individual collagen fibrils can be varied over a range of 2-200 MPa. Nanoindentation of reconstituted bovine Achilles tendon fibrils by atomic force microscopy (AFM) under different aqueous and ethanol environments was carried out. Titration of monovalent salts up to a concentration of 1 M at pH 7 causes E to increase from 2 to 5 MPa. This stiffening effect is more pronounced at lower pH where, at pH 5, e.g., there is an ∼7-fold increase in modulus on addition of 1 M KCl. An even larger increase in modulus, up to ∼200 MPa, can be achieved by using increasing concentrations of ethanol. Taken together, these results indicate that there are a number of intermolecular forces between tropocollagen monomers that govern the elastic response. These include hydration forces and hydrogen bonding, ion pairs, and possibly the hydrophobic effect. Tuning of the relative strengths of these forces allows rational tuning of the elastic modulus of the fibrils.     

Freezing of Conifer Xylem and the Cohesion-Tension Theory

The research explored the apparent contradiction between the cohesion-tension theory and the lack of blockage following freezing in conifers. Five to ten cm lengths of the main stem of actively transpiring Thuja occidentalis, Picea glauca and Pinus sylvestris were frozen. On thaw, there was no evidence of blockage in the formerly frozen xylem. Microscopic examination of frozen wood and hydrodynamic considerations showed that air was present in the xylem water and that bubbles were probably present upon freezing. On freezing about 9 per cent of the water irreversibly migrated into the unfrozen plant parts. Lack of blockage in the presence of air bubbles was reconciled to the cohesion-tension theory by a model in which only the largest bubble in a number of interconnected cells expands. As this bubble expands tension is released allowing all bubbles in adjoining tracheids to redissolve. Potential rate of resolution of unexpanded bubbles following release of tension was calculated to be less than 0. 1 second.