The ability of aquatic macrophytes to increase root porosity and radial oxygen loss determines their resistance to sediment anoxia

Radial oxygen loss (ROL) has been suggested to be a major process to protect plants exposed to root anaerobic stress. In the present study, we aimed to test the importance of root porosity and radial oxygen loss on the aquatic macrophyte resistance to sediment anoxia. We expected that species living in eutrophic environments characterized by anaerobic conditions in sediments exhibited higher root porosity and radial oxygen loss than species restrained to oligotrophic environments. In this way, we compared the responses to sediment anoxia of two hydrophyte species growing under meso-eutrophic conditions in the field (Myriophyllum spicatum L. and Vallisneria spiralis L.) and three species growing under oligotrophic conditions (Potamogeton coloratus Horne, Elodea canadensis Michx and Sparganium emersum Michx.). Under laboratory conditions, ROL, root porosity, plant metabolism (aerobic respiration, photosynthesis, root fermentative activity) and plant growth were analysed after 3?months of acclimation in anaerobic sediments and compared with control values obtained from aerobic sediments. The results showed that two meso-eutrophic species (M. spicatum and V. spiralis) survived in anaerobic sediments and maintained similar photosynthesis rates than those measured under aerobic conditions. In contrast, the three oligotrophic species (P. coloratus, E. canadensis and S. emersum) suffered net biomass loss and depressed their photosynthesis rates under anaerobic conditions. All variables associated with plant tolerance to anaerobic conditions (maintenance of photosynthesis, aerobic respiration and growth rate, and limitation of root fermentative activity) were positively linked to root porosity and ROL. According to our hypothesis, species that could survive to anaerobic conditions were the species able to increase their root porosity and ROL under these conditions. Thus, in ecological studies, it would be useful to use the root porosity and ROL plasticity as biological traits in order to model the distribution of macrophytes in river floodplains.     

Development of artificially induced biological soil crusts in fields and their effects on top soil

Aims

Biological soil crusts (BSCs) could improve severe environment ecological conditions by increasing soil moisture, soil nitrogen concentration, and so on. In order to control desertification and recover the destroyed soil fertility utilizing a new means using BSCs, the soil surface was artificially inoculated with Microcoleus vaginatus and Scytonema javanicum. Relationships between the development of the artificially induced biological soil crusts and the distribution and dynamic changes of nitrogen and phosphorus in the soil crusts have been analyzed.

Methods

Crusts of different ages were investigated by measuring soil physical and chemical factors, such as moisture, pH, total and available N content, and total and available P, which were correlated with the depths of the crusts.

Results

This study found that the types of color, shape, and species components of the algal crusts increased with crust development. Soil moisture, total N, available N, and available P increased gradually with crust growth. Soil with crusts was wetter than the controlled naked sandy soil, and a significant correlation was observed between biomass and total nitrogen (r?=?0.946, P?=?0.015). Soil pH was lower than that of control. The scytonemin on the soil surface was exceptionally higher than the other pigments, and all the pigments were mainly distributed at the soil surface level. Though the crusts were mainly distributed on soil surface, the available P was mainly stored below the crust layer.

Conclusions

Pearson correlation tests indicated that artificially inoculated biological crusts could improve soil fertility and micro-environment of the top soil: The development of artificially induced BSCs was very well, and this was favorable to inducing the following crust succession.     

Marine sponges and their microbial symbionts: love and other relationships

Many marine sponges harbour dense and diverse microbial communities of considerable ecological and biotechnological importance. While the past decade has seen tremendous advances in our understanding of the phylogenetic diversity of sponge-associated microorganisms (more than 25 bacterial phyla have now been reported from sponges), it is only in the past 3-4 years that the in situ activity and function of these microbes has become a major research focus. Already the rewards of this new emphasis are evident, with genomics and experimental approaches yielding novel insights into symbiont function. Key steps in the nitrogen cycle [denitrification, anaerobic ammonium oxidation (Anammox)] have recently been demonstrated in sponges for the first time, with diverse bacteria - including the sponge-associated candidate phylum 'Poribacteria'- being implicated in these processes. In this minireview we examine recent major developments in the microbiology of sponges, and identify several research areas (e.g. biology of viruses in sponges, effects of environmental stress) that we believe are deserving of increased attention.     

Foraging behavior and activity budgets of sea stars on a subtidal sediment bottom community

We dived at regular intervals (8, 12 or 14 h) over periods of 9 to 23 days to quantify the allocation of time to different activities by large adults of the four common sea stars foraging in a sediment bottom community (8 to 11 in depth) in the northern Gulf of St. Lawrence. The relative time spent at different activities was similar for Leptasterias polaris and Asterias vulgaris, the most frequent activity being moving over the bottom (42-46% of the time), presumably in search of prey, secondly being stationary (26-27%), and thirdly digesting prey (17-20%). Capture of prey was less frequent, 13% of the time for L. polaris (mainly digging for bivalves) and 5% for A. vulgaris. Compared to L. polaris and A. vulgaris, Crossaster papposus spent more time being stationary and periods of moving were of shorter duration. Solaster endeca differed from the other sea stars because of the high proportion of time spent digesting prey (40%) and the small proportion of time spent moving (22%). Capture events were not observed for C. papposus and S. endeca, likely because they exclusively fed on epifaunal species, which were rapidly captured. For all four species, the most frequent changes between activities were between moving and being stationary. The number of days per prey eaten (length of a feeding bout) was about 11 for S. endeca, 6.6 for L. polaris, 3.1 for C. papposus and 2.8 for A. vulgaris. All sea stars appear to be energy maximizers that spend most of their time finding, capturing and digesting prey.     

Bacterial symbionts on green hydra and their effect on phosphate uptake

Gram-negative rod-shaped bacteria (1.5–2m long and 0.5m wide) have been found associated with green hydra. They are always present on the hydra surface delineating the ectodermal cells, on animals in culture, and also on those sampled from a natural habitat. The bacteria could be removed by a 30-min treatment with antibiotics (50/ml polymyxin B and 50/ml streptomycin). Antibiotic-treated hydra took up 55% less phosphate from the medium than control hydra. The nutritional relationship between the bacteria and green hydra and possible routes of infection of the hydra by these prokaryotic symbionts are discussed. Their importance in interpreting results of certain types of physiological experiments using aquatic organisms is emphasized.     

Fetal and placental responses to artificially induced hyperthermia in rats.

Pregnant rats were utilized to study the effect of maternal hyperthermia on fetal development. Eight groups of six to eight rats were exposed to ambient temperatures of 43-44 degrees C at various stages of pregnancy. All rats were killed on day 20 of gestation. Edema, microencephaly and microphthalmia followed heat treatment on day 4, 6, or 8 and skeletal defects occurred on day 10 of gestation. Apparently heat stress of dams after day 14 of gestation had little or no effect on embryos. Most placentas from day 6-10 treatment groups were significantly heavier than control and exhibited extensive thickening and necrosis of decidua basalis. Our results suggest that the rat is a useful model for investigating maternal hyperthermia as a possible cause of human placentophathies and fetal retardation.     

Hepatopancreas gluconeogenesis during anoxia and post-anoxia recovery in Chasmagnathus granulata crabs maintained on high-protein or carbohydrate-rich diets

C. granulata is a semiterrestrial crab that lives in the mesolittoral and the supralittoral zones of estuaries and faces hypoxia and anoxia when exposed to atmospheric air. The carbohydrate or protein content of the diets administered to the crabs induced different metabolic adjustments during anoxia and post-anoxia recovery period. During the first hour in anoxia a marked increase in L-lactate concentration in hemolymph was induced, followed by a reduction in its levels accompanied by two peaks in hepatopancreas gluconeogenic capacity. Anoxia exposure did not induce a reduction in the hepatopancreas phosphoenolpyruvate carboxykinase activity in either dietary group. Our results suggest that in anaerobiosis this crab uses the conversion of lactate to glucose in hepatopancreas to maintain the acid-base balance and the glucose supply. In post-anoxia recovery, the fate of L-lactate is the hepatopancreas gluconeogenesis in high protein maintained crabs. On the other hand, in the crabs maintained on carbohydrate-rich diet the L-lactate levels decreased gradually in the hemolymph during the post-anoxia recovery; however, the hepatopancreas gluconeogenesis did not increase. In both dietary groups, an increase in the gluconeogenic capacity of hepatopancreas occurred at 30 h of post-anoxia recovery.     

Actin monoubiquitylation is induced in plants in response to pathogens and symbionts.

Most dramatic examples of actin reorganization have been described during host-microbe interactions. Plasticity of actin is, in part, due to posttranslational modifications such as phosphorylation or ubiquitylation. Here, we show for the first time that actins found in root nodules of Phaseolus vulgaris are modified transiently during nodule development by monoubiquitylation. This finding was extended to root nodules of other legumes and to other plants infected with mycorrhiza or plant pathogens such as members of the genera Pseudomonas and Phytophthora. However, neither viral infections nor diverse stressful conditions (heat shock, wounding, or osmotic stress) induced this response. Additionally, this phenomenon was mimicked by the addition of a yeast elicitor or H2O2 to Phaseolus vulgaris suspension culture cells. This modification seems to provide increased stability of the microfilaments to proteolytic degradation and seems to be found in fractions in which the actin cytoskeleton is associated with membranes. All together, these data suggest that actin monoubiquitylation may be considered an effector mechanism of a general plant response against microbes.     

Assembling a mutualism: ant symbionts locate their host plants by detecting volatile chemicals

In most mutualisms, partners disperse independently of each other. For instance, in ant-plant symbioses, plants disperse as seeds, and ants disperse as winged queens. For an ant-plant mutualism to persist, therefore, queens must be able to locate and colonise host plant saplings. It has been suggested that host plants emit volatile chemical cues that attract dispersing queens, but this has never been demonstrated experimentally. We used a Y-tube olfactometry protocol to test this hypothesis in the tropical understorey antplant Cordia nodosa Lam. (Boraginaceae), which associates with two genera of ants, Azteca (Dolichoderinae) and Allomerus (Myrmicinae). Both genera show significant attraction to the volatiles of C. nodosa over control understorey plant species that do not associate with ants. These results support the hypothesis that ants are attracted to volatiles emitted by their host plant and suggest a key preadaptation that promoted the evolution of ant-plant symbioses. Received 1 July 2005; revised 2 November 2005; accepted 8 November 2005.     

Effects of sediment anoxia and light on turion germination and early growth of Potamogeton crispus

Potamogeton crispus is a cosmopolitan aquatic species and is widely used as a pioneer species for vegetation restoration of eutrophic lakes. However, many restoration projects applying P. crispus turions have not been successful. Earlier studies focused on effects of light and temperature on turion germination. The purpose of this study was to determine whether sediment anoxia and light interactively affected the turion germination and early growth of P. crispus. Anoxic conditions in the experiment were produced by adding sucrose to the sediment. The germination rate of the turions was 68–73% lower in the highly anoxic condition treatment than in the control. Medium light intensity (10% of natural light at the water surface) was more favorable for germination under slightly anoxic conditions than either low or high light intensity. The growth of newly-formed sprouts was also significantly inhibited by sediment anoxia. Photosynthesis and shoot biomass were reduced under sediment anoxia, whereas total chlorophyll content, root biomass, and soluble protein content were highest in the low anoxic condition treatment. Medium light improved net photosynthesis and biomass production of the sprouts. We conclude that turion germination and sprout growth can be significantly inhibited by sediment anoxia. Medium light intensity may alleviate this inhibition by anoxia, but light has little effect when sediment anoxia is severe. For the purposes of vegetation restoration, more attention should be paid to the role of sediment anoxia, and it is necessary to improve sediment and light conditions for turion germination and early growth of P. crispus in eutrophic lakes. These results will contribute to a more complete understanding of turion germination dynamics of P. crispus and will be useful for future restoration programs. Handling editor: S. M. Thomaz     

Several deep-sea mussels and their associated symbionts are able to live both on wood and on whale falls

Bathymodiolin mussels occur at hydrothermal vents and cold seeps, where they thrive thanks to symbiotic associations with chemotrophic bacteria. Closely related genera Idas and Adipicola are associated with organic falls, ecosystems that have been suggested as potential evolutionary 'stepping stones' in the colonization of deeper and more sulphide-rich environments. Such a scenario should result from specializations to given environments from species with larger ecological niches. This study provides molecular-based evidence for the existence of two mussel species found both on sunken wood and bones. Each species specifically harbours one bacterial phylotype corresponding to thioautotrophic bacteria related to other bathymodiolin symbionts. Phylogenetic patterns between hosts and symbionts are partially congruent. However, active endocytosis and occurrences of minor symbiont lineages within species which are not their usual host suggest an environmental or horizontal rather than strictly vertical transmission of symbionts. Although the bacteria are close relatives, their localization is intracellular in one mussel species and extracellular in the other, suggesting that habitat choice is independent of the symbiont localization. The variation of bacterial densities in host tissues is related to the substrate on which specimens were sampled and could explain the abilities of host species to adapt to various substrates.     

Defending against parasites: fungus-growing ants combine specialized behaviours and microbial symbionts to protect their fungus gardens

Parasites influence host biology and population structure, and thus shape the evolution of their hosts. Parasites often accelerate the evolution of host defences, including direct defences such as evasion and sanitation and indirect defences such as the management of beneficial microbes that aid in the suppression or removal of pathogens. Fungus-growing ants are doubly burdened by parasites, needing to protect their crops as well as themselves from infection. We show that parasite removal from fungus gardens is more complex than previously realized. In response to infection of their fungal gardens by a specialized virulent parasite, ants gather and compress parasitic spores and hyphae in their infrabuccal pockets, then deposit the resulting pellet in piles near their gardens. We reveal that the ants' infrabuccal pocket functions as a specialized sterilization device, killing spores of the garden parasite Escovopsis. This is apparently achieved through a symbiotic association with actinomycetous bacteria in the infrabuccal pocket that produce antibiotics which inhibit Escovopsis. The use of the infrabuccal pocket as a receptacle to sequester Escovopsis, and as a location for antibiotic administration by the ants' bacterial mutualist, illustrates how the combination of behaviour and microbial symbionts can be a successful defence strategy for hosts.     

Natural and artificially induced ovulatory models related to lactation in the rat: role of prolactin

The presence and the importance of a preovulatory prolactin (PRL) peak was determined in four, natural or artificially induced, ovulatory models related to lactation in the rat. Gonadotrophin peaks were determined in the afternoon preceding ovulation in four models: postpartum ovulation (PPO), ovulation after the lactational period (AL) (natural models), ovulation after litter removal at midlactation (ML), and ovulation in lactating rats (LR) (artificially induced models). In PPO, AL, and ML rats a preovulatory PRL surge was detected, showing that its presence is a common characteristic of ovulation in the rat. Bromocriptine inhibition of PRL levels in PPO and AL rats did not modify the percentage of rats which ovulated. In contrast, this treatment was able to significantly increase ovulation percentage in ML rats. Moreover, in LR rats strong dopaminergic inhibition of PRL levels, induced by pergolide, was necessary for ovulation to take place, but if pergolide-treated rats were injected with ovine PRL ovulation was completely inhibited. These data suggest that while a PRL surge seems to be always present in natural ovulatory models, it is not essential for ovulation to take place. On the other hand, in artificially induced ovulatory models, suppression of prolactinemia is able to induce ovulation or to increase the percentage of rats which ovulated. This effect of PRL on ovulation may be direct or indirect.     

Ecological relations between hermit crabs and their shell-supplying gastropods: Constrained consumers

Hermit crabs are critically dependent upon gastropod shells for their survival and reproductive fitness. While anecdotal reports have suggested that hermit crabs may be capable of removing live gastropods from their shells to access the essential shell resource, no systematic experiments have been conducted to investigate this possibility. This paper reports experiments on both marine (Pagurus bernhardus) and terrestrial (Coenobita compressus) hermit crabs in which crabs were paired in the laboratory with the gastropods whose shells they inhabit in the field. Pairings included both shelled and naked crabs and spanned the full range of the gastropod life cycle. Neither marine nor terrestrial hermit crabs were successful at removing live gastropods from their shells. Furthermore, only a small fraction of the crabs (5.7%) were capable of accessing shells in which the gastropod had been killed in advance, with its body left intact inside the shell. Finally, although hermit crabs readily entered empty shells positioned on the surface, few crabs (14.3%) were able to access empty shells that were buried just centimeters beneath them. These results suggest that hermit crabs are constrained consumers, with the shells they seek only being accessible during a narrow time window, which begins following natural gastropod death and bodily decomposition and which typically ends when the gastropod's remnant shell has been buried by tidal forces. Further experiments are needed on more species of hermit crabs as well as fine-grained measurements of (i) the mechanical force required to pull a gastropod body from its shell and (ii) the maximum corresponding force that can be generated by different hermit crab species' chelipeds.     

Atmospheric phenanthrene transfer and effects on two grassland species and their root symbionts: A microcosm study

The objective of this work was to determine the transfer of phenanthrene (PHE) from air to grassland plants and soil compartments and its effects on the plant growth and symbiotic root microorganisms (arbuscular mycorrhizal fungi and Rhizobium nodules). The experimental procedure exposed Trifolium pratense L. or Lolium perenne L. to atmospheric PHE pollution (150 μg m⁻³) over the course of one month. PHE was transferred from the air to the leaves and to the soil surfaces. In leaves, PHE was mostly absorbed in the inner leaf tissues, representing 92% and 73% of the total PHE amount quantified in leaves, respectively for clover and ryegrass. In soils, most of PHE contamination was recovered in the top layer (0-1 cm) and did not readily diffuse into the deep layer (1-10 cm). The highest PHE concentration recovered in deep roots (1.8 and 4.5 μg g⁻¹ dry weight (DW), respectively for clover and ryegrass) related to the lowest PHE concentration recovered in its associated soil suggested a PHE translocation from shoots to roots within the two plant species. The large PHE amount quantified in clover shoots (124 μg g⁻¹ DW) induced a significant diminution by 30% of the shoot biomass whereas root biomass remained stable. Efficient mycorrhizal symbiosis was maintained during exposure whereas the Rhizobium nodule symbiosis was altered in the surface of soil. By contrast, neither biomass accumulation nor symbiotic association was affected in ryegrass, probably due to a lower sensitivity of this species to PHE exposure. Perspectives of carbon allocation and nitrogen nutrition perturbations are suggested in clovers.     

Contrasting physiological plasticity in response to environmental stress within different cnidarians and their respective symbionts

Given concerns surrounding coral bleaching and ocean acidification, there is renewed interest in characterizing the physiological differences across the multiple host–algal symbiont combinations commonly found on coral reefs. Elevated temperature and CO₂ were used to compare physiological responses within the scleractinian corals Montipora hirsuta (Symbiodinium C15) and Pocillopora damicornis (Symbiodinium D1), as well as the corallimorph (a non-calcifying anthozoan closely related to scleractinians) Discosoma nummiforme (Symbiodinium C3). Several physiological proxies were affected more by temperature than CO₂, including photochemistry, algal number and cellular chlorophyll a. Marked differences in symbiont number, chlorophyll and volume contributed to distinctive patterns of chlorophyll absorption among these animals. In contrast, carbon fixation either did not change or increased under elevated temperature. Also, the rate of photosynthetically fixed carbon translocated to each host did not change, and the percent of carbon translocated to the host increased in the corallimorph. Comparing all data revealed a significant negative correlation between photosynthetic rate and symbiont density that corroborates previous hypotheses about carbon limitation in these symbioses. The ratio of symbiont-normalized photosynthetic rate relative to the rate of symbiont-normalized carbon translocation (P:T) was compared in these organisms as well as the anemone, Exaiptasia pallida hosting Symbiodinium minutum, and revealed a P:T close to unity (D. nummiforme) to a range of 2.0–4.5, with the lowest carbon translocation in the sea anemone. Major differences in the thermal responses across these organisms provide further evidence of a range of acclimation potential and physiological plasticity that highlights the need for continued study of these symbioses across a larger group of host taxa.     

The impact of phloem nutrients on overwintering mountain pine beetles and their fungal symbionts

In the low nutrient environment of conifer bark, subcortical beetles often carry symbiotic fungi that concentrate nutrients in host tissues. Although bark beetles are known to benefit from these symbioses, whether this is because they survive better in nutrient-rich phloem is unknown. After manipulating phloem nutrition by fertilizing lodgepole pine trees (Pinus contorta Douglas var. latifolia), we found bolts from fertilized trees to contain more living individuals, and especially more pupae and teneral adults than bolts from unfertilized trees at our southern site. At our northern site, we found that a larger proportion of mountain pine beetle (Dendroctonus ponderosae Hopkins) larvae built pupal chambers in bolts from fertilized trees than in bolts from unfertilized trees. The symbiotic fungi of the mountain pine beetle also responded to fertilization. Two mutualistic fungi of bark beetles, Grosmannia clavigera (Rob.-Jeffr. & R. W. Davidson) Zipfel, Z. W. de Beer, & M. J. Wingf. and Leptographium longiclavatum Lee, S., J. J. Kim, & C. Breuil, doubled the nitrogen concentrations near the point of infection in the phloem of fertilized trees. These fungi were less capable of concentrating nitrogen in unfertilized trees. Thus, the fungal symbionts of mountain pine beetle enhance phloem nutrition and likely mediate the beneficial effects of fertilization on the survival and development of mountain pine beetle larvae.     

Ecology of sessile animals on sublittoral hard substrata: The need to measure variation

This paper concerns assemblages of sessile animals occupying shaded, commonly vertical hard substrata in the shallow subtidal zone. We are interested particularly in questions about the coexistence of species and about what influences their joint dynamics. We propose a conceptual model which focuses on variation in characteristics such as birth and death rates, competitive interactions and dispersal. We argue that, qualitatively, this model appears to be a satisfactory representation of the important characteristics of certain sessile assemblages. Further, we suggest that it may explain the coexistence of a large number of ecologically similar species in assemblages that appear in a sense ‘stable’ (but where assemblages at different sites differ in detail). There is support for this assertion from formal theoretical work on simpler versions of the model. It is not sufficient merely to argue that the model seems satisfactory or plausible, so we finally consider what kind of data is needed for the further development and testing of this kind of model.