Opportunistic feeding responses of the Yellow-legged Gull Larus michahellis to accessibility of refuse dumps

Capsule The gulls adjust their diet composition and diversity according to refuse dump accessibility.

Aims To examine the influence of the accessibility of open-air refuse dumps on the pre-laying diet of the adult Yellow-legged Gull.

Methods We studied six colonies settled on six rocky islands off the southeastern coast of France. A comparative study of the diet of breeding adults from the six colonies was made through pellet analysis (a total of 848 pellets). We determined the main foraging habitat used (refuse dumps, terrestrial habitats, marine habitat) and the number of foraging habitats used simultaneously (one, two or three), from which we deduced the mean diet diversity.

Results Refuse dumps were consistently the main foraging habitat (evidence in 53–74% of pellets) for the six colonies, even when refuse dump accessibility was low. The majority of pellets contained materials from two simultaneous foraging habitats (evidence in 50–64% of pellets). We demonstrated the influence of a gradient of refuse dump accessibility in terms of adjustment of the pre-laying adult's diet. Indeed, high refuse dump accessibility leads to a poorly diversified diet dominated by refuse. In contrast, when refuse dump accessibility is low, Yellow-legged Gulls broaden their trophic niche, with an increased exploitation of alternative foraging habitats, such as terrestrial habitats.

Conclusion These results show the species' opportunistic feeding and high adaptability, two parameters which need to be known to foresee the consequences on population dynamics, feeding and predatory behaviour of a sudden and severe food shortage, for example due to closure of open-air refuse dumps.     

Cellulose-Enriched Microbial Communities from Leaf-Cutter Ant (Atta colombica) Refuse Dumps Vary in Taxonomic Composition and Degradation Ability

Deconstruction of the cellulose in plant cell walls is critical for carbon flow through ecosystems and for the production of sustainable cellulosic biofuels. Our understanding of cellulose deconstruction is largely limited to the study of microbes in isolation, but in nature, this process is driven by microbes within complex communities. In Neotropical forests, microbes in leaf-cutter ant refuse dumps are important for carbon turnover. These dumps consist of decaying plant material and a diverse bacterial community, as shown here by electron microscopy. To study the portion of the community capable of cellulose degradation, we performed enrichments on cellulose using material from five Atta colombica refuse dumps. The ability of enriched communities to degrade cellulose varied significantly across refuse dumps. 16S rRNA gene amplicon sequencing of enriched samples identified that the community structure correlated with refuse dump and with degradation ability. Overall, samples were dominated by Bacteroidetes, Gammaproteobacteria, and Betaproteobacteria. Half of abundant operational taxonomic units (OTUs) across samples were classified within genera containing known cellulose degraders, including Acidovorax, the most abundant OTU detected across samples, which was positively correlated with cellulolytic ability. A representative Acidovorax strain was isolated, but did not grow on cellulose alone. Phenotypic and compositional analyses of enrichment cultures, such as those presented here, help link community composition with cellulolytic ability and provide insight into the complexity of community-based cellulose degradation.     

Moisture enhances the positive effect of leaf‐cutting ant refuse dumps on soil biota activity

Soil biota activity in arid lands is often limited by the availability of water and organic matter. We experimentally explored whether small changes in soil moisture affect the activity of soil biota in external refuse dumps of the leaf‐cutting ant Acromyrmex lobicornis, one of the most important sources of organic matter in a semi‐arid land of north‐western Patagonia. We estimated CO₂ consumption in refuse dumps and in adjacent, non‐nest soil samples at two moisture levels, after 48 and 72 h. Soil biota activity, estimated by respiration rates, was up to 160 times greater in refuse dumps than in adjacent, non‐nest soils. Activity of soil biota in non‐nest soil did not change through time and was not affected by moisture. Conversely, soil biota increased their activity in refuse dump samples only at high moisture condition after 72 h. As the activity of microorganisms is key for soil nutrient generation and availability, refuse dumps may be considered as ‘islands of fertility’ for plants. This effect may be especially important after sporadic spring rainfalls, when the beneficial effect of refuse dumps on soil biota is enhanced. In addition, as refuse dumps generate several times more CO₂ than non‐nest soils, nest areas may be considered also as hot spots of CO₂ emissions. These results illustrate the potential importance of ant nests for nutrient cycling, maintenance of plant cover and carbon balance in arid ecosystems.     

Relationship of water potential to growth of leaves

A thermocouple psychrometer that measures water potentials of intact leaves was used to study the water potentials at which leaves grow. Water potentials and water uptake during recovery from water deficits were measured simultaneously with leaves of sunflower (Helianthus annuus L.), tomato (Lycopersicon esculentum Mill.), papaya (Carica papaya L.), and Abutilon striatum Dickson. Recovery occurred in 2 phases. The first was associated with elimination of water deficits; the second with cell enlargement. The second phase was characterized by a steady rate of water uptake and a relatively constant leaf water potential. Enlargement was 70% irreversible and could be inhibited by puromycin and actinomycin D. During this time, leaves growing with their petioles in contact with pure water remained at a water potential of —1.5 to —2.5 bars regardless of the length of the experiment. It was not possible to obtain growing leaf tissue with a water potential of zero. It was concluded that leaves are not in equilibrium with the potential of the water which is absorbed during growth. The nonequilibrium is brought about by a resistance to water flow which requires a potential difference of 1.5 to 2.5 bars in order to supply water at the rate necessary for maximum growth.

Leaf growth occurred in sunflower only when leaf water potentials were above —3.5 bars. Sunflower leaves therefore require a minimum turgor for enlargement, in this instance equivalent to a turgor of about 6.5 bars. The high water potentials required for growth favored rapid leaf growth at night and reduced growth during the day.

     

Seedling recruitment in a semi‐arid Patagonian steppe: Facilitative effects of refuse dumps of leaf‐cutting ants

Question: What is the influence of refuse dumps of leaf‐cutting ants on seedling recruitment under contrasting moisture conditions in a semi‐arid steppe? Location: Northwestern Patagonia, Argentina. Methods: In a greenhouse experiment, we monitored seedling recruitment in soil samples from refuse dumps of nests of the leaf‐cutting ant Acromyrmex lobicornis and non‐nest sites, under contrasting moisture conditions simulating wet and dry growing seasons. Results: The mean number of seedling species and individuals were higher in wet than in dry plots, and higher in refuse dump plots than in non‐nest soil plots. The positive effect of refuse dumps on seedling recruitment was greater under low moisture conditions. Both the accumulation of discarded seeds by leaf‐cutting ants and the passive trapping of blowing‐seeds seems not explain the increased number of seeds in refuse dumps. Conversely, refuse dumps have higher water retention capacity and nutrient content than adjacent non‐nest soils, allowing the recruitment of a greater number of species and individual seedlings. Conclusions: Nests of A. lobicornis may play an important role in plant recruitment in the study area, allowing a greater number of seedlings and species to be present, hence resulting in a more diverse community. Moreover, leaf‐cutting ant nests may function as nurse elements, generating safe sites that enhance the performance of neighbouring seedlings mainly during the driest, stressful periods.     

EFFECT OF WATER POTENTIAL ON A MICROCOLEUS (CYANOPHYCEAE) FROM A DESERT CRUST1

The effect of water potential, on the growth and photosynthesis of a species of Microcoleus forming a desert crust was determined, using both osmotic and matric variations in water potential. The alga was quite sensitive to moisture stress, partial inhibition of growth being observed at -7 bars, and complete inhibition at -18 bars. Photosynthesis was markedly inhibited at -18 bars, and virtually completely at, -28 bars (water potential of seawater) and lower. The alga was more sensitive to matric reduction in water potential than osmotic. By comparisons of these results with those obtained with other algae, it is concluded that this desert crust alga is not especially adapted to grow and photosynthesize at low water potentials, although it shows considerable ability to survive severe drought conditions.     

The role of a plant/fungal consortium in the degradation of bituminous hard coal

The sporadic growth of Cynodon dactylon was observed to occur directly on the surface of hard coal in dumps of the Witbank coal mining area of South Africa with the surface coal being broken down into a humic-like particulate material. Microorganism analysis of plants and rhizosphere material from the dumps revealed the presence of arbuscular mycorrhizal fungi and the coal solubilising fungus, Neosartorya fischeri. Studies established to replicate the dump environment revealed increased coal degradation in the form of humic acid production and an increase in small size particles as a result of Cynodon dactylon growth in association with arbuscular mycorrhizal fungi and Neosartorya fischeri. Results suggest that interactions between Cynodon dactylon, arbuscular mycorrhizal fungi, Neosartorya fischeri and other coal-degrading rhizosphere fungi could lead to the degradation of hard coal in situ and that the application of these organisms to discard dumps could be a novel method of coal dump rehabilitation.     

How plants may benefit from their consumers: leaf-cutting ants indirectly improve anti-herbivore defenses in <Emphasis Type="Italic">Carduus nutans</Emphasis> L

Although herbivores often have a negative impact on plant fitness, sometimes plants may benefit from their consumers. However, these positive interactions usually occur as a result of plant damage (e.g., overcompensation, defense induction). I present evidence of a novel way by which plants may benefit from their consumers without being eaten. Plants of Carduus nutans increased their physical defenses when grown in external refuse dumps of the leaf-cutting ant Acromyrmex lobicornis. Seedlings planted in refuse exhibited longer spines and tougher leaves than those planted in control soils. Pick-up assays with entire leaves and leaf discs demonstrated that these enhanced physical defenses prevented leaf-cutting ant harvest. Additionally, plants established in refuse dumps showed fewer insect herbivory than those in non-nest soils. The nutrient-rich refuse dump appeared to reduce the stage at which leaves are tender and thus more vulnerable to herbivory. This is the first case where plants may benefit from insect herbivores via waste products without the cost of being eaten. This illustrates how plants may plastically respond to reliable cues of the risk of herbivory.     

Microbial Community Structure of Leaf-Cutter Ant Fungus Gardens and Refuse Dumps

Background

Leaf-cutter ants use fresh plant material to grow a mutualistic fungus that serves as the ants'' primary food source. Within fungus gardens, various plant compounds are metabolized and transformed into nutrients suitable for ant consumption. This symbiotic association produces a large amount of refuse consisting primarily of partly degraded plant material. A leaf-cutter ant colony is thus divided into two spatially and chemically distinct environments that together represent a plant biomass degradation gradient. Little is known about the microbial community structure in gardens and dumps or variation between lab and field colonies.

Methodology/Principal Findings

Using microbial membrane lipid analysis and a variety of community metrics, we assessed and compared the microbiota of fungus gardens and refuse dumps from both laboratory-maintained and field-collected colonies. We found that gardens contained a diverse and consistent community of microbes, dominated by Gram-negative bacteria, particularly γ-Proteobacteria and Bacteroidetes. These findings were consistent across lab and field gardens, as well as host ant taxa. In contrast, dumps were enriched for Gram-positive and anaerobic bacteria. Broad-scale clustering analyses revealed that community relatedness between samples reflected system component (gardens/dumps) rather than colony source (lab/field). At finer scales samples clustered according to colony source.

Conclusions/Significance

Here we report the first comparative analysis of the microbiota from leaf-cutter ant colonies. Our work reveals the presence of two distinct communities: one in the fungus garden and the other in the refuse dump. Though we find some effect of colony source on community structure, our data indicate the presence of consistently associated microbes within gardens and dumps. Substrate composition and system component appear to be the most important factor in structuring the microbial communities. These results thus suggest that resident communities are shaped by the plant degradation gradient created by ant behavior, specifically their fungiculture and waste management.     

The interactive effects of temperature and osmotic potential on the growth of marine isolates of <Emphasis Type="Italic">Fusarium solani</Emphasis>

The mycelial growth of 18 Fusarium solani strains isolated from sea beds of the south-eastern coast of Spain was tested on potato-dextrose-agar adjusted to different osmotic potentials with either KCl or NaCl (−1.50 to −144.54 bars) in 10 °C intervals ranging from 15 to 35 °C. Fungal growth was determined by measuring colony diameter after 4 days of incubation. Mycelial growth was maximal at 25 °C. The quantity and frequency pattern of mycelial growth of F. solani differ significantly at 15 and 25 °C, with maximal growth occurring at the highest water potential tested (−1.50 bars); and at 35 °C, with a maximal mycelial growth at −13.79 bars. The effect of water potential was independent of salt composition. The general growth pattern of F. solani showed declining growth at potentials below −41.79 bars. Fungal growth at 35 °C was always higher than that grow at 15 °C, of all the water potentials tested. Significant differences observed in the response of mycelia to water potential and temperature as main and interactive effects. The viability of cultures was increasingly inhibited as the water potential dropped, but some growth was still observed at −99.56 bars. These findings could indicate that marine strains of F. solani have a physiological mechanism that permits survival in environments with low water potential. The observed differences in viability and the magnitude of growth could indicate that the biological factors governing potential and actual growth are affected by osmotic potential in different ways.     

Chloroplast response to low leaf water potentials: I. Role of turgor

The effect of decreases in turgor on chloroplast activity was studied by measuring the photochemical activity of intact sunflower (Helianthus annuus L. cv. Russian Mammoth) leaves having low water potentials. Leaf turgor, calculated from leaf water potential and osmotic potential, was found to be affected by the dilution of cell contents by water in the cell walls, when osmotic potentials were measured with a thermocouple psychrometer. After the correction of measurements of leaf osmotic potential, both the thermocouple psychrometer and a pressure chamber indicated that turgor became zero in sunflower leaves at leaf water potentials of −10 bars. Since most of the loss in photochemical activity occurred at water potentials below −10 bars, it was concluded that turgor had little effect on the photochemical activity of the leaves.     

Chloroplast Response to Low Leaf Water Potentials: IV. Quantum Yield Is Reduced

Quantum yields were measured for CO₂ fixation by sunflower (Helianthus annuus L.) leaves having various water potentials and for dichlorophenolindophenol photoreduction by chloroplasts isolated from similar leaves having various water potentials. In red radiation, the quantum yield for CO₂ was 0.076 for an attached sunflower leaf at a water potential of −3 to −4 bars but was 0.020 for the same leaf at −15.3 bars. After recovery to a water potential of −5 bars, the quantum yield rose to 0.060. Soybean (Glycine max L. [Merr.]) leaves behaved similarly. Chloroplasts from a sunflower leaf with a water potential of −3.6 bars had a quantum yield for 4 equivalents of 0.079, but when tissue from the same leaf had a water potential of −14.8 bars, the quantum yield of the chloroplasts decreased to 0.028. The decrease could not be attributed to differences in rates of respiration by the leaves or the chlorophyll content or absorption spectrum of the leaves and chloroplasts.     

The Effect of Soil Water Regimes on Leaf Water Potential, Growth and Development of Soybeans

The growth and development of soybeans (Glycine max L. cv. Amsoy) was studied at soil matric potentials of ?0.1 to ?1.0 bars. Chlorophyll, photosynthesis, and leaf nitrogen per plant was greatest at ?4 bars leaf water potential. Leaf area, number of internodes, plant height and dry weight of vegetative parts declined as leaf water potential decreased from ?2 to ?19 bars. Nitrogen content and nitrate reductase activity per g fresh weight determined the percentage protein of individual seeds but nitrogen content and nitrate reductase activity per plant determined the amount of total seed protein. The protein synthesized in the seed changed little in amino acid composition with changes in leaf water potential. Leaf water potentials above or below ?4 bars decreased yield, total protein and total lipid but plants produced the largest percentage of individual seed protein at ?19 bars leaf water potential.     

Effect of water potential on seed germination

The response of seed germination to substrate water potential was determined for several plant species of the arctic tundra. Seeds were collected from Cape Thompson and Eagle Summit, Alaska and germinated on dialysis membranes over water solutions of polyethylene glycol with osmotic potentials of 0 to −6 bars. Germination did not occur with potentials below −3 bars, except for three fellfield species. Germination was delayed at lower osmotic potentials. Because the response of most species was similar, substrate water potential is probably not a factor affecting the establishment of most tundra plant species from seeds.     

Water potentials induced by growth in soybean hypocotyls

Gradients in water potential form the driving force for the movement of water for cell enlargement. In stems, they are oriented radially around the vascular system but should also be present along the stem. To test this possibility, growth, water potential, osmotic potential, and turgor were determined at intervals along the length of dark-grown soybean (Glycine max L. Merr., cv. Wayne) hypocotyls. Transpiration was negligible in the dark, humid conditions, so that all water uptake was for growth. Elongation occurred in the terminal 1.5 centimeters of the hypocotyl. Water potential was −3.5 bars in the elongating region but −0.5 bar in the mature region, both in intact plants and detached tissue. There was a gradual transition between these values that was related to the growth profile along the hypocotyl. Tissue osmotic potentials generally paralleled tissue water potentials, so that turgor was the same throughout the length of the hypocotyl. If the elongating zone was excised, growth ceased immediately. If the elongating zone was excised along with mature tissue, however, growth continued, which confirmed the presence of a water-potential gradient that caused longitudinal water movement from the mature zone to the elongating zone. When the plants were grown in vermiculite having low water potentials, tissue water potentials and osmotic potentials both decreased, so that water potential gradients and turgor remained undiminished. It is concluded that growth-induced water potentials reflect the local activity for cell enlargement and are supported by appropriate osmotic potentials.     

Refuse dumps from leaf-cutting ant nests reduce the intensity of above-ground competition among neighboring plants in a Patagonian steppe

In arid environments, the high availability of sunlight due to the scarcity of trees suggests that plant competition take place mainly belowground for water and nutrients. However, the occurrence of soil disturbances that increase nutrient availability and thereby promote plant growth may enhance shoot competition between neighboring plants. We conducted a greenhouse experiment to evaluate the influence of the enriched soil patches generated by the leaf-cutting ant, Acromyrmex lobicornis, on the performance of the alien forb Carduus thoermeri (Asteraceae) under different intraspecific competition scenarios. Our results showed that substrate type and competition scenario affected mainly aboveground plant growth. As expected, plants growing without neighbors and in nutrient-rich ant refuse dumps showed more aboveground biomass than plants growing with neighbors and in nutrient-poor steppe soils. However, aboveground competition was more intense in nutrient-poor substrates: plants under shoot and full competition growing in the nutrient-rich ant refuse dumps showed higher biomass than those growing on steppe soils. Belowground biomass was similar among focal plants growing under different substrate type. Our results support the traditional view that increments in resource availability reduce competition intensity. Moreover, the fact that seedlings in this sunny habitat mainly compete aboveground illustrates how limiting factors may be scale-dependent and change in importance as plants grow.     

Stomatal diffusion resistance of snap beans. I. Influence of leaf-water potential

Concurrent measurements of abaxial and adaxial stomatal resistance and leaf-water potentials of snap beans (Phaseolus vulgaris L.) in the field and growth chamber show that the stomata on the 2 surfaces of the leaflet react differently to water deficit. The stomata on the abaxial surface, which are about 7 times more numerous than on the adaxial surface, are not significantly affected at leaf-water potentials greater than —11 bars, but with further decrease in leaf-water potential, the resistance rapidly increases. On the other hand, the resistance of the adaxial stomata increases sharply at a leaf-water potential of about —8 bars and is constant at higher water potentials. The average stomatal resistance for both surfaces of the leaf, which is the major diffusive resistance to water vapor, to a first approximation acts as an on-off switch and helps prevent further decline in leaf-water potential. The relation between the leaf-water potential and the stomatal resistance links the soil-water potential to the transpiration stream as needed for soil-plant-atmosphere models.     

Immunofluorescence Approach to the Study of the Ecology of Thermoplasma acidophilum In Coal Refuse Material

Specific immunofluorescence staining was applied to the study of the localization, distribution, and growth of Thermoplasma acidophilum in its natural habitat, the coal refuse pile. Different antigenic groups of T. acidophilum could be isolated from the same refuse pile, and the same antigenic groups were isolated from piles from different geographical areas. No correlation could be established between the antigenic groups and the pH or temperature of the habitats. Brightly fluorescing cells of T. acidophilum were detected on microscope slides buried in contact with the coal refuse material or immersed in the water in the stream draining a refuse pile. T. acidophilum grew when inoculated into either coal refuse material and/or an aqueous extract of coal refuse when incubated at its optimal temperature of 55 C, but not when incubated at room temperature or 37 C. The coal refuse pile appears to be a primary habitat for T. acidophilum.     

Growth-induced Water Potentials in Plant Cells and Tissues

A physical analysis of water movement through elongating soybean (Glycine max L. Merr.) hypocotyls was made to determine why significant water potentials persist in growing tissues even though the external water potentials were zero and transpiration is virtually zero. The analysis was based on a water transport theory modified for growth and assumed that water for growing cells would move through and along the cells in proportion to the conductivity of the various pathways.

Water potentials calculated for individual cells were nearly in local equilibrium with the water potentials of the immediate cell surroundings during growth. However, water potentials calculated for growing tissue were 1.2 to 3.3 bars below the water potential of the vascular supply in those cells farthest from the xylem. Only cells closest to the xylem had water potentials close to that of the vascular supply. Gradients in water potential were steepest close to the xylem because all of the growth-sustaining water had to move through this part of the tissue. Average water potentials calculated for the entire growing region were −0.9 to −2.2 bars depending on the tissue diffusivity.

For comparison with the calculations, average water potentials were measured in elongating soybean hypocotyls using isopiestic thermocouple psychrometers for intact and excised tissue. In plants having virtually no transpiration and growing in Vermiculite with a water potential of −0.1 bar, rapidly growing hypocotyl tissue had water potentials of −1.7 to −2.1 bars when intact and −2.5 bars when excised. In mature, nongrowing hypocotyl tissue, average water potentials were −0.4 bar regardless of whether the tissue was intact or excised.

The close correspondence between predicted and measured water potentials in growing tissue indicates that significant gradients in water potential are required to move growth-associated water through and around cells over macroscopic distances. The presence of such gradients during growth indicates that cells must have different cell wall and/or osmotic properties at different positions in the tissue in order for organized growth to occur. The mathematical development used in this study represents the philosophy that would have to be followed for the application of contemporary growth theory when significant tissue water potential gradients are present.

     

Biotechnology in hydrometallurgical processes

The presence of microorganisms in metal sulfide leaching operations has been found to be beneficial in catalysing the dissolution process. Currently this technology is applied on a commercial scale in the recovery of copper and uranium in heap, dump and in-situ leach techniques. However, the biotechnological principles are not limited to the extraction of these two elements. Cobalt, nickel and zinc could be solubilized from their respective sulfides, and coal, silver and gold resources purified from pyrite and arsenopyrite inclusions.