High Nutrient Environments Research Articles

To eat or not to eat: novel stable isotope models reveal a shift in carnivory with nutrient availability for aquatic Utricularia spp.

Freshwater nitrogen inputs are increasing globally, altering the structure and function of wetland ecosystems adapted to low nutrient conditions. Carnivorous wetland plants of the genus Utricularia are hypothesized to reduce their reliance on carnivory and increase their assimilation of environmental nutrients when the supply of ambient nutrients increases. Despite success in using stable isotope approaches to quantify carnivory of terrestrial carnivorous plants, quantifying carnivory of aquatic Utricularia requires improvement. We developed stable isotope mixing models to quantify aquatic plant carnivory and used these models to measure dietary changes of three Utricularia species, Utricularia australis, U. gibba and U. uliginosa, in 11 wetlands across a 794-km gradient in eastern Australia. Diet was assessed using multiple models that compared variations in the natural-abundance nitrogen isotope composition (δ15N) of Utricularia spp. with that of non-carnivorous plants, and environmental and carnivorous nitrogen sources. Carnivory supplied 40-100 % of plant nitrogen. The lowest carnivory rates coincided with the highest availability of ammonium and dissolved organic carbon. Our findings suggest that Utricularia populations may adapt to high nutrient environments by shifting away from energetically costly carnivory. This has implications for species conservation as anthropogenic impacts continue to affect global wetland ecosystems.

Does high parasite load contribute to limitation of the poleward range of Acropora corals?

The role of species interactions in setting species range limits is rarely empirically explored. Here, we quantify host and parasite densities in subtropical eastern Australia (26.65°–30.20°S) to examine whether parasitism might contribute to range limitation of Acropora corals at their cold-range boundary. 79% of Acropora corals had endolithic barnacles (family Pyrgomatidae), with higher parasite load in larger corals and up to 141 barnacles per coral. Parasite load increased poleward and closer to the mainland and was greater in cooler and high nutrient environments. Parasite burden was higher at sites with fewer Acropora corals, broadly consistent with the hypothesis that parasites can fragment host populations where host densities are low, and the parasite is a better disperser than the host. Whilst the mechanism is unclear, our findings suggest that at the high densities recorded here, coral-barnacles could influence range dynamics of Acropora corals at their poleward range limit.

Intraspecific alternative phenotypes contribute to variation in species' strategies for growth.

Ecologists have historically sought to identify the mechanisms underlying the maintenance of local species diversity. High-dimensional trait-based relationships, such as alternative phenotypes, have been hypothesized as important for maintaining species diversity such that phenotypically dissimilar individuals compete less for resources but have similar performance in a given environment. The presence of alternative phenotypes has primarily been investigated at the community level, despite the importance of intraspecific variation to diversity maintenance. The aims of this research are to (1) determine the presence or absence of intraspecific alternative phenotypes in three species of tropical tree seedlings, (2) investigate if these different species use the same alternative phenotypes for growth success, and (3) evaluate how findings align with species co-occurrence patterns. We model species-specific relative growth rate with individual-level measurements of leaf mass per area (LMA) and root mass fraction (RMF), environmental data, and their interactions. We find that two of the three species have intraspecific alternative phenotypes, with individuals within species having different functional forms leading to similar growth. Interestingly, individuals within these species use the same trait combinations, high LMA low RMF and low LMA high RMF, in high soil nutrient environments to acquire resources for higher growth. This similarity among species in intraspecific alternative phenotypes and variables that contribute most to growth may lead to their negative spatial co-occurrence. Overall, we find that multiple traits or interactions between traits and the environment drive species-specific strategies for growth, but that individuals within species leverage this multi-dimensionality in different ways for growth success.

Taxonomy, paleoecology, and paleobiogeographical significance of the middle-late Eocene molluscs from the Garet Gehannem section in the Fayoum depression, Egypt

The studied middle-upper Eocene Garet Gehannam section exhibits moderate macrofossil diversity and notably high macrofossil abundance. Eleven molluscan species were identified and described, including five bivalves and six gastropods. Three macrobenthic assemblages were recognized: The turritelline-dominated assemblage, Carolia placunoides assemblage and Turkostrea multicostata assemblage. The Turritelline-dominated assemblage indicates high-nutrient environments, which typically exhibit normal to slightly below normal marine salinities, low sedimentation rates, and high-energy environments. Carolia placunoides assemblage indicates brackish water environment during times of sea level regression. Turkostrea multicostata assemblage provides compelling evidence of a prevailing hot climate and an environment rich in calcium, characterized by nutrient-rich water and low to moderate energy levels and high light availability. The investigation of the articulation of Carolia placunoides Cantraine1838 and Turkostrea multicostata (Deshayes, 1818) shells strongly suggests a parautochthonous assemblage. In terms of paleobiogeography, Turkostrea multicostata (Deshayes, 1818) first appeared during the Paleocene in the southern Tethys, specifically in Tunisia and Algeria, as well as in the Trans-Saharan Seaway in Mali. Consequently, its paleobiogeographic distribution can be allied to current circulation along the Tethys Ocean, firstly from its original area in the southern Tethys regions and Trans-Saharan Seaway to neighboring regions of the southern and northern Tethys, as well as the West Africa provinces. The paleobiogeographic map of the identified species shows that the identified species were prevailing in two bioprovinces during the middle – late Eocene epochs: i) the North-West, North, and South Tethyan Margins Province, ii) and the West Africa Province.

Nitrogen and phosphorus availability alters tree‐grass competition intensity in savannas

Abstract Plant essential macronutrients like nitrogen (N) and phosphorus (P) can limit savanna tree growth and are important determinants of savanna vegetation dynamics, along with rainfall, fire and herbivory. How nitrogen and phosphorus shape tree‐grass competition and their coexistence remain unclear, hindering our ability to predict how savannas may respond to altered nutrient cycling. Here, we evaluate (1) if trees and grasses respond differently to N versus P availability, or (2) if grasses are more competitive in low nutrient environments while trees are more competitive in high nutrient environments. To do this, we grew saplings of 6 tree and 1 grass species from the Kruger National Park, South Africa, for 16 weeks under fully factorial nutrient and competition treatments (with/without competitors, low/high rate of N supply and low/high rate of P supply) under a watering regime designed to mimic wet season rainfall in a mesic savanna. Trees and grasses foraged most aggressively for nitrogen and allocated biomass differently depending on nitrogen availability. Overall, tree growth decreased in competition with grass, even in high nutrient environments where they grew faster. Grasses were always better below‐ground competitors, utilising aggressive nutrient foraging strategies, including high root phosphatase activity in response to nitrogen and large root biomass allocation. Synthesis. In low nutrient environments (e.g. on nutrient‐poor sandy soils), nutrients may limit tree growth. Nutrient rich environments enable tree growth, but grasses continue to compete effectively with trees. Understanding what this means for ecosystem responses to nutrient availability is not trivial, especially in the context of fire and herbivory. However, it is clear that soil nutrients likely affect tree and grass growth and competition in savannas, which suggests that future changes in nutrient cycling, such as N deposition, may have important effects on savanna vegetation.

Selection for oligotrophy among bacteria inhabiting host microbiomes.

Understanding how natural selection has historically shaped the traits of microbial populations comprising host microbiomes would help predict how the functions of these microbes may continue to evolve over space and time. Numerous host-associated microbes have been found to adapt to their host, sometimes becoming obligate symbionts, whereas free-living microbes are best known to adapt to their surrounding environment. Our study assessed the selective pressures of both the host environment and the surrounding external environment in shaping the functional potential of host-associated bacteria. Despite residing within the resource-rich microbiome of their hosts, we demonstrate that host-associated heterotrophic bacteria show evidence of trait selection that matches the nutrient availability of their broader surrounding environment. These findings illustrate the complex mix of selective pressures that likely shape the present-day function of bacteria found inhabiting host microbiomes. Our study lends insight into the shifts in function that may occur as environments fluctuate over time.

The upper ocean silicon cycle of the subarctic Pacific during the EXPORTS field campaign

Diatoms are major contributors to marine primary productivity and carbon export due to their rapid growth in high-nutrient environments and their heavy silica ballast. Their contributions are highly modified in high-nutrient low-chlorophyll regions due to the decoupling of upper-ocean silicon and carbon cycling caused by low iron (Fe). The Si cycle and the role of diatoms in the biological carbon pump was examined at Ocean Station Papa (OSP) in the HNLC region of the northeastern subarctic Pacific during the NASA EXport Processes in the Ocean from RemoTe Sensing (EXPORTS) field study. Sampling occurred during the annual minimum in surface silicic acid (Si(OH)4) concentration. Biogenic silica (bSi) concentrations were low, being in the tens of nanomolar range, despite high Si(OH)4 concentrations of about 15 μM. On average, the >5.0-µm particle size fraction dominated Si dynamics, accounting for 65% of bSi stocks and 81% of Si uptake compared to the small fraction (0.6–5.0 μm). Limitation of Si uptake was detected in the small, but not the large, size fraction. Growth rate in small diatoms was limited by Fe, while their Si uptake was restricted by Si(OH)4 concentration, whereas larger diatoms were only growth-limited by Fe. About a third of bSi production was exported out of the upper 100 m. The contribution of diatoms to carbon export (9–13%) was about twice their contribution to primary productivity (3–7%). The combination of low bSi production, low diatom primary productivity and high bSi export efficiency at OSP was more similar to the dynamics in the subtropical gyres than to other high-nutrient low-chlorophyll regions.

The fern economics spectrum is unaffected by the environment.

The plant economics spectrum describes the trade-off between plant resource acquisition and storage, and sheds light on plant responses to environmental changes. However, the data used to construct the plant economics spectrum comes mainly from seed plants, thereby neglecting vascular non-seed plant lineages such as the ferns. To address this omission, we evaluated whether a fern economics spectrum exists using leaf and root traits of 23 fern species living under three subtropical forest conditions differing in light intensity and nutrient gradients. The fern leaf and root traits were found to be highly correlated and formed a plant economics spectrum. Specific leaf mass and root tissue density were found to be on one side of the spectrum (conservative strategy), whereas photosynthesis rate, specific root area, and specific root length were on the other side of the spectrum (acquisitive strategy). Ferns had higher photosynthesis and respiration rates, and photosynthetic nitrogen-use efficiency under high light conditions and higher specific root area and lower root tissue density in high nutrient environments. However, environmental changes did not significantly affect their resource acquisition strategies. Thus, the plant economics spectrum can be broadened to include ferns, which expands its phylogenetic and ecological implications and utility.

Phenotypic plasticity and exotic plant invasions: Effects of soil nutrients, species nutrient requirements, and types of traits.

High-phenotypic plasticity has long been considered as a characteristic promoting exotic plant invasions. However, the results of the studies testing this hypothesis are still inconsistent. Overlooking the effects of species resource requirements and environmental resource availability may be the main reasons for the ambiguous conclusions. Here, we compared phenotypic plasticity between five noxious invasive species with different nutrient requirements (evaluated using the soil nutrient status of their natural distribution ranges) and their phylogenetically related natives under five nutrient levels. We found that species with high-nutrient requirements showed greater plasticity of total biomass than species with low-nutrient requirements, regardless of their status (invasive or native). Invasives with high-nutrient requirements had greater growth plasticity than their related natives, which may contribute to their invasiveness under high-nutrient environments. However, compared with the related natives, a higher growth plasticity may not help exotic species with low-nutrient requirements to invade nutrient-rich habitats, and exotic species with high-nutrient requirements to invade nutrient-limited habitats. In contrast, invasives with low-nutrient requirements exhibited lower growth plasticity than their related natives, contributing to their invasiveness under nutrient-limited habitats. Functional traits showed growth-related plasticity in only 10 cases (3.8%), and there was no functional trait whose plastic response to soil nutrients was beneficial to exotic plant invasions. Our study indicates that low-growth plasticity could also promote exotic plant invasions, high plasticity may not necessarily lead to invasiveness. We must test the adaptive significance of plasticity of functional traits when studying its biological roles.

Improper Diving Behavior Affects Physiological Responses of Acropora hyacinthus and Porites cylindrica

Human activities beyond ecosystem capacity have resulted in serious effects on corals worldwide. Nowadays, many studies have focused on the influence of diving activities on coral communities, while the knowledge of physiological changes under corresponding environmental stresses remains largely undetermined. In the study, we aimed to investigate the physiological effects of touching, ammonia nitrogen enrichment (5 μmol⋅L–1), and sediment cover (particle size of less than 0.3 mm), which simulated improper self-contained underwater breathing apparatus (SCUBA) diving behaviors, on Acropora hyacinthus and Porites cylindrica in Wuzhizhou Island, the South China Sea. For A. hyacinthus, continuous touching caused the tentacles to shrink and secrete mucus, which consumed energy and dissolved oxygen. The skeletal growth rate was decreased by 72% compared with the control group. There was a rapid decline of Fv/Fm and alpha under the dual impacts of high ammonia nitrogen and touching, while the Chl a concentration and tissue biomass were decreased by 36 and 28% compared with touching alone, respectively. High ammonia nitrogen and touching increased the concentrations of lipid and protein. Nevertheless, zooxanthellae density was increased by 23% to relieve the effects of a lower concentration of Chl a in a high nutrient environment. Constant touching and sediment cover in diving areas with elevated ammonia content affected the photosynthesis and respiration of corals, and a significant decrease was observed for lipid, zooxanthellae density, and Chl a concentration. Coral bleaching occurred on day 7. For P. cylindrica, the decreasing magnitude of Fv/Fm and alpha under different stresses in the subsequent phase was less compared with A. hyacinthus. The contents of carbohydrate and protein under continuous touching were decreased by 7 and 15% compared with the control group, respectively, causing negative growth. Under the dual influences of high ammonia nitrogen and continuous touching, all energy reserves were significantly lower. Repeated touching and sediment cover in diving areas containing high ammonia content increased the concentrations of lipid and protein compared with the touching and high nutrient treatment group likely because that Porites associated with C15 zooxanthella increased heterotrophic feeding to compensate for restricted symbiodiniaceae photosynthesis. Additionally, P. cylindrica produced mucus to aid the removal of sediment, so that corals didn’t obviously bleach during the experiment. Collectively, P. cylindrica was more resistant to diving activities than A. hyacinthus which only relies on photoautotrophy. To ensure the sustainable development of coral reef dive tourism, it is necessary to strengthen the supervision of diving behaviors, rotate the diving areas, and conduct regular assessments on the coral status.

Gone and forgotten: facilitative effects of intercropping combinations did not carry over to affect barley performance in a follow-up crop rotation

AimIntercropping often leads to improved productivity of individual species compared to monocultures. We have practically little knowledge of facilitation effects in different intercropping systems and their importance in creating soil legacies that can indirectly affect the succeeding crop in a crop rotation through plant-soil feedback (PSF) effects.MethodsTo test this, we used a two-phased field experiment where we combined intercropping and crop rotation. During intercropping, we grew maize, faba bean, and lupine in monocultures or two-species crop combinations. The following season, we grew winter barley on the soil previously used for intercropping to test PSF effects under field conditions.ResultsWe found evidence for facilitative effects on aboveground biomass production that were species-specific with faba bean and maize biomass benefitting when intercropped compared to their expected biomasses in monocultures. Lupine, in contrast, performed best in monocultures. After the intercropping phase, total soil mineral nitrogen was higher in legume monocultures creating soil legacies but this did not affect soil microbial parameters and barley biomass production in the follow-up rotation phase.ConclusionsWe found support for species-specific positive and negative interactions in intercropping. Our results also demonstrated that soil legacies play no significant role under moderately high nutrient environments.

Deer browsing effects on temperate forest soil nitrogen cycling shift from positive to negative across fertility gradients

Herbivores impact soil biogeochemical processes, often increasing nutrient cycling rates under high nutrient availability and decreasing nutrient cycling rates under low nutrient availability. These patterns are far from universal, and interactions between habitat fertility and herbivore effects are under continuing investigation. By sampling inside and outside a network of deer exclosures, we determined deer browsing effects on temperate forest soil nitrogen (N) and carbon (C) cycling along a gradient of soil and litter C–N ratios across our network of sites. Deer browsing increased net N mineralization rates in high nutrient environments and decreased N mineralization rates in low nutrient environments, whereas browsing decreased CO2 respiration rates in high nutrient environments and increased CO2 respiration rates in low nutrient environments. Differences in deer browsing effects on soil processes could be explained by plant responses to herbivory across gradients of resource availability. To our knowledge, our study is one of the first to show that deer browsing can have significant effects on net N mineralization and C respiration in temperate forest soils and that the direction and magnitude of deer browsing effects on soil N and C cycling vary across fertility gradients.

Long‐term Dynamics and Drivers of Coral and Macroalgal Cover on Inshore Reefs of the Great Barrier Reef Marine Park

Quantifying the role of biophysical and anthropogenic drivers of coral reef ecosystem processes can inform management strategies that aim to maintain or restore ecosystem structure and productivity. However, few studies have examined the combined effects of multiple drivers, partitioned their impacts, or established threshold values that may trigger shifts in benthic cover. Inshore fringing reefs of the Great Barrier Reef Marine Park (GBRMP) occur in high-sediment, high-nutrient environments and are under increasing pressure from multiple acute and chronic stressors. Despite world-leading management, including networks of no-take marine reserves, relative declines in hard coral cover of 40–50% have occurred in recent years, with localized but persistent shifts from coral to macroalgal dominance on some reefs. Here we use boosted regression tree analyses to test the relative importance of multiple biophysical drivers on coral and macroalgal cover using a long-term (12–18 yr) data set collected from reefs at four island groups. Coral and macroalgal cover were negatively correlated at all island groups, and particularly when macroalgal cover was above 20%. Although reefs at each island group had different disturbance-and-recovery histories, degree heating weeks (DHW) and routine wave exposure consistently emerged as common drivers of coral and macroalgal cover. In addition, different combinations of sea-surface temperature, nutrient and turbidity parameters, exposure to high turbidity (primary) floodwater, depth, grazing fish density, farming damselfish density, and management zoning variously contributed to changes in coral and macroalgal cover at each island group. Clear threshold values were apparent for multiple drivers including wave exposure, depth, and degree heating weeks for coral cover, and depth, degree heating weeks, chlorophyll a, and cyclone exposure for macroalgal cover, however, all threshold values were variable among island groups. Our findings demonstrate that inshore coral reef communities are typically structured by broadscale climatic perturbations, superimposed upon unique sets of local-scale drivers. Although rapidly escalating climate change impacts are the largest threat to coral reefs of the GBRMP and globally, our findings suggest that proactive management actions that effectively reduce chronic stressors at local scales should contribute to improved reef resistance and recovery potential following acute climatic disturbances.

Long-term dynamics and drivers of coral and macroalgal cover on inshore reefs of the Great Barrier Reef Marine Park.

Quantifying the role of biophysical and anthropogenic drivers of coral reef ecosystem processes can inform management strategies that aim to maintain or restore ecosystem structure and productivity. However, few studies have examined the combined effects of multiple drivers, partitioned their impacts, or established threshold values that may trigger shifts in benthic cover. Inshore fringing reefs of the Great Barrier Reef Marine Park (GBRMP) occur in high-sediment, high-nutrient environments and are under increasing pressure from multiple acute and chronic stressors. Despite world-leading management, including networks of no-take marine reserves, relative declines in hard coral cover of 40-50% have occurred in recent years, with localized but persistent shifts from coral to macroalgal dominance on some reefs. Here we use boosted regression tree analyses to test the relative importance of multiple biophysical drivers on coral and macroalgal cover using a long-term (12-18yr) data set collected from reefs at four island groups. Coral and macroalgal cover were negatively correlated at all island groups, and particularly when macroalgal cover was above 20%. Although reefs at each island group had different disturbance-and-recovery histories, degree heating weeks (DHW) and routine wave exposure consistently emerged as common drivers of coral and macroalgal cover. In addition, different combinations of sea-surface temperature, nutrient and turbidity parameters, exposure to high turbidity (primary) floodwater, depth, grazing fish density, farming damselfish density, and management zoning variously contributed to changes in coral and macroalgal cover at each island group. Clear threshold values were apparent for multiple drivers including wave exposure, depth, and degree heating weeks for coral cover, and depth, degree heating weeks, chlorophyll a, and cyclone exposure for macroalgal cover, however, all threshold values were variable among island groups. Our findings demonstrate that inshore coral reef communities are typically structured by broadscale climatic perturbations, superimposed upon unique sets of local-scale drivers. Although rapidly escalating climate change impacts are the largest threat to coral reefs of the GBRMP and globally, our findings suggest that proactive management actions that effectively reduce chronic stressors at local scales should contribute to improved reef resistance and recovery potential following acute climatic disturbances.

Transfer of coliform bacteria to duckweed harvested from anaerobic baffled reactor effluent

Efforts to recover nutrients from wastewater have drawn interest in effluent polishing with duckweed, a small angiosperm known to grow in high nutrient environments. One important consideration is that pathogens may be transferred to suspended duckweed biomass and ultimately spread into agricultural or other environmental settings. Our objective was to evaluate transfer of fecal indicator bacteria to duckweed (Lemna minor and Wolffia arrhiza) from anaerobic baffled reactor effluent. The presence of duckweed resulted in significant removal (~1.5-log) of Escherichia coli and >65% decrease in turbidity in reactors. There was E. coli transfer of ~7-log CFU/g from effluent to harvested duckweed, and viable E. coli persisted (at 3.6-log CFU/g) even after several days of biomass drying, representing a health risk for agricultural applications.

Competitive reversal between plant species is driven by species‐specific tolerance to flooding stress and nutrient acquisition during early marsh succession

Abstract Understanding plant species interactions along successional trajectories is critical for managing and restoring ecosystems, as both resource availability and abiotic stresses change over time to affect competitive outcomes and species distributions. Newly created ecosystems experience a succession of plant species and rapid changes in resource availability, which may influence the outcome of biotic interactions. How these biotic interactions vary along abiotic gradients in early successional systems is not well understood. Here, we tested the hypothesis that species‐specific tolerances to flooding would influence their relative ability to capture resources (i.e. nutrients) and affect competition intensity between pioneer and secondary successional species in an early successional created tidal marsh. We transplanted a competitively dominant higher marsh species, saltmeadow cordgrass (Spartina patens), across an elevation gradient within and outside of clones of a pioneer stress‐tolerant low marsh species, smooth cordgrass (S. alterniflora). Within 6 months, S. alterniflora had suppressed the stature and growth of S. patens; however, the magnitude of this competitive effect increased at lower marsh elevations where S. alterniflora was more efficient at capturing available nitrogen (N). In unvegetated areas, where S. patens vigour was high, the amount of available N was approximately 40 times greater than within S. alterniflora clones. Synthesis and applications. Our results demonstrate that competition intensity of the stress‐tolerant species over the competitive species depended on relative resource capture in response to abiotic stress. Managing for specific vegetation communities in marsh restoration, therefore, requires insight into these relationships and interactions. Specifically, marsh restoration in high nutrient environments will limit the succession to high density competitive species due to competition with stress‐tolerant pioneer species, particularly at lower elevations.

Plant-soil feedback is shut down when nutrients come to town

Background and aimsThe concept of plant-soil feedback is increasingly used to explain plant community assembly processes. Soil nutrient availability can be expected to play a critical role on these processes. However, little is known about the effects of nutrient availability on feedback direction and strength.MethodsA plant-soil feedback experiment was performed with the grasses Anthoxanthum odoratum and Festuca rubra, and the forbs Leontodon hispidus and Plantago lanceolata, on soil with either low or high nutrient availability. Additionally, we tested if plant-soil feedback of the two forbs under these conditions changed by inoculation of the soil with spores of an arbuscular mycorrhizal fungus.ResultsIncreased nutrient availability neutralised plant-soil feedback based on shoot biomass independent of its negative or positive direction, whereas the effects on root biomass were either not altered or turned negative. Mycorrhizal fungi spore addition decreased negative feedback and increased positive feedback.ConclusionsOur results suggest that negative plant-soil feedback on low nutrient soil can be overcome with nutrient addition, and that positive soil biota associations on low nutrient soil may become superfluous with nutrient increase. We hypothesize that species-specific, microbial mediated plant community assembly processes occur in low rather than high nutrient environments.

Bellamya aeruginosa (Reeve) promote the growth of Elodea nuttallii (Planch.) H. St John in high nutrient environment

Elodea nuttallii (Planch.) H. St John is an invasive alien submerged macrophyte, which grows very fast and dominates in many Chinese waters. Bellamya aeruginosa (Reeve) is a widely distributed benthonic organism in China. This snail species rarely grazes submerged macrophytes and may promote their growth by feeding on epiphyte and phytoplankton. The above two species can commonly be found together in nature waters and their interaction may promote the growth of both species, which could disturb the ecological balance. In this paper, effects of different densities of B. aeruginosa at two different nutrient stages on water quality and the growth of E. nuttallii were studied. The results showed that the growth rates (GRs) of E. nuttallii were not significantly affected by different B. aeruginosa densities in the low nutrient (LN) stage. However, in the high nutrient (HN) stage, the GRs of the aboveground parts of E. nuttallii in the high density (HD) groups were considerably higher than the control (CK) and low density (LD) groups. The water chlorophyll (Chl) and nitrate-nitrogen (NO3-N) contents increased substantially with increasing B. aeruginosa density in the LN stage, while the Chl and NO3-N contents in the LD groups were significantly higher than in the HD and CK groups in the HN stage. The results of this paper indicated that B. aeruginosa could promote the growth of E. nuttallii by reducing the Chl contents in the water in high-nutrient environment rather than in low-nutrient environment, which highlighted that B. aeruginosa may strengthen the invisibility of E. nuttallii in eutrophic water caused by human activities.

Hydrogen cyanide produced by the soil bacterium Chromobacterium sp. Panama contributes to mortality in Anopheles gambiae mosquito larvae

Mosquito larvae continuously encounter microbes in their aquatic environment, which serve as food and play a critical role in successful development. In previous work, we isolated a Chromobacterium sp. (C.sp_P) with larvicidal activity from the midgut of dengue vector Aedes mosquitoes in Panama. In this study, we found a positive correlation between initial concentrations of C.sp_P and larval mortality rates, and that C.sp_P is more efficient at inducing larval mortality in a high nutrient environment. Multiple Chromobacterium species induce larval mortality with similar efficacy to C.sp_P except for C. subtsugae. We also found that a non-lethal dose of C.sp_P lengthens development time and increases mortality over multiple developmental stages, suggesting persistent effects of exposure. Additionally, we showed that larvicidal activity persists in the larval breeding water after removal of live bacteria, and that the larvicidal factor in C.sp_P-treated water is smaller than 3 kDa, heat resistant to 90 °C, and lost after vacuum centrifugation. We showed that C.sp_P produces hydrogen cyanide in culture and in larval water at concentrations sufficient to kill An. gambiae larvae, and treatment of the larval water with a cyanide antidote eliminated larvicidal activity. We conclude that a potential mechanism by which C.sp_P can induce larval mortality is via production of hydrogen cyanide.

Acceleration or deceleration of litter decomposition by herbivory depends on nutrient availability through intraspecific differences in induced plant resistance traits

Abstract Herbivores often induce changes in plant defensive chemistry or nutrient content that may respectively inhibit or promote microbial decomposition of senesced litter. Often the directional impact of herbivores on decomposition is considered to be a property of a species or ecosystem. While rarely explored, plasticity in the induction of defensive strategies across environmental gradients may also result in divergent impacts of herbivores on decomposition (deceleration vs. acceleration). Here, we examined how soil nutrient conditions determine legacy effects of herbivory, using nine goldenrod genotypes grown across four levels of nutrient supply and with or without grasshopper herbivory. In this species, herbivory induces defensive traits in genotypes grown in high nutrient conditions but induces tolerance (compensatory growth) in low nutrient conditions. We combined senesced litter from each treatment with a common soil inoculum in microcosms and measured soil respiration and litter mass loss over 100 days as estimates of decomposition. Plant genotype, nutrient environment, and herbivory each altered decomposition. The legacy effect of herbivory overwhelmed the positive effect of high soil nutrient supply on decomposition. This significant herbivory × nutrient environment interaction meant that herbivore‐induced plants grown in high nutrient environments produced litter that was more resistant to microbial breakdown than litter from the same genotype not exposed to herbivores. But the opposite occurred at low nutrient levels where litter from herbivore‐induced plants was most readily decomposed. Furthermore, we mechanistically tie treatment‐induced changes in leaf traits to decomposition rates. Lastly, we demonstrate a significant correlation between herbivore growth rates on living tissue and decomposition efficiency by the microbial community of senesced tissue, suggesting that herbivores and microbes perceive the “quality” of the induced substrate similarly. Synthesis. Herbivore‐induced changes in leaf palatability and trait expression due to defense induction or compensatory growth can cascade through to either inhibit or promote the decomposability of leaf litter within a single species. Here, the key determinant of which occurs is the soil fertility environment of the goldenrod clone. These findings offer mechanistic understanding of how spatial heterogeneity in ecosystem process rates may be generated by spatial variation in herbivory and nutrient availability.