Trophic Strategies Research Articles

Mixoplankton and mixotrophy: future research priorities.

Phago-mixotrophy, the combination of photoautotrophy and phagotrophy in mixoplankton, organisms that can combine both trophic strategies, have gained increasing attention over the past decade. It is now recognized that a substantial number of protistan plankton species engage in phago-mixotrophy to obtain nutrients for growth and reproduction under a range of environmental conditions. Unfortunately, our current understanding of mixoplankton in aquatic systems significantly lags behind our understanding of zooplankton and phytoplankton, limiting our ability to fully comprehend the role of mixoplankton (and phago-mixotrophy) in the plankton food web and biogeochemical cycling. Here, we put forward five research directions that we believe will lead to major advancement in the field: (i) evolution: understanding mixotrophy in the context of the evolutionary transition from phagotrophy to photoautotrophy; (ii) traits and trade-offs: identifying the key traits and trade-offs constraining mixotrophic metabolisms; (iii) biogeography: large-scale patterns of mixoplankton distribution; (iv) biogeochemistry and trophic transfer: understanding mixoplankton as conduits of nutrients and energy; and (v) in situ methods: improving the identification of in situ mixoplankton and their phago-mixotrophic activity.

Combined use of carbon, nitrogen and sulfur stable isotopes reveal trophic structure and connections in deep-sea mesopelagic and demersal fish communities from the Southeastern Pacific Ocean

The trophic structure of deep-sea fish communities has been poorly studied in the Southeastern Pacific Ocean. Here, using the analysis of multiple (CNS) stable isotopes, we provide a first approach to characterize the structure of a marine food web including mesopelagic (9 species) and demersal (13 species) fishes. At a species level, Dissostichus eleginoides were the most 13C-depleted and the grenadier Coryphaenoides ariommus the most 13C-enriched species. The most 15N-depleted species was the lanternfish Diogenichthys laternatus and macrourid fishes Trachyrincus villagai and C. ariommus the most 15N-enriched. δ34S values showed less variation, but ranged considerably, varying between 10.5 and 17.7‰: C. ariommus was the most 34S-depleted species and D. eleginoides the most 34S-enriched. At a whole study level, individual δ13C and δ15N values were positively correlated. Relationships between δ13C and δ34S, and δ15N and δ34S were both negative. When analyses were conducted separately for demersal and mesopelagic fishes, δ13C showed a positive correlation with δ15N. There was a negative relationship between δ13C and δ34S in demersal fish but no apparent relationship between δ15N and δ34S. In mesopelagic fishes, there was no statistical support for relationships between δ13C and δ34S or δ15N and δ34S. Mean log-transformed total length (TL) and mean δ15N were related in mesopelagic fishes, but there was no apparent relationship between TL and δ13C or δ34S values. In the case of demersal fishes there was no relationship between TL and either δ15N, δ13C or δ34S. Using cluster analysis, species were classified into 4 distinct trophic groups (G1 - G4) that display different trophic strategies according to the literature. Groups G1 and G2 included zooplanktivorous, micronektonivorous and gelatinovorous fishes associated with pelagic habitats. Groups G3 and G4 include micronektonivorous, piscivorous and generalist fishes foraging in benthic habitats. The demersal fish Dissostichus eleginoides was classified as a member of G1, suggesting that its diet in northern Chile is likely dominated by pelagic prey. In addition, Macrourus holotrachys (G3) showed very marked individual variability in δ15N values indicating considerable intraspecific variation in diet and foraging behavior. Given that this species had isotope values that extended across values typical of benthic and pelagic habitats, it suggests that some individuals may perform vertical migrations. Our results also highlighted that some demersal species (M. holotrachys and Coryphaenoides ariommus) had δ34S values that indicate the assimilation of some chemosynthetic-derived sulfur.Our results provide isotopic information on deep-water species in this region and show likely coupling between the demersal and mesopelagic assemblages. This information improves our understanding of deep-sea ecosystem functioning and provides key information for fisheries management and conservation initiatives.

Levels of autotrophy and heterotrophy in mesophotic corals near the end photic zone

Mesophotic corals live at ~30-150 m depth and can sustain metabolic processes under light-limited conditions by enhancing autotrophy through specialized photoadaptations or increasing heterotrophic nutrient acquisition. These acclimatory processes are often species-specific, however mesophotic ecosystems are largely unexplored and acclimation limits for most species are unknown. This study examined mesophotic coral ecosystems using a remotely operated vehicle (Ashmore Reef, Western Australia at 40–75m depth) to investigate the trophic ecology of five species of scleractinian coral (from genera Leptoseris, Pachyseris, and Craterastrea) using stable isotope analyses (δ13C and δ15N) of host and symbiont tissues and protein concentration. Trophic strategies were analyzed between species and between overall corals sampled above and below the end-photic point, where light is only 1% of surface irradiance. Results showed species-specific differences in resource use. Leptoseris hawaiiensis, L. scabra, and P. speciosa had similar Δ13C values (δ13C host - δ13C symbiont) approaching zero (< 0.5 ‰) which indicated greater dependence on symbiont autotrophy. In contrast, Leptoseris glabra and Craterastrea levis had higher Δ13C values (1.4 to 3.5 ‰) which indicated a greater reliance on external carbon sources. The latter two species also demonstrated tight nitrogen recycling within the holobiont, exhibiting low Δ15N values (host δ15N - symbiont δ15N =< 0.5 ‰), compared to more autotrophic species (Δ15N = >1.2 ‰). Some species demonstrated the ability to maintain metabolic processes despite substantially reduced light availability (0.5 – 2% of surface irradiance). This research challenges our knowledge of acclimation limits for many scleractinian corals and contributes novel information for Ashmore Reef, the Western Australia region and mesophotic ecosystems in general, and critically examines common methods used to interpretate trophic ecology with bulk stable isotopes δ13C and δ15N.

Paleoenvironmental significance of benthic foraminiferal associations of the Hauterivian to Barremian marine beds of the Neuquén Basin, Patagonia, Argentina

The upper Hauterivian-lower Barremian sedimentary sequences of the Agua de la Mula Member of the Agrio Formation (Neuquén Basin, Argentina) represent shallow marine environments characterized by mixed siliciclastic‑carbonate deposition. Ten benthic foraminiferal associations based on species composition were defined and evaluated in relation to explanatory variables such as facies, sequence stratigraphy, location and age. Variations in sedimentation rates, food and oxygen availability related to relative sea-level changes largely control distribution of the benthic foraminiferal associations. Four associations (Guttulina sp., Polymorphinids, Spiroplectammina sp.2-Haplophragmoides sp.1, and Planularia madagascariensis) are mostly composed by shallow infaunal, bacterial and detritivorous foraminiferal scavengers that largely occur in sandy facies within 3rd-order regressive systems tracts. These associations are thought to be indicative of well oxygenated and organic matter-rich sediments below the sediment-water interface. The remaining six associations consist of epifaunal, mainly opportunistic foraminifera with active herbivorous and detritivorous trophic strategies. Among them, Epistomina hechti - Epistomina australis and Reinholdella hofkeri associations occur in fine-grained, dark shales throughout 3rd-order transgressive systems tracts, and are considered related to oxygen and food stressed environment. An integrated approach, using benthic foraminifera (alpha diversity analysis, epifaunal/infaunal ratio) and multivariate statistical methods (cluster analysis, RDA, partial-RDA, PERMANOVA, PCoA), allowed to evaluate the paleosynecological significance of benthic associations, and to relate certain paleoenvironmental settings to relative sea-level changes recorded in the Agua de la Mula Member. The application of foraminiferal associations analysis in the paleoenvironmental interpretations led to recognize the influence of major relative sea-level changes in shaping higher-frequency sequences for this sector of the Neuquén Basin. Such approach can also be applied to other sectors of the basin and to high-frequency sedimentary sequences of comparable shallow marine settings.

Plants changed the response of bacterial community to the nitrogen and phosphorus addition ratio.

Human activities have increased the nitrogen (N) and phosphorus (P) supply ratio of the natural ecosystem, which affects the growth of plants and the circulation of soil nutrients. However, the effect of the N and P supply ratio and the effect of plant on the soil microbial community are still unclear. In this study, 16s rRNA sequencing was used to characterize the response of bacterial communities in Phragmites communis (P.communis) rhizosphere and non-rhizosphere soil to N and P addition ratio. The results showed that the a-diversity of the P.communis rhizosphere soil bacterial community increased with increasing N and P addition ratio, which was caused by the increased salt and microbially available C content by the N and P ratio. N and P addition ratio decreased the pH of non-rhizosphere soil, which consequently decreased the a-diversity of the bacterial community. With increasing N and P addition ratio, the relative abundance of Proteobacteria and Bacteroidetes increased, while that of Actinobacteria and Acidobacteria decreased, which reflected the trophic strategy of the bacterial community. The bacterial community composition of the non-rhizosphere soil was significantly affected by salt, pH and total carbon (TC) content. Salt limited the relative abundance of Actinobacteria, and increased the relative abundance of Bacteroidetes. The symbiotic network of the rhizosphere soil bacterial community had lower robustness. This is attributed to the greater selective effect of plants on the bacterial community influenced by nutrient addition. Plants played a regulatory role in the process of N and P addition affecting the bacterial community, and nutrient uptake by the root system reduced the negative impact of N and P addition on the bacterial community. The variations in the rhizosphere soil bacterial community were mainly caused by the response of the plant to the N and P addition ratio.

Assembly rules of coral reef fish communities along the depth gradient

Coral reefs are home to some of the most studied ecological assemblages on the planet. However, differences in large-scale assembly rules have never been studied using empirical quantitative data stratified along the depth gradient of reefs. Consequently, little is known about the small- and regional-scale effects of depth on coral reef assemblages. Using a large dataset of underwater surveys, we observed that the influence of classic biogeographic drivers on the species richness of coral reef fishes changes significantly with depth, shaping distinct assemblages governed by different rules in mesophotic coral ecosystems. We show that a general pattern of decreased taxonomic and functional richness of reef fish assemblages with depth results from convergent filtering of species composition and trophic strategies on deeper reefs across ocean basins and that at smaller scales deep-reef communities are less influenced by regional factors than shallower reefs.

From cell size and first principles to structure and function of unicellular plankton communities

Here we review, synthesize, and analyse the size-based approach to model unicellular plankton cells and communities. We first review how cell size influences processes of the individual the cell: uptake of dissolved nutrients and dissolved organic carbon, phototrophy, phagotrophy, and metabolism. We parameterize processes primarily from first principles, using a synthesis of existing data only when needed, and show how these processes determine minimum and maximum cell size and limiting resource concentrations. The cell level processes scale directly up to the structure and function of the entire unicellular plankton ecosystem, from heterotrophic bacteria to zooplankton. Community structure is described by the size spectrum and by the emergent trophic strategies. We develop an analytical approximate solution of the size spectrum and show how the trophic strategies of osmotrophy, light- and nutrient-limited phototrophy, mixotrophy, and pure phagotrophy depend on the resource environment. We further develop expressions to quantify the functions of the plankton community: production, respiration, and carbon available to production of higher trophic levels, and show how the plankton community responds to changes in temperature and grazing from higher trophic levels. We finally discuss strengths and limitations of size-based representations of plankton communities and which additional trait axes will improve the description of plankton functional diversity.

Ingestion kinetics of mixotrophic and heterotrophic flagellates

AbstractIn sunlit waters, significant predation is performed by unicellular, phagotrophic mixotrophs, that is, predators that also possess plastids. The success of a mixotrophic lifestyle will depend in part on how well mixotrophs acquire prey relative to specialized heterotrophs. Likewise, consequences of mixotrophy for productivity and element cycling will depend on the rate and efficiency at which mixotrophs consume prey biomass relative to heterotrophs. However, trait differences between mixotrophs and heterotrophs are not well characterized. In addition, cell size of mixotrophs varies widely, and constitutive mixotrophs include small flagellates deriving from diverse taxa, while larger species are primarily dinoflagellates. To determine whether similar constraints apply to phagotrophs across this broad range of size and taxa, we compiled 83 measurements of flagellate functional responses and compared maximum clearance rates (Cmax) and maximum ingestion rates (Imax) between trophic modes. We found that the average mixotroph has a 3.7‐fold lower Cmax and 7.8‐fold lower Imax than the average heterotroph, after controlling for cell size. The smaller penalty for Cmax suggests that relative fitness of mixotrophs will be enhanced under dilute prey concentrations that are common in pelagic ecosystems. We also find that growth efficiency is greater for mixotrophs and for flagellates with lower Cmax, indicating a spectrum of trophic strategies that may be driven by phototrophy vs. phagotrophy allocation as well as fast vs. slow metabolic variation. Allometric scaling shows that Imax is constrained by a common relationship among dinoflagellates and other taxa, but dinoflagellates achieve a greater volume‐specific Cmax. These results should aid in interpreting protistan communities and modeling mixotrophy.

Body size, depth of occurrence, and local oceanography shape trophic structure in a diverse deep-pelagic micronekton assemblage

The Gulf of Mexico’s (GOM) deep-pelagic realm is one of the planet’s most speciose pelagic ecoregions, but detailed knowledge of ecosystem structure and function is lacking. Understanding trophic structure is critical to understanding ecosystem dynamics as trophic interactions regulate the flow of energy through ecosystems and influence the resilience of ecosystems to disturbance. Using novel stable isotope (SIA) data and historical stomach content (SCA) data, we examined deep-pelagic trophic structure using 58 species of micronekton from the GOM that encompassed a variety of migratory behaviors, depth distributions, and trophic strategies. We identified major trophic groupings, explored the extent that differences in diet, body size, vertical migration, and presence of a mesoscale feature (Loop Current) explained micronekton isotopic variation and estimated species-specific trophic positions. Cluster analysis of SIA data identified four trophic groups, although species were not strictly clustered by diet. Specifically, non-migratory zooplanktivores with elevated δ15N values were grouped with vertically migrating piscivores, suggesting some non-migratory species could be feeding within food chains with elevated isotopic baselines. The mean δ13C values of species encompassed a narrow range from −21.6 ‰ to −18.1 ‰, with variation in δ13C values of vertically migrating species explained by a positive relationship with body size and higher δ13C values in Loop Current water. In contrast, variation in δ13C values of non-migrators was primarily explained by elevated values in deeper-dwelling species and in larger species. The mean δ15N values of species ranged between 5.0 ‰ and 11.5 ‰, with variation in δ15N values of vertical migrators explained by a positive relationship with body size and lower δ15N values in Loop Current water. Variation in the δ15N values of non-migrators was largely explained by elevated δ15N values in deeper-dwelling species and in larger-bodied species. Trophic position (TP) estimations for the assemblage ranged between 2.6 and 4.9. The elevated δ15N values in non-migratory species led to higher TP estimates relative to estimates derived from stomach content data, but agreement between TP estimates using SIA and SCA was high for vertically migrating taxa. This discrepancy may be a factor of vertical migrating species having a more similar feeding depth than non-migrators. Our results provide important insight into the trophic organization of low-latitude oligotrophic ecosystems and demonstrate that trophic variation within micronekton assemblages is primarily driven by differences in body size, location in the water column and position relative to salient oceanographic features.

Necrophagy by insects in Oculudentavis and other lizard body fossils preserved in Cretaceous amber

When a vertebrate carcass begins its decay in terrestrial environments, a succession of different necrophagous arthropod species, mainly insects, are attracted. Trophic aspects of the Mesozoic environments are of great comparative interest, to understand similarities and differences with extant counterparts. Here, we comprehensively study several exceptional Cretaceous amber pieces, in order to determine the early necrophagy by insects (flies in our case) on lizard specimens, ca. 99 Ma old. To obtain well-supported palaeoecological data from our amber assemblages, special attention has been paid in the analysis of the taphonomy, succession (stratigraphy), and content of the different amber layers, originally resin flows. In this respect, we revisited the concept of syninclusion, establishing two categories to make the palaeoecological inferences more accurate: eusyninclusions and parasyninclusions. We observe that resin acted as a “necrophagous trap”. The lack of dipteran larvae and the presence of phorid flies indicates decay was in an early stage when the process was recorded. Similar patterns to those in our Cretaceous cases have been observed in Miocene ambers and actualistic experiments using sticky traps, which also act as “necrophagous traps”; for example, we observed that flies were indicative of the early necrophagous stage, but also ants. In contrast, the absence of ants in our Late Cretaceous cases confirms the rareness of ants during the Cretaceous and suggests that early ants lacked this trophic strategy, possibly related to their sociability and recruitment foraging strategies, which developed later in the dimensions we know them today. This situation potentially made necrophagy by insects less efficient in the Mesozoic.

Trophic ecology of midwater zooplankton along a productivity gradient in the Southeast Pacific

The environmental regulation of planktonic food web structure is a key determinant for the efficiency of energy transfer through trophic levels, with direct implications for the amount of energy that is available to top-consumers and deep-sea communities. Yet, the complex trophic interplay between the different components of plankton communities under contrasting environmental conditions remains unresolved. Here, we present a comprehensive field study based on compound-specific isotope analysis of amino acids that aims to explore depth-resolved changes in the trophic strategies of various size-classes of mesozooplankton from distinct habitats across the Southeast Pacific. According to the δ15N values of the so-called source amino acids such as phenylalanine, we first identified a clear shift in nitrogen sources for biological production, from nitrate in the productive upwelling waters of the Humboldt Current System to increased inputs of diazotroph-N within the ultra-oligotrophic South Pacific gyre. These shifts in the productivity and in the source of N at the base of the food web were not accompanied by significant changes in the trophic position (TP) of mesozooplankton, which were mainly omnivores (TP = 2.8 ± 0.3, n = 65) in all ecoregions. However, although the planktonic food web length remained relatively unaltered along the productivity gradient, the microbial contribution to mid-trophic level consumers appeared to be more important at relatively high productivities, likely due to more intense remineralization processes in a late successional stage of the upwelling throughout the coastal-transition zone. Altogether, this research work will contribute to a better understanding of the food web functioning in one of the least explored marine regions of the world’s oceans.

The interplay between Microsetella norvegica (Harpacticoida, Ectinosomatidae) and Pleurobrachia bachei (Ctenophora: Pleurobrachiidae) reveals novel trophic interactions

Despite the wide geographic distribution of Microsetella norvegica little is known regarding its interspecies interactions. In this work, we provide the first description of the interaction between the harpacticoid copepod M. norvegica and the ctenophore Pleurobrachia bachei in the southeastern Gulf of California. M. norvegica exhibited two remarkable trophic strategies during the period of this study. A parasitic strategy is proposed since M. norvegica undergoes ontogenetic changes from egg to late-copepodite in the mesoglea of the ctenophore. In contrast, adult females exhibited a short-time interaction with the ctenophore, consistent with a micropredation trophic strategy. Females occurred inside the ctenophore in three stages of maturity: lacking egg-sac (presumably fertilized), ovigerous, and with egg-sacs just detached from the female genital segment. This suggests that females enter into the ctenophore’s mesoglea to release egg-sacs, and then leave the host to continue a free life where mating occurs. Accordingly, we propose that M. norvegica is able to exhibit a protelean life history in which larvae are parasitic, but adults are free living. Copepods invaded all stages of ctenophores (i.e. from larval to adult stage ≤ 15.1 mm in sagittal diameter) with low prevalence, ranging from 0.01 to 0.4% and intensity between 1 to 3 adult females. In contrast, developmental stages from egg to copepodites reached up to 18 individuals per host. Density of M. norvegica was relatively high attaining up to 29.2 ind. 100 m−3. Positive correlation between abundance of ctenophores and copepods (r2=0.6, p < 0.05) suggests a synchronization of the interaction. Abundances were highest during the coldest season at shallow sampling stations with oligotrophic water characterized by low levels of chlorophyll-a, and moderate oxygenated waters.

Resource Partitioning among "Ancillary" Pelagic Fishes (Scomber spp., Trachurus spp.) in the Adriatic Sea.

The Mediterranean is one of the most overfished seas of the world where mesopredators are severely threatened. The trophic strategies of four pelagic species that inhabit the Adriatic Sea (Scomber spp. and Trachurus spp.) were investigated through an integrated approach of stomach contents and stable isotopes analyses. Our study demonstrated that Scomber colias feeds mainly on strictly pelagic prey, with fish larvae as a secondary prey in the Southern Adriatic Sea, while S. scombrus feeds on prey belonging to higher trophic levels. Smaller specimens of Trachurus mediterraneus have a diet mainly based on pelagic prey, while larger fishes rely on prey such as benthic decapods, showing an ontogenetic shift in the diet of the species. Trachurus trachurus shows a preference for offshore and deeper areas and a diet such as that of its congeneric, but no clear ontogenetic shift was observed. This spatial segregation allows the co-existence of these two species of Trachurus. Scomber colias mainly inhabits southern areas and S. scombrus shows a preference for the northern sectors. This latitudinal gradient avoids the overlap of their trophic niches. Bayesian mixing models confirmed that the trophic niches of these species only partially overlap in the middle of the trophic web.

Trophic strategies explain the ocean niches of small eukaryotic phytoplankton.

A large fraction of marine primary production is performed by diverse small protists, and many of these phytoplankton are phagotrophic mixotrophs that vary widely in their capacity to consume bacterial prey. Prior analyses suggest that mixotrophic protists as a group vary in importance across ocean environments, but the mechanisms leading to broad functional diversity among mixotrophs, and the biogeochemical consequences of this, are less clear. Here we use isolates from seven major taxa to demonstrate a tradeoff between phototrophic performance (growth in the absence of prey) and phagotrophic performance (clearance rate when consuming Prochlorococcus). We then show that trophic strategy along the autotrophy-mixotrophy spectrum correlates strongly with global niche differences, across depths and across gradients of stratification and chlorophyll a. A model of competition shows that community shifts can be explained by greater fitness of faster-grazing mixotrophs when nutrients are scarce and light is plentiful. Our results illustrate how basic physiological constraints and principles of resource competition can organize complexity in the surface ocean ecosystem.

Microbial assemblies with distinct trophic strategies drive changes in soil microbial carbon use efficiency along vegetation primary succession in a glacier retreat area of the southeastern Tibetan Plateau

Soil microbial carbon use efficiency (CUE) is a vital physiological parameter in assessing carbon turnover. Yet, how the microbial assemblies with distinct trophic strategies regulate the soil microbial CUE remains elusive. Based on the oligotrophic-copiotrophic framework, we explored the role of microbial taxa with different trophic strategies in mediating microbial CUE (determined by a 13C-labeled approach) along the vegetation primary succession in Hailuogou glacier retreat area of the southeastern Tibetan Plateau. Results showed that soil microbial CUE ranged from 0.54 to 0.72 (averaging 0.62 ± 0.01 across all samples) and increased staggeringly along the vegetation succession. Microbial assemblies with distinct trophic strategies were crucial regulators of soil microbial CUE. Specifically, microbial CUE increased with microbial oligotroph: copiotroph ratios, oligotroph-dominated stage had a higher microbial CUE than copiotroph-dominated ones. The prevalence of oligotrophic members would be the underlying microbial mechanism for the high microbial CUE. Given that oligotrophs predominate in more recalcitrant carbon soils and their higher microbial CUE, we speculate that oligotrophs are likely to potentially enhance carbon sequestration in soils. In addition, the responses of the microbial CUE to fungal oligotroph: copiotroph ratios were higher than bacterial ones. Fungal taxa may play a dominant role in shaping microbial CUE relative to bacterial members. Overall, our results constructed close associations between microbial trophic strategies and CUE and provide direct evidence regarding how microbial trophic strategies regulate microbial CUE. This study is a significant step forward for elucidating the physiological mechanisms regulating microbial CUE and has significant implications for understanding microbial-mediated carbon cycling processes.

Grassland Alterations Do Not Affect Breeding Success, But Lead to Dietary Shifts of Eastern Imperial Eagle, a Top Predator: a Case of Successful Adaptation

Habitat transformation is identified as a global threat to biodiversity, affecting threatened raptors. In our study area, habitat change dramatically affected permanent grasslands, shrinking their availability. As we expected, the decrease in share of grasslands in eagles’ territories significantly affected occupancy rate, but not productivity: we found that occupancy rate decreased significantly, while productivity showed no trend. Understanding the adaptive capacity of top predators and how they respond to shifts in prey abundance and availability is crucial for their conservation. We investigated the diet pattern of the endangered Eastern Imperial Eagle (Aquila heliaca) facing long-term and large-scale changes. We studied the abundance variation of its profitable prey, European Souslik (Spermophilus citellus), and how it reflected on eagle population trajectories in a regional and temporal context. We found a significant diet alteration expressed in large decrease of brown hare (β2=-0.83), poultry (β2=-0.81), gulls (β2=-0.71), and water birds (β2=-0.57) and an obvious increase of northern white-breasted hedgehog (β2=0.61) and doves (β2=0.60). Raptors, including owls, raised their share among the prey (β2=0.44), but white stork and different reptiles supplied more biomass. Abundance of European souslik decreased through the studied periods (adjusted R2=0.25, p&lt;0.001), which accounted for the lower proportion of this prey in the eagle’s diet. Nevertheless, the eagle population successfully adapted and significantly increased (β2=0.97) in most of its distribution area. The trophic strategy used by this top predator related to opportunistic foraging represents an ecological advantage that allows the species to adapt to different habitats and guarantees its future. In addition, we assessed whether human-driven habitat alterations mediated dietary shifts of the Eastern imperial eagle. Following a bottom-up conception (before–after), we evaluated the effect of grassland change on the eagle’s dietary shift and breeding success. Land use patterns underwent a significant transformation over the study period, creating a large decrease in grasslands (on average, 25.79% loss in grasslands on eagle territories). Habitat alteration mediated dietary shifts, but had no reproductive consequences for eagles. Eagles became 1.90 times more likely to predate on northern white-breasted hedgehog and 1.62 times more likely to forage on white stork in the period after grassland alteration. The share of land tortoises as eagle’s prey also increased, and they were 4.04 times more likely to be predated on in the years after transformation. Conversely, brown hare was 0.51 times less likely to be consumed in the grassland loss period, while this likelihood was 0.54 times lower for rodents and 0.64 times lower for the European souslik. Doves, meanwhile, were 2.73 times more likely to be predated on in the years following grassland destruction. We found that the presence and biomass of songbirds and European Souslik correlated negatively with the breeding success of eagles, while the White Stork’s (Ciconia ciconia) presence and biomass resulted in more progeny. Diet diversity did not have an effect on the eagle’s reproductive success. The responses of the eagles may vary across territories, depending on how they rank their prey, as the territory effect was a powerful factor shaping dietary shifts for this top predator. Our results offer new evidence of the link between habitat alteration, dietary shifts, and reproductive success, contributing to our understanding of the enigmatic mechanism, through which an apex predator successfully adapts to large-scale land use pattern transformation by increasing dietary specialization. We recommend restoration of habitat complexity, including preservation of field margins, grassland patches with scattered small shrub formations, and grassland margins between medium-sized arable lands, promotion of measures for traditional grassland management through gradual grazing, and a ban on the use of shredders.

Organic and inorganic nutrients modulate taxonomic diversity and trophic strategies of small eukaryotes in oligotrophic oceans.

As the oligotrophic gyres expand due to global warming, exacerbating resource limitation impacts on primary producers, predicting changes to microbial assemblages and productivity requires knowledge of the community response to nutrient availability. This study examines how organic and inorganic nutrients influence the taxonomic and trophic composition (18S metabarcoding) of small eukaryotic plankton communities (< 200µm) within the euphotic zone of the oligotrophic Sargasso Sea. The study was conducted by means of field sampling of natural microbial communities and laboratory incubation of these communities under different nutrient regimes. Dissimilarity in community composition increased along a depth gradient, with a homogeneous protist community within the mixed layer and distinct microbial assemblages at different depths below the deep chlorophyll maximum. A nutrient enrichment assay revealed the potential of natural microbial communities to rapidly shift in composition in response to nutrient addition. Results highlighted the importance of inorganic phosphorus availability, largely understudied compared to nitrogen, in constraining microbial diversity. Dissolved organic matter addition led to a loss of diversity, benefiting a limited number of phagotrophic and mixotrophic taxa. Nutrient history of the community sets the physiological responsiveness of the eukaryotic community to changing nutrient regimes and needs to be considered in future studies.

Does Temporal and Spatial Diet Alteration Lead to Successful Adaptation of the Eastern Imperial Eagle, a Top Predator?

Predator–prey interactions may be linked to different temporal or spatial patterns, including dynamics in prey populations. Therefore, understanding the adaptive capacity and how top predators respond to shifts in prey abundance and availability is crucial for their conservation. In this study, we investigated the diet pattern of the endangered Eastern Imperial Eagle facing long-term and large-scale changes. We studied the abundance variation of its profitable prey, sousliks, and how it reflected on eagle population trajectories in a regional and temporal context. We found a significant diet alteration expressed in large decrease of brown hare (β2 = −0.83), poultry (β2 = −0.81), gulls (β2 = −0.71), and water birds (β2 = −0.57), and an obvious increase of northern white-breasted hedgehog (β2 = 0.61) and doves (β2 = 0.60). Raptors and owls raised their participation (β2 = 0.44), but white stork and different reptiles supplied more biomass. Abundance of European souslik decreased through the studied periods (adjusted R2 = 0.25, p &lt; 0.001) which accounted for the lower proportion of this prey in the eagle’s diet. Nevertheless, the eagle population successfully adapted and significantly increased (β2 = 0.97) in most of the distribution area. The trophic strategy used by this top predator related to opportunistic foraging represents an ecological advantage that allows the species to adapt to different habitats and guarantees its future. The observed prolonged diet alteration could result in a significant negative attitude among different groups such as hunters, pigeon fanciers, and poultry keepers towards eagles. Therefore, enhanced communication with key stakeholders is needed. Conservation efforts should be focused also on the preservation of the species’ main foraging habitats and the restoration of damaged ones so as to maintain the good conditions of both primary food source and subsequent prey.

Trophic strategies of intertidal foraminifera explored with single-cell microbiome metabarcoding and morphological methods: What is on the menu?

In mudflats, interactions and transfers of nutrients and secondary metabolites may drive ecosystems and biodiversity. Foraminifera have complex trophic strategies as they often rely on bacteria and eukaryotes or on potential symbionts for carbon and nitrogen resources. The capacity of these protists to use a wide range of adaptive mechanisms requires clarifying the relationships between them and their microbial associates. Here, we investigate the interactions of three foraminiferal species with nearby organisms in situ, by coupling molecular (cloning/Sanger and high-throughput sequencing) and direct counting and morphological identification with microscopy. This coupling allows the identification of the organisms found in or around three foraminiferal species through molecular tools combined with a direct counting of foraminifera and diatoms present in situ through microscopy methods. Depending on foraminiferal species, and in addition to diatom biomass, diatom frustule shape, size and species are key factors driving the abundance and diversity of foraminifera in mudflat habitats. Three different trophic strategies were deduced for the foraminifera investigated in this study: Ammonia sp. T6 has an opportunistic strategy and is feeding on bacteria, nematoda, fungi, and diatoms when abundant; Elphidium oceanense is feeding mainly on diatoms, mixed with other preys when they are less abundant; and Haynesina germanica is feeding almost solely on medium-large pennate diatoms. Although there are limitations due to the lack of species coverage in DNA sequence databases and to the difficulty to compare morphological and molecular data, this study highlights the relevance of combining molecular with morphological tools to study trophic interactions and microbiome communities of protists at the single-cell scale.

Peculiar polycyclic aromatic hydrocarbons accumulation patterns in a non-zooxanthellate scleractinian coral.

Assessing the sources and accumulation patterns of polycyclic aromatic hydrocarbons (PAHs) in corals is critical, as they threaten coral ecosystem resilience in addition to other anthropogenic pressures. We determined acenaphthene, fluorene, fluoranthene, and pyrene concentration in the skeleton and soft tissue of 7 adult and 29 old specimens of the non-zooxanthellate coral Leptopsammia pruvoti from the Mediterranean Sea. Leptopsammia pruvoti accumulated 2–72 times higher PAH concentrations than the previously investigated zooxanthellate Balanophyllia europaea living at the same site at shallower depth, likely in relation to the different trophic strategy. Low molecular weight PAHs were preferentially accumulated compared to high molecular weight PAHs. Detected PAHs were mainly petrogenic, consistently with local pollution sources. Populations of L. pruvoti immobilized PAHs in the skeleton 3–4 orders of magnitude more efficiently than B. europaea . This highlights the need to investigate other non-zooxanthellate species, which represent the majority of Mediterranean scleractinians, but are widely overlooked with respect to the few zooxanthellate species. • Coral trophic strategy influences PAH accumulation patterns in skeleton and tissue. • Low molecular weight PAHs were preferentially accumulated. • Populations of heterotrophic corals store PAH in the skeletons very efficiently.