Stable nitrogen isotopic composition of amino acids reveals food web structure in stream ecosystems

New insight into biomagnification factor of mercury based on food web structure using stable isotopes of amino acids

Although many attempts have been carried out to elaborate trophic magnification factor (TMF) and biomagnification factor (BMF), such as normalizing the concentration of pollutants and averaging diet sources, the uncertainty of the indexes still need to be improved to assess the bioaccumulation of pollutants. This study first suggests an improved BMF (i.e., BMF') applied to mercury bioaccumulation in freshwater fish from four sites before and after rainfall. The diet source and TP of each fish were identified using nitrogen stable isotope of amino acids (δ15NAAs) combined with bulk carbon stable isotope (δ13C). The BMF' was calculated normalizing with TP and diet contributions derived from MixSIAR. The BMF' values (1.3-27.2 and 1.2-27.8), which are representative of the entire food web, were generally higher than TMF (1.5-13.9 and 1.5-14.5) for both total mercury and methyl mercury, respectively. The BMF' implying actual mercury transfer pathway is more reliable index than relatively underestimated TMF for risk assessment. The ecological approach for BMF calculations provides novel insight into the behavior and trophic transfer of pollutants like mercury.

Ecological Traits of Fish for Mercury Biomonitoring: Insights from Compound-Specific Nitrogen and Stable Mercury Isotopes

We coupled compound-specific isotopic analyses of nitrogen (N) in amino acids (δ¹⁵N_Glu, δ¹⁵N_Phe) and mercury stable isotopes (δ²⁰²Hg, Δ¹⁹⁹Hg) to quantify ecological traits governing the concentration, variability, and source of Hg in largemouth bass (LB) and pike gudgeon (PG) across four rivers, South Korea. PG displayed uniform Hg concentration (56-137 ng/g), trophic position (TP_corrected; 2.6-3.0, n = 9), and N isotopes in the source amino acid (δ¹⁵N_Phe; 7-13‰), consistent with their specialist feeding on benthic insects. LB showed wide ranges in Hg concentration (45-693 ng/g), TP_corrected (2.8-3.8, n = 14), and δ¹⁵N_Phe (1.3-16‰), reflecting their opportunistic feeding behavior. Hg sources assessed using Hg isotopes reveal low and uniform Δ¹⁹⁹Hg in PG (0.20-0.49‰), similar to Δ¹⁹⁹Hg reported in sediments. LB displayed site-specific δ²⁰²Hg (-0.61 to -0.04‰) and Δ¹⁹⁹Hg (0.53-1.09‰). At the Yeongsan River, LB displayed elevated Δ¹⁹⁹Hg and low δ¹⁵N_Phe, consistent with Hg and N sourced from the atmosphere. LB at the Geum River displayed low Δ¹⁹⁹Hg and high δ¹⁵N_Phe, both similar to the isotope values of anthropogenic sources. Our results suggest that a specialist fish (PG) with consistent ecological traits and Hg concentration is an effective bioindicator species for Hg. When accounting for Hg sources, however, LB better captures site-specific Hg sources.

Combining food web theory and population dynamics to assess the impact of invasive species

The impacts of invasive species on resident communities are driven by a tangle of ecological interactions difficult to quantify empirically. Combining a niche model with a population dynamic model, both allometrically parametrized, may represent a consistent framework to investigate invasive species impacts on resident communities in a food web context when empirical data are scarce. We used this framework to assess the ecological consequences of an invasive apex predator (Silurus glanis) in peri-Alpine lake food webs. Both increases and decreases of resident species abundances were highlighted and differed when accounting for different S. glanis body sizes. Complementarily, the prominence of indirect effects, such as trophic cascades, suggested that common approaches may only capture a restricted fraction of invasion consequences through direct predation or competition. By leveraging widely available biodiversity data, our approach may provide relevant insights for a comprehensive assessment and management of invasive species impacts on aquatic ecosystems.

Amino acid nitrogen and carbon isotope data: Potential and implications for ecological studies

Explaining food web dynamics, stability, and functioning depend substantially on understanding of feeding relations within a community. Bulk stable isotope ratios (SIRs) in natural abundance are well‐established tools to express direct and indirect feeding relations as continuous variables across time and space. Along with bulk SIRs, the SIRs of individual amino acids (AAs) are now emerging as a promising and complementary method to characterize the flow and transformation of resources across a diversity of organisms, from microbial domains to macroscopic consumers. This significant AA‐SIR capacity is based on empirical evidence that a consumer's SIR, specific to an individual AA, reflects its diet SIR coupled with a certain degree of isotopic differences between the consumer and its diet. However, many empirical ecologists are still unfamiliar with the scope of applicability and the interpretative power of AA‐SIR. To fill these knowledge gaps, we here describe a comprehensive approach to both carbon and nitrogen AA‐SIR assessment focusing on two key topics: pattern in AA‐isotope composition across spatial and temporal scales, and a certain variability of AA‐specific isotope differences between the diet and the consumer. On this basis we review the versatile applicability of AA‐SIR to improve our understanding of physiological processes as well as food web functioning, allowing us to reconstruct dominant basal dietary sources and trace their trophic transfers at the specimen and community levels. Given the insightful and opportunities of AA‐SIR, we suggest future applications for the dual use of carbon and nitrogen AA‐SIR to study more realistic food web structures and robust consumer niches, which are often very difficult to explain in nature.

Magnesium isotopes reveal bedrock impacts on stream organisms

Magnesium is an essential element to aquatic organisms and understanding the origin of Mg is important for understanding their growth. Ultimately, Mg in streams is derived from the chemical weathering of bedrock in the catchment. In this study, we used Mg stable isotope ratios (δ²⁶Mg) to test whether stream organisms reflect lithological sources in stream catchments. In November 2017 and May 2018, we sampled aquatic insects and small gobies from six temperate streams in the Lake Biwa area (central Japan). Three of these streams had up to 38% limestone in their catchment (limestone streams), and three streams lacked limestone (non-limestone streams). We hypothesised that stream organisms from limestone streams had significantly lower δ²⁶Mg values compared to those of the same organisms from non-limestone streams. Aquatic insects from limestone streams had an average of 0.78‰ lower δ²⁶Mg values than those of the same organisms from non-limestone streams, thereby indicating a lithological control on the δ²⁶Mg of aquatic insects. Aquatic insects often showed an offset to higher δ²⁶Mg values compared to those of stream water, thereby pointing to a ²⁶Mg-enriched diet as an additional Mg source to water and/or Mg isotope fractionation during Mg accumulation. Instead, stream water was the main Mg source for small gobies, as their bones reflected the δ²⁶Mg of water. We concluded that δ²⁶Mg could trace Mg sources of aquatic organisms, and the same methodology can be applied to other metals.

New light in the dark - a proposed multidisciplinary framework for studying functional ecology of groundwater fauna

Groundwaters provide the vast majority of unfrozen freshwater resources on the planet, but our knowledge of subsurface ecosystems is surprisingly limited. Stygofauna, or stygobionts -subterranean obligate aquatic animals - provide ecosystem services such as grazing biofilms and maintaining water quality, but we know little about how their ecosystems function. The cryptic nature of groundwaters, together with the high degree of local endemism and stygofaunal site-specific adaptations, represent major obstacles for the field. To overcome these challenges, and integrate biodiversity and ecosystem function, requires a holistic design drawing on classical ecology, taxonomy, molecular ecology and geochemistry. This study presents an approach based on the integration of existing concepts in groundwater ecology with three more novel scientific techniques: compound specific stable isotope analysis (CSIA) of amino acids, radiocarbon analysis (¹⁴C) and DNA analyses of environmental samples, stygofauna and gut contents. The combination of these techniques allows elucidation of aspects of ecosystem function that are often obscured in small invertebrates and cryptic systems. Carbon (δ¹³C) and nitrogen (δ¹⁵N) CSIA provides a linkage between biogeochemical patterns and ecological dynamics. It allows the identification of stygofaunal food web structures and energy flows based on the metabolic pathway of specific amino groups. Concurrently, ¹⁴C provides complementary data on the carbon recycling and incorporation within the stygobiotic trophic webs. Changes in groundwater environmental conditions (e.g. aquifer recharge), and subsequent community adaptations, can be pinpointed via the measurementof the radiocarbon fingerprint of water, sediment and specimens. DNA analyses are a rapidly expanding approach in ecology. eDNA is mainly employed as a biomonitoring tool, while metabarcoding of individuals and/or gut contents provides insight into diet regimes. In all cases, the application of the approaches in combination provides more powerful data than any one alone. By combining quantitative (CSIA and ¹⁴C) and qualitative (eDNA and DNA metabarcoding) approaches via Bayesian Mixing Models (BMM), linkages can be made between community composition, energy and nutrient sources in the system, and trophic function. This suggested multidisciplinary design will contribute to a more thorough comprehension of the biogeochemical and ecological patterns within these undervalued but essential ecosystems.

Stable isotope compounds - production, detection, and application

Stable isotopes are used in wide fields of application from natural tracers in biology, geology and archeology through studies of metabolic fluxes to their application as tracers in quantitative proteomics and structural biology. We review the use of stable isotopes of biogenic elements (H, C, N, O, S, Mg, Se) with the emphasis on hydrogen and its heavy isotope deuterium. We will discuss the limitations of enriching various compounds in stable isotopes when produced in living organisms. Finally, we overview methods for measuring stable isotopes, focusing on methods for detection in single cells in situ and their exploitation in modern biotechnologies.

Integrated trophic position decreases in more diverse communities of stream food webs

The relationship between biodiversity and ecosystem functioning is an important theme in environmental sciences. We propose a new index for configuration of the biomass pyramid in an ecosystem, named integrated trophic position (iTP). The iTP is defined as a sum of trophic positions (i.e. the average number of steps involved in biomass transfer) of all the animals in a food web integrated by their individual biomass. The observed iTP for stream macroinvertebrates ranged from 2.39 to 2.79 and was negatively correlated with the species density and the Shannon–Wiener diversity index of the local community. The results indicate a lower efficiency of biomass transfer in more diverse communities, which may be explained by the variance in edibility hypothesis and/or the trophic omnivory hypothesis. We found a negative effect of biodiversity on ecosystem functioning.

Trophic discrimination factor of nitrogen isotopes within amino acids in the dobsonfly Protohermes grandis (Megaloptera: Corydalidae) larvae in a controlled feeding experiment

The trophic discrimination factor (TDF) of nitrogen isotopes (¹⁵N/¹⁴N) within amino acids, between a stream-dwelling dobsonfly larva (Protohermes grandis: Megaloptera; Corydalidae) and its diet (chironomid larvae), was determined in controlled feeding experiments. Last-instar larvae of P. grandis were collected from the Yozawa-gawa River, central Japan, and reared in the laboratory. After fed to satiation for 1 month, one group of larvae was each fed one living chironomid larva per day for 4 weeks, while a second group was starved for 8 weeks. The larvae were harvested at intervals and the nitrogen isotopic composition of glutamic acid (δ¹⁵N_Glu) and phenylalanine (δ¹⁵N_Phe) were determined to calculate TDF. The mean TDF of satiated and starved larvae were 7.1‰ ± 0.5‰ (n = 3) and 7.3‰ ± 0.5‰ (n = 5), respectively. Thus, the TDF for P. grandis larvae in this study was similar to that reported for other arthropods (approximately 7‰) and was independent of satiation or starvation. A previous study of wild P. grandis larvae, based on the δ¹⁵N_Glu and δ¹⁵N_Phe values, estimated its trophic position (TP) as approximately 2.0 ± 0.1 (n = 5), a low value close to that of algivores, although they are generally characterized as carnivores (usually accepted as TP ≥ 3). The TDF for P. grandis larvae suggests that their low TPs in nature were caused by incorporation of vascular plant-derived amino acids (with a different δ¹⁵N profile from that of algae) and not by an unusually low TDF or by the effects of the satiation/starvation on amino acid metabolism.