Abstract Compound‐specific nitrogen isotope ( δ 15 N) analysis of amino acids is a powerful tool for estimating the trophic positions (TPs) of animals. The TP of an animal can be represented as a linear function of the isotopic difference between glutamic acid ( δ 15 N Glu ) and phenylalanine ( δ 15 N Phe ). However, the method using δ 15 N Glu and δ 15 N Phe cannot be applied to animals in mixed food webs where basal resources are derived from both terrestrial and aquatic primary producers, because the mean value of δ 15 N Phe relative to δ 15 N Glu differs greatly between terrestrial plants (+8.4‰) and aquatic algae (−3.4‰). To resolve this problem, the δ 15 N of methionine ( δ 15 N Met ) is useful. Because the C–N bond of methionine is not cleaved in its initial metabolic step, theoretically there should be little diversity in δ 15 N Met relative to δ 15 N Glu among primary producers and a small trophic discrimination factor for methionine in animal metabolism. We developed a dual‐column‐coupled GC‐C‐IRMS method to determine δ 15 N Met . Data collected from controlled feeding experiments and wild samples demonstrated that the isotopic difference between methionine and phenylalanine in terrestrial food webs (Δ Met−Phe = −16.5 ± 0.5‰) is clearly distinguishable from that in aquatic food webs (Δ Met−Phe = −5.0 ± 0.5‰). This approach allowed us to determine ecologically reasonable TP values for carnivores in a stream food web, which were substantially underestimated with the conventional method. This method has potential utility in assessing TP for animals that rely on varying proportions of both terrestrial‐ and aquatic‐derived resources, with no requirement to characterize δ 15 N in their basal resources.
Abstract 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.
Abstract Nitrogen isotope analysis of chloropigments provides information on the sources of nitrogenous nutrients assimilated by phytoplankton. The abundant, ubiquitous chlorophyll a records nitrogen isotopic compositions (δ 15 N) of eukaryotic phytoplankton and cyanobacteria, whereas more source‐specific chloropigments, such as the divinylchlorophylls exclusively possessed by the marine picocyanobacterium Prochlorococcus , can potentially resolve isotopic variability within the phytoplankton community. In this study, we analyzed the δ 15 N of both chlorophyll a and divinylchlorophyll a isolated from suspended particulate material collected at the subsurface chlorophyll maximum (SCM) along a meridional transect at 88°E in the oligotrophic eastern Indian Ocean. The nitrogen isotopic compositions of Prochlorococcus (δ 15 N PRO ) and the combined biomass of eukaryotic phytoplankton and Synechococcus (δ 15 N EU+SYN ) estimated from the δ 15 N of divinylchlorophyll a and chlorophyll a , respectively, revealed systematic variations that were not apparent from bulk isotope measurements. Whereas consistently low δ 15 N PRO indicated reliance of Prochlorococcus on regenerated nitrogen throughout the transect, elevation in δ 15 N EU+SYN values at several stations was interpreted to reflect assimilation of subsurface NO 3 − by eukaryotic phytoplankton. The δ 15 N distributions revealed subtle differences in NO 3 − availability at the SCM along the transect, which were consistently explained by the occurrence of mesoscale eddies in the Bay of Bengal, deepening of the mixed layer induced by a seasonal Wyrtki Jet in the equatorial region, and substantial deepening of the nutricline in the South Indian Ocean gyre. Our results highlight the utility of compound‐specific isotopic measurements of multiple species of chlorophylls in obtaining essential non‐incubation‐based biogeochemical constraints on the primary production in the ocean.
Abstract Compound‐specific isotope analysis of nitrogen (δ 15 N) in amino acids (CSIA‐AA) has significantly contributed to environmental sciences such as anthropology, biogeochemistry, and ecology. Several methods exist for determining δ 15 N of amino acids (AAs). Although these methods have their own strength and weakness, they have not been intercalibrated yet, especially for biological samples with matrices. To address this issue, we systematically compared AA δ 15 N values among three methods using gas chromatography/combustion/isotope ratio mass spectrometry (GC/C/IRMS), preparative liquid chromatography (LC) separation followed by elemental analyzer/IRMS (LC × EA/IRMS), and LC separation followed by GC/C/IRMS (LC × GC/C/IRMS). The δ 15 N values of glutamic acid (δ 15 N Glu ) and phenylalanine (δ 15 N Phe ) in fish muscle, two crucial AAs for estimating the trophic positions (TPs) of organisms, were compared among methods. Although a significant difference in fish muscle δ 15 N Glu values was found among the three analytical methods, their δ 15 N Glu and δ 15 N Phe values were fairly consistent between all pairs of methods ( n = 8, R 2 = 0.9968 for GC/C/IRMS vs. LC × GC/C/IRMS; 0.9936 for LC × EA/IRMS vs. LC × GC/C/IRMS; and 0.9912 for GC/C/IRMS vs. LC × EA/IRMS), which resulted in similar TP estimates among the methods. Thus, the results provide empirical validation that the CSIA‐AA is comparable among different methods in interdisciplinary research fields. We also highlighted some critical features of each of the three analytical methods that can be used as a guideline for future CSIA‐AA research.
Cellular membranes are heterogeneous, and this has a great impact on cellular function. Despite the central role of membrane functions in multiple cellular processes in sperm, their molecular mechanisms are poorly understood. Membrane rafts are specific membrane domains enriched in cholesterol, ganglioside GM1, and functional proteins, and they are involved in the regulation of a variety of cellular functions. Studies of the functional characterization of membrane rafts in mammalian sperm have demonstrated roles in sperm-egg binding and the acrosomal reaction. Recently, our biochemical and cell biological studies showed that membrane rafts are present and might play functional roles in chicken sperm. In this study, we isolated membrane rafts from chicken sperm as a detergent-resistant membranes (DRM) floating on a density gradient in the presence of 1% Triton X-100, and characterized the function and proteomes associated with these domains. Biochemical comparison of the DRM between fresh and cryopreserved sperm demonstrated that cryopreservation induces cholesterol loss specifically from membrane rafts, indicating the functional connection with reduced post-thaw fertility in chicken sperm. Furthermore, using an avidin-biotin system, we found that sperm DRM is highly enriched in a 60 KDa single protein able to bind to the inner perivitelline layer. To identify possible roles of membrane rafts, quantitative proteomics, combined with a stable isotope dimethyl labeling approach, identified 82 proteins exclusively or relatively more associated with membrane rafts. Our results demonstrate the functional distinctions between membrane domains and provide compelling evidence that membrane rafts are involved in various cellular pathways inherent to chicken sperm.
Abstract Integration of inland waters into regional and global carbon (C) budgets requires a comprehensive understanding of factors regulating organic carbon (OC) delivery and in situ processing. This study reviews advances in optical, molecular, and isotopic approaches to resolve the sources, ages, and transformations of OC in aquatic systems. OC characterization using excitation emission matrix spectra, Fourier transform ion cyclotron mass spectrometry, and nuclear magnetic resonance provides detailed molecular level insight. Radiocarbon isotopic approaches and compound‐specific techniques resolve the input, metabolic fate, and turnover time of OC in ecosystems ranging in size from streams to the open ocean. Accumulating evidence suggests that aquatic OC is composed of diverse biogeochemical components. We conclude with enduring and emerging questions that underscore the role of inland systems in the global C cycle and propose unique combinations of approaches to better discern their role in the delivery and transformation of OC from soils to seas.
