New paper on advances in amino acid nitrogen isotopic analysis (Ohkouchi et al., Organic Geochemsitry)

Advances in the application of amino acid nitrogen isotopic analysis in ecological and biogeochemical studies

Naohiko Ohkouchi1,*, Yoshito Chikaraishi1,13, Hilary G. Close2, Brian Fry3, Thomas Larsen4, Daniel J. Madigan5, Matthew D. McCarthy6, Kelton W. McMahon7, Toshi Nagata8, Yuichi I. Naito1,14, Nanako O. Ogawa1, Brian N. Popp9, Shawn Steffan10,11, Yoshinori Takano1, Ichiro Tayasu12, Alex S.J. Wyatt8, Yasuhiko T. Yamaguchi8,15, Yusuke Yokoyama8

1 Department of Biogeochemistry, Japan Agency for Marine-Earth Science and Technology, Japan; 2 Rosenstiel School of Marine and Atmospheric Science, University of Miami, USA3 Australian Rivers Institute, Griffith University, Australia; 4 Leibniz-Laboratory, University of Kiel, Germany; 5 Harvard University Center for the Environment, USA; 6 Department of Ocean Sciences, University of California, Santa Cruz, USA; 7 Graduate School of Oceanography, University of Rhode Island, USA; 8 Atmosphere and Ocean Research Institute, The University of Tokyo, Japan; 9 Department of Geology and Geophysics, University of Hawaii, USA; 10 US Department of Agriculture, Agricultural Research Service, USA; 11 Department of Entomology, University of Wisconsin-Madison, USA; 12 Research Institute of Humanity and Nature, Japan; 13 Present address: Institute of Low Temperature Science, Hokkaido University, Japan; 14 Present address: Nagoya University Museum, Japan; 15 Present address: Lake Biwa Environmental Research Institute, Japan

Compound-specific isotopic analysis of amino acids (CSIA-AA) has emerged in the last decade as a powerful approach for tracing the origins and fate of nitrogen in ecological and biogeochemical studies. This approach is based on the empirical observation that source amino acids (AAs) (i.e., phenylalanine), fractionate 15N very little (< 0.5‰) during trophic transfer, whereas trophic AAs (i.e., glutamic acid), are greatly (∼6–8‰) enriched in 15N during each trophic step. The differential fractionation of these two AA groups can provide a valuable estimate of consumer trophic position that is internally indexed to the baseline δ15N value of the integrated food web. In this paper, we critically review the analytical methods for determining the nitrogen isotopic composition of AAs by gas chromatography–isotope-ratio mass spectrometry. We also discuss methodological considerations for accurate trophic position assessment of organisms using CSIA-AA. We then discuss the advantages and challenges of the CSIA-AA approach using published case studies across a range of topics, including trophic position assessment in various ecosystems, reconstruction of ancient human diets, reconstruction of animal migration and environmental variability, and assessment of marine organic matter dynamics with new classification of microbial fractionation patterns. It is clear that the CSIA-AA approach can provide unique insight into the sources, cycling, and trophic modification of organic nitrogen as it flows through systems. However, this approach will be greatly improved through continued exploration into how biochemical, physiological, and ecological mechanisms affect isotopic fractionation of individual AAs. We end this review with a perspective on future work that will promote the evolution of the rapidly growing field of CSIA-AA.

Adding to our isotope axes @ IsoEcol 2016 (Dr Wyatt)

Amino acid and radiocarbon insights from captive whale sharks

Alex S.J. WYATT1*, Rui Matsumoto2, Yoshito Chikaraishi3, Yosuke Miyari1, Yusuke Yokoyama1, Keiichi Sato2, Nao Ohkouchi3, Toshi Nagata1

1Atmosphere and Ocean Research Institute, The University of Tokyo, Kashiwa, Chiba, JAPAN
2Okinawa Churaumi Aquarium, Motobu, Okinawa, JAPAN
3Japan Agency for Marine-Earth Science and Technology, Yokosuka, JAPAN
*Presenting author

Stable isotope analyses (SIA) have the potential to provide novel insights into spatial and temporal patterns in the trophic ecology of poorly understood organisms like whale sharks Rhincodon typus. However, interpreting SIA depends on accurate diet-tissue discrimination factors (DTDF) to quantify diets and trophic positions, with experimental derivations of DTDF rare for such large-bodied organisms. Captive R. typus have provided a unique opportunity to validate a range of SIA, compound-specific isotope analyses (CSIA) and radioisotope approaches in the world’s largest fish and one of three planktivorous sharks. Diet records over the past five years revealed a diet dominated by North Pacific and Antarctic krill, 44% and 49% of weighted diet for Euphausia pacifica and E. superba, respectively. Despite the well-known diet, SIA of fin tissue from three captive R. typus (7.1, 7.2, and 8.4 m in length) proved hard to reconcile, especially for bulk carbon. In contrast, CSIA of amino acid (AA) nitrogen in the sharks’ tissue was relatively stable over time, despite evidence of variation in AA compositions and δ15N-AA of diet components. Tissue radiocarbon further suggested either long turnover in fin tissues (27 months), or the preferential assimilation of the smaller E. pacifica14C of 3 ‰ compared to -112 ‰ for E. superba). Daily-scale analysis of radiocarbon in R. typus faeces may support the preferential assimilation hypothesis, faeces generally being depleted relative to diet. Together, CSIA-AA and radiocarbon analyses add multiple addtional axes to our isotope space and may alleviate some of the complications involved in interpreting bulk SIA in ecological studies.

Isotopic tools for planktivorous megafauna @ ASLO 2015 (Dr Wyatt)

Isotopic Tools for Assessing Oceanic Versus Reef-Scale Drivers of Planktivorous Megauna Aggregations

Alex S.J. WYATT1*, Rui Matsumoto2, Yoshito Chikaraishi3, Keiichi Sato2, Nao Ohkouchi3, Toshi Nagata1

1Marine Biogeochemistry Laboratory, Department of Chemical Oceanography, Atmosphere & Ocean Research Institute, The University of Tokyo, Kashiwa, Chiba, JAPAN.
2Okinawa Churaumi Aquarium, Motobu, Okinawa, JAPAN.
3Japan Agency for Marine-Earth Science and Technology, Yokosuka, JAPAN
*Presenting author

Stable isotope analyses (SIA) have the potential to provide novel insights into spatial and temporal patterns in the trophic ecology of poorly understood planktivorous megafauna, especially the regional oceanic versus local reef-scale drivers of whale shark and manta ray aggregations that occur along coral reefs worldwide. However, interpreting SIA depends on accurate diet-tissue discrimination factors (DTDF) to quantify diets and trophic positions, with experimental derivations of DTDF rare for such large-bodied organisms. Captive whale sharks Rhincodon typus have provided a unique opportunity to validate a range of SIA, compound-specific isotope analyses (CSIA) and radioisotope approaches in the world’s largest fish and one of three planktivorous sharks. Combining SIA and CSIA with depth-specific radioisotope markers such as iodine ratios (129I/127I) are expected to offer a promising path towards elucidating the regional to local scale divers of planktivore aggregations Although I will focus on the implications of multi-tissue differences in DTDF and turnover times in three captive whale sharks (7.1, 7.2, and 8.4 m in length) the concepts and techniques are highly applicable to studying a wide range of species in diverse environments. An example will be provided of application to a wild caught (4.4 m) specimen of the smallest planktivorous shark, the rarely encountered megamouth shark Megachasma pelagios.

Isotope discrimination in captive whale sharks @ IsoEcol 2014 (Dr Wyatt)

Isotope Discrimination in Planktivorous Elasmobranchs Focusing on the World’s Largest Fish, Captive Whale Sharks Rhincodon typus

Alex S.J. WYATT1* Rui Matsumoto2 Yoshito Chikaraishi3 Keiichi Sato2 Nao Ohkouchi3 Toshi Nagata1

1Marine Biogeochemistry Laboratory, Department of Chemical Oceanography, Atmosphere & Ocean Research Institute, The University of Tokyo, Kashiwa, Chiba, JAPAN.
2Okinawa Churaumi Aquarium, Motobu, Okinawa, JAPAN.
3Japan Agency for Marine-Earth Science and Technology, Yokosuka, JAPAN
*Presenting author

Accurate diet-tissue discrimination factors (DTDF) are essential for quantifying diets and trophic positions (TP) using stable isotope analyses (SIA), with potential variation between diets, tissues, organisms and environments arguing against untested application of meta-analysis averages (e.g. 3.4 ‰ for bulk nitrogen (Δ15Nbulk), ~0.5 ‰ for bulk carbon (Δ13Cbulk), and 7.6 ‰ and 0.4 ‰ for nitrogen of glutamic acid (Δ15Nglu) and phenylalanine (Δ15Nphe), respectively). Experimental derivations of DTDF in elasmobranchs (sharks and rays) are scarce, with large-bodied organisms difficult to maintain in captivity and non-lethal multi-tissue sampling problematic for both captive and wild individuals. SIA of captive whale sharks Rhincodon typus, one male (8.5 m in length) and two females (7.1 and 7.2 m), fed a mixed diet composed mainly (~ 48 % each) of Antarctic krill Euphausia superba15N = 3.45 ‰, δ13C = -26.3 ‰) and North Pacific krill E. pacifica15N = 5.88 ‰, δ13C = -21.6 ‰), provide an opportunity to examine DTDF in the world’s largest fish and one of three planktivorous sharks. DTDFs estimated based on temporally averaged diets for easily sampled but slow turnover fin tissue were close to previous observations, but varied between individuals, perhaps reflecting differing growth rates with size or physiological differences between the sexes: Δ15Nbulk (2.6, 3.3, 3.1 ‰), Δ13Cbulk (3.9, 4.5, 5.9 ‰), Δ15Nglu (7.6, 6.5, n.d. ‰) and Δ15Nphe (0.3, 0.2, n.d. ‰). Short turnover tissues, such as liver or blood, may be difficult or impossible to obtain for these species, requiring non-lethal isotopic proxies to examine diet and TP at higher temporal resolution. For instance, SIA of faecal material was highly variable but reflected day-to-day variation in minor (<3 %) components of the sharks’ diets. DTDF will be discussed in the context of sampling constraints related to multi-tissue SIA and recent radioisotope approaches for understanding feeding and aggregations of planktivorous elasmobranchs, including recent application to a wild caught (4.4 m) specimen of the smallest planktivorous shark, the rare megamouth shark Megachasma pelagios.