Lipid extraction has tissue-dependent effects on isotopic values (δ34 S, δ13 C, and δ15 N) from different marine predators

Discrimination Factor of Sulphur Stable Isotope Ratios Between Pregnant Fin Whales and Their Foetuses

RATIONALE

In-utero synthesised tissues of mammals have often been used to infer maternal behaviour during gestation. Differences in δ15N or δ13C values between foetal and maternal tissues (foetus-mother discrimination factors) are well established, but they remain uncertain for δ34S values. This study addresses this gap by investigating such discrimination in δ34S values of fin whale muscle (Balaenoptera physalus) and its potential variation throughout gestation.

METHODS

We analysed muscle δ34S values in 11 pregnant fin whales and their respective foetuses. Samples were obtained from individuals feeding off northwestern (NW) Spain during the 1983-1985 summer seasons. Yearday (0-365) and foetal length at the moment of sampling were considered proxies of the gestation stage and their effect on discrimination factors was examined. The δ34S values were determined by continuous flow isotope ratio mass spectrometry.

RESULTS

δ34S values in foetal and maternal muscle were positively correlated. δ34S values were higher in foetal muscle (M ± SD = 19.2 ± 0.3 ‰) compared to the maternal one (M ± SD = 18.6 ± 0.4 ‰), with a foetus-mother discrimination of Δ34S = 0.59 ± 0.15 ‰. This observed enrichment may be due to differences in isotopic turnover rates, amino acid metabolism, and/or maternal dietary patterns during gestation. The foetus-mother δ34S discrimination values did not change with yearday or foetus length.

CONCLUSIONS

These findings are relevant for understanding foetal-maternal δ34S discrimination and drawing ecological inferences from foetal tissues. Further research is needed to understand the mechanisms driving δ34S fractionation under different scenarios.

Why aquatic scientists should use sulfur stable isotope ratios (ẟ34S) more often

Over the last few decades, measurements of light stable isotope ratios have been increasingly used to answer questions across physiology, biology, ecology, and archaeology. The vast majority analyse carbon (δ13C) and nitrogen (δ15N) stable isotopes as the 'default' isotopes, omitting sulfur (δ34S) due to time, cost, or perceived lack of benefits and instrumentation capabilities. Using just carbon and nitrogen isotopic ratios can produce results that are inconclusive, uncertain, or in the worst cases, even misleading, especially for scientists that are new to the use and interpretation of stable isotope data. Using sulfur isotope values more regularly has the potential to mitigate these issues, especially given recent advancements that have lowered measurement barriers. Here we provide a review documenting case studies with real-world data, re-analysing different biological topics (i.e. niche, physiology, diet, movement and bioarchaeology) with and without sulfur isotopes to highlight the various strengths of this stable isotope for various applications. We also include a preliminary meta-analysis of the trophic discrimination factor (TDF) for sulfur isotopes, which suggest small (mean -0.4 ± 1.7 ‰ SD) but taxa-dependent mean trophic discrimination. Each case study demonstrates how the exclusion of sulfur comes at the detriment of the results, often leading to very different outputs, or missing valuable discoveries entirely. Given that studies relying on carbon and nitrogen stable isotopes currently underpin most of our understanding of various ecological processes, this has concerning implications. Collectively, these examples strongly suggest that researchers planning to use carbon and nitrogen stable isotopes for their research should incorporate sulfur where possible, and that the new 'default' isotope systems for aquatic science should now be carbon, nitrogen, and sulfur.

Lipid-extracted muscle and liver tissues: Can they reveal mercury exposure of pelagic sharks?

Pelagic sharks are apex predators in oceanic ecosystems and tend to accumulate high amounts of mercury (Hg). The conventional method for assessing Hg exposure in sharks involves analyzing tissue samples without any chemical treatment. However, a substantial number of chemically treated tissue samples are still being preserved in laboratories or museums. It is critical to maximize the utilization of existing samples to reduce the need for additional sampling of pelagic sharks, especially endangered species. Lipid extraction is a widely employed pretreatment process for carbon isotope analysis in shark trophic ecology, while its impact on Hg quantification remains uncertain. Here, we evaluated the feasibility of using lipid-free muscle and liver tissues for investigation of Hg exposure in four endangered pelagic sharks inhabiting the eastern Pacific, including bigeye thresher (Alopias superciliosus), pelagic thresher (A. pelagicus), blue shark (Prionace glauca) and silky shark (Carcharhinus falciformis). Results showed that total Hg concentrations (THg) differed between untreated (THgbulk) and lipid-free (THglipid-free) samples for each tissue type of each species. In addition, dichloromethane-methanol extractions significantly altered the amount of Hg. This may result from the removal of lipoprotein compounds that vary between tissues and species. The THgbulk can be calculated by THglipid-free using the following formulas, THgbulk = 1.14 × THglipid-free + 0.30 and THgbulk = 0.33 × THglipid-free + 0.18, for muscle and liver tissues, respectively. These findings emphasize the applications of lipid-free tissues in THg analysis. This study may have important implications for improving evaluation of Hg exposure in endangered pelagic sharks.

Stable isotopes elucidate body-size and seasonal fluctuations in the feeding strategies of planktivorous fishes across a semi-enclosed tropical embayment

Reef fish may switch feeding strategies due to fluctuations in resource availability or through ontogeny. A number of studies have explored these trophodynamics using carbon (δ13C) and nitrogen (δ15N) stable isotopes, but additional tracers such as sulfur isotopes (δ34S) show strong potential in systems, where δ13C and δ15N results are ambiguous. We tested the utility of adding δ34S to conventional δ13C and δ15N analysis to detect seasonal and body size changes in resource use of two planktivorous damselfish, Dascyllus reticulatus and Dascyllus trimaculatus across the Puerto Galera embayment in the Philippines. We analyzed stable isotope ratios (δ13C, δ15N, and δ34S) in multiple fish tissues (liver, eye, and muscle) to represent different dietary time frames. We then compared fish tissue isotopes against particulate organic matter (POM) (δ13C and δ15N) and POM suspension feeder (the tunicate Polycarpa aurata: δ13C, δ15N, and δ34S) across the same sites. There were size-based and seasonal differences in damselfish resource use, the latter of which was most pronounced in the fast-turnover liver. Small fish (<70 mm) demonstrated significant seasonality, appearing to switch their resource use between the rainy season and the dry season, while there was no seasonal variation in larger fish (>70 mm). This suggests that smaller fish across the embayment employ an opportunistic feeding strategy to take advantage of fluctuating resource availability, while larger fish exhibits more consistent resource use. Isotope ratios of tunicates and POM further confirmed strong seasonality in this system and a lack of a spatial isotopic gradient. δ15N did not seem to contribute to consumer resource use patterns, while by contrast, δ34S fluctuated significantly between sampling periods and was crucial for demonstrating seasonality in resource use. We recommend including δ34S when attempting to disentangle seasonal differences in resource use in aquatic food webs using stable isotopes.