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Evidence that Pacific tuna mercury levels are driven by marine methylmercury production and anthropogenic inputs

Abstract

Pacific Ocean tuna is among the most-consumed seafood products but contains relatively high levels of the neurotoxin methylmercury. Limited observations suggest tuna mercury levels vary in space and time, yet the drivers are not well understood. Here, we map mercury concentrations in skipjack tuna across the Pacific Ocean and build generalized additive models to quantify the anthropogenic, ecological, and biogeochemical drivers. Skipjack mercury levels display a fivefold spatial gradient, with maximum concentrations in the northwest near Asia, intermediate values in the east, and the lowest levels in the west, southwest, and central Pacific. Large spatial differences can be explained by the depth of the seawater methylmercury peak near low-oxygen zones, leading to enhanced tuna mercury concentrations in regions where oxygen depletion is shallow. Despite this natural biogeochemical control, the mercury hotspot in tuna caught near Asia is explained by elevated atmospheric mercury concentrations and/or mercury river inputs to the coastal shelf. While we cannot ignore the legacy mercury contribution from other regions to the Pacific Ocean (e.g., North America and Europe), our results suggest that recent anthropogenic mercury release, which is currently largest in Asia, contributes directly to present-day human mercury exposure.

 

Graphical abstract


Spatial variability of skipjack mercury concentrations. Smoothed spatial contour maps of (A) observed and (B) standardized Hg concentrations (micrograms ⋅ grams−1, dw) in skipjack white muscle samples from the Pacific Ocean. The black dots represent the location of skipjack samples. Ocean areas correspond to the sample origin: NWPO, CNPO, NEPO, EPO, SWPO, and WCPO. The transparent dots represent the location of seawater samples with available and published MeHg data

 

Highlights

Humans are exposed to toxic methylmercury mainly by consuming marine fish. New environmental policies under the Minamata Convention rely on a yet-poorly-known understanding of how mercury emissions translate into fish methylmercury levels. Here, we provide the first detailed map of mercury concentrations from skipjack tuna across the Pacific. Our study shows that the natural functioning of the global ocean has an important influence on tuna mercury concentrations, specifically in relation to the depth at which methylmercury concentrations peak in the water column. However, mercury inputs originating from anthropogenic sources are also detectable, leading to enhanced tuna mercury levels in the northwestern Pacific Ocean that cannot be explained solely by oceanic processes.

 

Reference

Anaïs Médieu, David Point, Takaaki Itai, Hélène Angot, Pearse J. Buchanan, Valérie Allain, Leanne Fuller, Shane Griffiths, David P. Gillikin, Jeroen E. Sonke, Lars-Eric Heimbürger-Boavida, Marie-Maëlle Desgranges, Christophe E. Menkes, Daniel J. Madigan, Pablo Brosset, Olivier Gauthier, Alessandro Tagliabue, Laurent Bopp, Anouk Verheyden, Anne Lorrain. Evidence that Pacific tuna mercury levels are driven by marine methylmercury production and anthropogenic inputs. Proceedings of the National Academy of Sciences Jan 2022, 119 (2) e2113032119; DOI: 10.1073/pnas.2113032119

Read the article on PNAS website

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Recent advances in bivalve-microbiota interactions for disease prevention in aquaculture

Abstract
In bivalves, no clear-cut functional role of microbiota has yet been identified, although many publications suggest that they could be involved in nutrition or immunity of their host. In the context of climate change, integrative approaches at the crossroads of disciplines have been developed to explore the environment-host-pathogen-microbiota system. Here, we attempt to synthesize work on (1) the current methodologies to analyse bivalve microbiota, (2) the comparison of microbiota between species, between host compartments and their surrounding habitat, (3) how the bivalve microbiota are governed by environmental factors and host genetics and (4) how host-associated microorganisms act as a buffer against pathogens and/or promote recovery, and could thereby play a role in the prevention of disease or mortalities.

Graphical abstract

Figure 2. Host and environmental factors acting on bivalve microbiota.
Some of these factors, alone or in combination, have been shown to lead to dysbiosis, diseases and recovery.

Highlights :

  • High-throughput sequencing technologies enabled research on bivalve-microbiota interactions.
  • Bivalve-microbiota interactions depend on host factors nested within each other.
  • Microbiota interaction with surrounding environment is important for bivalve health.
  • Recent studies highlight the involvement of oyster microbiota in disease resistance.
  • Microbiome studies will provide new strategies to improve bivalve aquaculture.

Référence :

Christine Paillard, Yannick Gueguen, K Mathias Wegner, David Bass, Alberto Pallavicini, Luigi Vezzulli, Isabelle Arzul. Recent advances in bivalve-microbiota interactions for disease prevention in aquaculture. Current Opinion in Biotechnology, Volume 73, 2022, Pages 225-232. ISSN 0958-1669. https://doi.org/10.1016/j.copbio.2021.07.026.

 

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Post-mortem storage conditions and cooking methods affect long-chain omega-3 fatty acid content in Atlantic mackerel (Scomber scombrus)

Omega-3 fatty acids are essential for human health and are found especially in marine fish. They degrade easily over time and at high temperatures, but to what extent? This is what we tested on mackerel. The loss of omega-3 was not particularly high in the first few hours after the fishes were caught, but it was still best to store them as cold as possible. For cooking, it is better to grill the mackerel fillet with its skin than to steam it. Bon appétit !

 

Abstract

Long-chain omega-3 fatty acids such as eicosapentaenoic (EPA) and docosahexaenoic acids (DHA) are health beneficial lipids found in high concentration in pelagic fishes, including Atlantic mackerel. While EPA and DHA are sensitive to oxidation during fish storage and processing, post-mortem degradation in the first hours following fish death is poorly documented. Here, we stored fish at two temperatures (2–4 °C and 18–20 °C) and monitored EPA + DHA content in dorsal fillet 6, 12 and 24 h after fish death and after cooking (grill or steam). Storage duration was the only influencing factor, and EPA + DHA loss was faster at 18–20 °C. Six hours after fish death, EPA + DHA content decreased by 1.3 ± 1.3 mg.g−1 dw (9.6 ± 9.5% of the initial content) but it was highly variable among individuals. Handling between fishing and storage should be as short and as cool as possible to preserve EPA + DHA and food safety. Regarding cooking, EPA + DHA and mono-unsaturated fatty acids increased in grilled fillets.

 

Graphical abstract

Fig.1: Outline of the sampling design testing for the influence of storage temperature (2–4 ◦C and 18–20 ◦C), storage duration (from T6 to T24 hours), and cooking method (grill and steam) on the fatty acid content of the Atlantic mackerel Scomber scombrus. Dark squares indicate the muscle sampling position at each step, and the grey ones the previously sampled positions. For cooking, we sampled the left side of the fish.

 

Highlights

  • Storage duration (< 24h) had a higher impact than storage temperature on EPA + DHA content in mackerel dorsal fillet.
  • EPA + DHA and mono-unsaturated fatty acids content increased in grilled fillets.
  • EPA + DHA losses in mackerel dorsal fillet were highly variable among individuals.

 

Reference

Fany Sardenne, Eleonora Puccinelli, Marie Vagner, Laure Pecquerie, Antoine Bideau, et al.. Post-mortem storage conditions and cooking methods affect long-chain omega-3 fatty acid content in Atlantic mackerel (Scomber scombrus). Food Chemistry, Elsevier, 2021, 359, pp.129828. ⟨10.1016/j.foodchem.2021.129828⟩.

The article is also available in open access on HAL : https://hal.archives-ouvertes.fr/hal-03215360

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Foraging depths of pelagic sharks: Insights from mercury stable isotopes

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Blue shark (Prionace glauca)

Abstract

The decline of shark populations in the world ocean is affecting ecosystem structure and function in an unpredictable way and new ecological information is today needed to better understand the role of sharks in their habitats. In particular, the characterization of foraging patterns is crucial to understand and foresee the evolution of dynamics between sharks and their prey. Many shark species use the mesopelagic area as a major foraging ground but the degree to which different pelagic sharks rely on this habitat remains overlooked. In order to depict the vertical dimension of their trophic ecology, we used mercury stable isotopes in the muscle of three pelagic shark species (the blue shark Prionace glauca, the shortfin mako shark Isurus oxyrinchus and the smooth hammerhead shark Sphyrna zygaena) from the northeastern Pacific region. The Δ199Hg values, ranging from 1.40 to 2.13‰ in sharks, suggested a diet mostly based on mesopelagic prey in oceanic habitats. We additionally used carbon and nitrogen stable isotopes (δ13C, δ15N) alone or in combination with Δ199Hg values, to assess resource partitioning between the three shark species. Adding Δ199Hg resulted in a decrease in trophic overlap estimates compared to those based on δ13C/δ15N alone, demonstrating that multi-isotope modeling is needed for accurate trophic description of the three species. Mainly, it reveals that they forage at different average depths and that resource partitioning is mostly expressed through the vertical dimension within pelagic shark assemblages. Concomitantly, muscle total mercury concentration (THg) differed between species and increased with feeding depth. Overall, this study highlights the key role of the mesopelagic zone for shark species foraging among important depth gradients and reports new ecological information on trophic competition using mercury isotopes. It also suggests that foraging depth may play a pivotal role in the differences between muscle THg from co-occurring high trophic level shark species.

 

Graphical abtsract

 

Highlights

– δ13C, δ15N, Δ199Hg and δ202Hg were determined in three pelagic shark species.
– Hg isotopes suggest that these shark species forage on mesopelagic prey.
– δ13C and δ15N overestimate overlaps between trophic niches.
– Differences in foraging depth better explain resource partitioning.
– Foraging depth influences mercury contamination level.

 

Reference

Lucien Besnard, Gaël Le Croizier, Felipe Galván-Magaña,David Point,Edouard Kraffe, James Ketchum, Raul Octavio Martinez Rincon, Gauthier Schaal. Foraging depth depicts resource partitioning and contamination level in a pelagic shark assemblage: Insights from mercury stable isotopes. Environmental Pollution, Volume 283, 2021, 117066. ISSN 0269-7491. https://doi.org/10.1016/j.envpol.2021.117066.

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Sound detection by the American lobster (Homarus americanus)

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Abstract
Although many crustaceans produce sounds, their hearing abilities and mechanisms are poorly understood, leaving uncertainties regarding whether or how these animals use sound for acoustic communication. Marine invertebrates lack gas-filled organs required for sound pressure detection, but some of them are known to be sensitive to particle motion. Here, we examined whether the American lobster (Homarus americanus) could detect sound and subsequently sought to discern the auditory mechanisms. Acoustic stimuli responses were measured using auditory evoked potential (AEP) methods. Neurophysiological responses were obtained from the brain using tone pips between 80 and 250 Hz, with best sensitivity at 80–120 Hz. There were no significant differences between the auditory thresholds of males and females. Repeated controls (recordings from deceased lobsters, moving electrodes away from the brain and reducing seawater temperature) indicated the evoked potentials’ neuronal origin. In addition, AEP responses were similar before and after antennules (including statocysts) were ablated, demonstrating that the statocysts, a long-proposed auditory structure in crustaceans, are not the sensory organs responsible for lobster sound detection. However, AEPs could be eliminated (or highly reduced) after immobilizing hairfans, which cover much of lobster bodies. These results suggest that these external cuticular hairs are likely to be responsible for sound detection, and imply that hearing is mechanistically possible in a wider array of invertebrates than previously considered. Because the lobsters’ hearing range encompasses the fundamental frequency of their buzzing sounds, it is likely that they use sound for intraspecific communication, broadening our understanding of the sensory ecology of this commercially vital species. The lobsters’ low-frequency acoustic sensitivity also underscores clear concerns about the potential impacts of anthropogenic noise.

Fig. 5. AEP responses from H. americanus to acoustic stimuli similar to the buzzing sounds they are known to produce. AEP responses from three lobsters (blue, purple and black curves) are shown.

Reference
Youenn Jézéquel, Ian T. Jones, Julien Bonnel, Laurent Chauvaud, Jelle Atema, T. Aran Mooney.Sound detection by the American lobster (Homarus americanus). Journal of Experimental Biology 2021 224: jeb240747. doi: 10.1242/jeb.240747 Published 25 March 2021
https://jeb.biologists.org/content/224/6/jeb240747

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