publication – Page 5 – Laboratoire LEMAR UMR 6539

Sound detection by the American lobster (Homarus americanus)

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

29/03/2021/by sherve@univ-brest.fr

Relationship between membrane n-3 HUFA content and mitochondrial efficiency

The relationship between membrane fatty acid content and mitochondrial efficiency differs within- and between- omega-3 dietary treatments

Abstract

An important, but underappreciated, consequence of climate change is the reduction in crucial nutrient production at the base of the marine food chain: the long-chain omega-3 highly unsaturated fatty acids (n-3 HUFA). This can have dramatic consequences on consumers, such as fish as they have limited capacity to synthesise n-3 HUFA de novo. The n-3 HUFA, such as docosahexaenoic acid (DHA, 22:6n-3) and eicosapentaenoic acid (EPA, 20:5n-3), are critical for the structure and function of all biological membranes. There is increasing evidence that fish will be badly affected by reductions in n-3 HUFA dietary availability, however the underlying mechanisms remain obscure. Hypotheses for how mitochondrial function should change with dietary n-3 HUFA availability have generally ignored ATP production, despite its importance to a cell’s total energetics capacity, and in turn, whole-animal performance. Here we (i) quantified individual variation in mitochondrial efficiency (ATP/O ratio) of muscle and (ii) examined its relationship with content in EPA and DHA in muscle membrane of a primary consumer fish, the golden grey mullet Chelon auratus, receiving either a high or low n-3 HUFA diet. Mitochondria of fish fed on the low n-3 HUFA diet had higher ATP/O ratio than those of fish maintained on the high n-3 HUFA diet. Yet, mitochondrial efficiency varied up about 2-fold among individuals on the same dietary treatment, resulting in some fish consuming half the oxygen and energy substrate to produce the similar amount of ATP than conspecific on similar diet. This variation in mitochondrial efficiency among individuals from the same diet treatment was related to individual differences in fatty acid composition of the membranes: a high ATP/O ratio was associated with a high content in EPA and DHA in biological membranes. Our results highlight the existence of interindividual differences in mitochondrial efficiency and its potential importance in explaining intraspecific variation in response to food chain changes.

Figure 2: A golden grey mullet’s mitochondrial efficiency of ATP production under Low and High n-3 HUFA diet is related to the n-3 content (docosahexaenoic acid (DHA, 22:6n-3) and eicosapentaenoic acid (EPA, 20:5n-3)) of its membrane phospholipids, with individuals that have mitochondria with higher ability to make ATP had higher membrane n-3 content.

Reference

Salin K, Mathieu-Resuge M, Graziano N, Dubillot E, Le Grand F, Soudant P, Vagner M. 2020. The relationship between membrane fatty acid content and mitochondrial efficiency differs within- and between- omega-3 dietary treatments. Marine Environmental Research:105205.

https://doi.org/10.1016/j.marenvres.2020.105205

27/11/2020/by sherve@univ-brest.fr

Identification of Polyunsaturated Fatty Acids Synthesis Pathways in the Toxic Dinophyte Alexandrium minutum Using 13C-Labelling

publication

Abstract

The synthetic pathways responsible for the production of the polyunsaturated fatty acids 22:6n-3 and 20:5n-3 were studied in the Dinophyte Alexandrium minutum. The purpose of this work was to follow the progressive incorporation of an isotopic label (¹³CO₂) into 11 fatty acids to better understand the fatty acid synthesis pathways in A. minutum. The Dinophyte growth was monitored for 54 h using high-frequency sampling. A. minutum presented a growth in two phases. A lag phase was observed during the first 30 h of development and had been associated with the probable temporary encystment of Dinophyte cells. An exponential growth phase was then observed after t₃₀. A. minutum rapidly incorporated ¹³C into 22:6n-3, which ended up being the most ¹³C-enriched polyunsaturated fatty acid (PUFA) in this experiment, with a higher ¹³C atomic enrichment than 18:4n-3, 18:5n-3, 20:5n-3, and 22:5n-3. Overall, the ¹³C atomic enrichment (AE) was inversely proportional to number of carbons in n-3 PUFA. C₁₈ PUFAs, 18:4n-3, and 18:5n-3, were indeed among the least ¹³C-enriched FAs during this experiment. They were assumed to be produced by the n-3 PUFA pathway. However, they could not be further elongated or desaturated to produce n-3 C₂₀-C₂₂ PUFA, because the AEs of the n-3 C₁₈ PUFAs were lower than those of the n-3 C₂₀-C₂₂ PUFAs. Thus, the especially high atomic enrichment of 22:6n-3 (55.8% and 54.9% in neutral lipids (NLs) and polar lipids (PLs), respectively) led us to hypothesize that this major PUFA was synthesized by an O₂-independent Polyketide Synthase (PKS) pathway. Another parallel PKS, independent of the one leading to 22:6n-3, was also supposed to produce 20:5n-3. The inverse order of the ¹³C atomic enrichment for n-3 PUFAs was also suspected to be related to the possible β-oxidation of long-chain n-3 PUFAs occurring during A. minutum encystment.

Figure 1: Experimental design of the enrichment experiment. The ¹³CO₂ is supplied to the culture depending on its pH. To sample the algae, pliers are used to close/open the tubes/ways needed to first put the balloon under pressure and then allow sampling and finally rinse the tubes after sampling.

Figure 8: Hypothesized pathways to produce C₁₈ FA in A. minutum. Numbers in boxes correspond to the final AE value (white for neutral lipids and black for polar lipids). The triangles symbolize the desaturases (front-end in yellow and methyl-end in purple), the circles the enzymes involved in the PKS pathway (KS: 3-ketoacyl synthase; KR: 3-ketoacyl-ACP-reductase; DH: dehydrase; 2.2I: 2-trans, 2-cis isomerase; 2.3I: 2-trans, 3-cis isomerase; ER: enoyl reductase). The ways with dashed arrows appear to be unlikely or cannot be proven with the enrichment dynamics. NL and PL written on the routes allowing the synthesis of 18:5n-3 indicate the fraction considered for each pathway.

Conclusions:

The fatty acid synthesis in the Dinophyte Alexandrium minutum went through different routes. The PKS pathway appeared to be a particularly fast synthetic process, responsible for the high enrichment and production of the polyunsaturated fatty acid 22:6n-3. 22:6n-3 seemed to have a central role in maintaining a good physiological state, including during encystment. In our study, we assumed that the lag phase observed reflected the possibility of temporary encystment in A. minutum. It was characterized by a significant decrease in the proportion of neutral lipids, corresponding to the consumption of the reserve lipids. The 22:6n-3 might be degraded during this process, and its degradation products might be involved in the re-synthesis of triacylglycerol during algae excystment. The enrichment dynamics of C18 PUFAs revealed that they are unlikely to be involved in the further desaturation and elongation steps of n-3 C₂₀-C₂₂ PUFAs. They appeared to be the final products of the classical n-3 pathway. The 18:5n-3 atomic enrichment makes possible its origin from the desaturation of 18:4n-3, the degradation of longer fatty acids such as 20:5n-3, or the PKS pathway. These C₁₈ PUFAs have been assumed to play some role in A. minutum, such as in its toxicity. Further studies are needed to better constrain the PUFA synthesis pathway in A. minutum, and especially to further demonstrate the involvement of the PKS pathway following 22:6n-3 and 20:5n-3 synthesis.

References:

Remize, M., Planchon, F., Loh, A.N., Le Grand, F., Lambert, C., Bideau, A., Bidault, A., Corvaisier, R., Volety, A., and Soudant, P. 2020. Identification of Polyunsaturated Fatty Acids Synthesis Pathways in the Toxic Dinophyte Alexandrium minutum Using 13C-Labelling. Biomolecules 10(10): 1428. doi:10.3390/biom10101428.

Open Access on the journal page:

https://www.mdpi.com/2218-273X/10/10/1428/htm

01/11/2020/by titodemora

A current synthesis on the effects of electric and magnetic fields emitted by submarine power cables on invertebrates

publication

Abstract

The goal of clean renewable energy production has promoted the large-scale deployment of marine renewable energy devices, and their associated submarine cable network. Power cables produce both electric and magnetic fields that raise environmental concerns as many marine organisms have magneto and electroreception abilities used for vital purposes. Magnetic and electric fields’ intensities decrease with distance away from the cable. Accordingly, the benthic and the sedimentary compartments are exposed to the highest field values. Although marine invertebrate species are the major fauna of these potentially exposed areas, they have so far received little attention. We provide extensive background knowledge on natural and anthropogenic marine sources of magnetic and electric fields. We then compile evidence for magneto- and electro-sensitivity in marine invertebrates and further highlight what is currently known about their interactions with artificial sources of magnetic and electric fields. Finally we discuss the main gaps and future challenges that require further investigation.

Fig. 2. General distribution of some invertebrate species according to the theoretical values of magnetic fields emitted by 225 kV buried (1 m) and unburied single-conductor cables, energised with an intensity of 1000 A (diameter: 27 cm). Magnetic field intensities were calculated with the following formula: B = (µ.µ₀)/(2πr) B is the magnetic induction (T), μ is the relative magnetic permeability of the medium, μ₀ is the vacuum permeability (4π・10−7 V s A−1 m−1), I is the current intensity (A) and r is the distance from the centre of the wire (m) (formula from Otremba et al., 2019).

Highlights

Submarine power cables produce both magnetic and electric fields.
Marine invertebrate species inhabit the benthic or sediment compartment where cables are laid or buried.
Evidence shows magneto and electro-sensitivity in some invertebrates but their response to artificial fields is poorly known.
Invertebrate species are likely to experience the highest and longest exposures and should be prioritised in future studies.

References

Albert L., Deschamps F., Jolivet A., Olivier F., Chauvaud L., Chauvaud S. 2020. A current synthesis on the effects of electric and magnetic fields emitted by submarine power cables on invertebrates, Marine Environmental Research, Vol.159. https://doi.org/10.1016/j.marenvres.2020.104958

28/04/2020/by sherve@univ-brest.fr

Bivalve δ15N isoscapes provide a baseline for urban nitrogen footprint at the edge of a World Heritage coral reef

publication

Abstract

Eutrophication is a major threat to world’s coral reefs. Here, we mapped the distribution of the anthropogenic nitrogen footprint around Nouméa, a coastal city surrounded by 15,743 km2 of UNESCO listed reefs. We measured the δ15N signature of 348 long-lived benthic bivalves from 12 species at 27 sites and interpolated these to generate a δ15N isoscape. We evaluated the influence of water residence times on nitrogen enrichment and predicted an eutrophication risk at the UNESCO core area. Nitrogen isoscapes revealed a strong spatial gradient (4.3 to 11.7‰) from the outer lagoon to three highly exposed bays of Nouméa. Several protected reefs would benefit from an improved management of wastewater outputs, while one bay in the UNESCO core area may suffer a high eutrophication risk in the future. Our study reinforces the usefulness of using benthic animals to characterize the anthropogenic N-footprint and provide a necessary baseline for both ecologists and policy makers.

Fig. 3. δ¹⁵N isoscape in 2012, estimated at 27 stations in the southwest lagoon of New Caledonia from muscle samples of 12 bivalve species.

Highlights

• Benthic filter-feeding bivalve communities are good bio monitors of spatial variations in anthropogenic-based eutrophication.

• The baseline δ¹⁵N value found in bivalves from the outer lagoon of New Caledonia was 4.7 ± 0.4‰.

• The δ¹⁵N signature of benthic bivalves reached 11.7‰ in the most exposed station.

• Isoscapes might be used both for long-term monitoring and to predict risks of at-sea anthropogenic pollution.

Reference

Thibault M., Duprey N., Gillikin D.P., Thébault J., Douillet P., Chauvaud L., Amice E., Munaron J.M., Lorrain A. 2020. Bivalve δ¹⁵N isoscapes provide a baseline for urban nitrogen footprint at the edge of a World Heritage coral reef. Marine Pollution Bulletin vol. 152. https://doi.org/10.1016/j.marpolbul.2019.110870

17/02/2020/by sherve@univ-brest.fr