Gaëtan Richard

Glucosylation of α-butyl- and α-octyl-d-glucopyranosides by dextransucrase and alternansucrase from Leuconostoc mesenteroides

Abstract For the first time, glucosylation of α-butyl- and α-octylglucopyranoside was achieved using dextransucrase (DS) of various specificities, and alternansucrase (AS) from Leuconostoc mesenteroides . All the glucansucrases (GS) tested used α-butylglucopyranoside as acceptor; in particular, DS produced α- d -glucopyranosyl-(1→6)- O -butyl-α- d -glucopyranoside and α- d -glucopyranosyl-(1→6)-α- d -glucopyranosyl-(1→6)- O -butyl-α- d -glucopyranoside. In contrast, α-octylglucopyranoside was glucosylated only by AS which was shown to be the most efficient catalyst. The conversion rates, obtained with this enzyme at sucrose to acceptor molar ratio of 2:1 reached 81 and 61% for α-butylglucopyranoside and α-octylglucopyranoside, respectively. Analyses obtained from liquid chromatography coupled with mass spectrometry revealed that different series of α-alkylpolyglucopyranosides regioisomers of increasing polymerization degree can be formed depending on the specificity of the catalyst.

Variation in body condition during the post-moult foraging trip of southern elephant seals and its consequences on diving behaviour

Mature female southern elephant seals (Mirounga leonina) come ashore only in October to breed and in January to moult, spending the rest of the year foraging at sea. Mature females may lose as much as 50% of their body mass, mostly in lipid stores, during the breeding season due to fasting and lactation. When departing to sea, post-breeding females are negatively buoyant, and the relative change in body condition (i.e. density) during the foraging trip has previously been assessed by monitoring the descent rate during drift dives. However, relatively few drift dives are performed, resulting in low resolution of the temporal reconstruction of body condition change. In this study, six post-breeding females were equipped with time–depth recorders and accelerometers to investigate whether changes in active swimming effort and speed could be used as an alternative method of monitoring density variations throughout the foraging trip. In addition, we assessed the consequences of density change on the swimming efforts of individuals while diving and investigated the effects on dive duration. Both descent swimming speed and ascent swimming effort were found to be strongly correlated to descent rate during drift dives, enabling the fine-scale monitoring of seal density change over the whole trip. Negatively buoyant seals minimized swimming effort during descents, gliding down at slower speeds, and reduced their ascent swimming effort to maintain a nearly constant swimming speed as their buoyancy increased. One per cent of seal density variation over time was found to induce a 20% variation in swimming effort during dives with direct consequences on dive duration.

Adjustment of diving behaviour with prey encounters and body condition in a deep diving predator: the Southern Elephant Seal

Summary 1. Optimal diving models have been developed to investigate how air-breathing predators should adjust their diving behaviour to optimize their foraging efficiency. Using time-depth recorders and 3D accelerometers, we addressed this question on six free-ranging Southern Elephant Seal (SES) females equipped on Kerguelen Island. 2. We hypothesize that seals would initially increase their foraging time with distance to the foraging patches before reducing it for physiological reasons, regardless of the prey encountered. We expect that SES spends more time at depths where more Prey Catch Attempts (PCA) occur, that is at the bottom. We also hypothesize that bottom time should be related to both the seal body density and the swimming effort dedicated to catching prey, as we expect seals to be more active when catching prey. Finally, because oxygen is acquired at the surface only, we expect that recovery times increase with the duration of the previous dives. 3. A total of 72� 6% of PCA detected by accelerometer occurred at the bottom of the dive. At shallow depths (<300 m), seals spent more time at the bottom in dives where PCA occurred compared to non-PCA dives. At deeper depths, SES had shorter bottom times in PCA dives due to higher swimming effort. When only dives associated with PCA were considered, the time spent at the bottom increased with the number of PCA. In addition, the closer the seal was to neutral buoyancy, the longer was the bottom duration. Body density, that is buoyancy, was found to be a critical factor in controlling variations in the dive duration through the swimming effort to access the prey at the bottom of the dive. Finally, post-dive surface intervals were related to the duration and swimming effort of the previous dive. 4. This study reveals how a marine top predator adjusts the time spent at the bottom depending on its body density, prey encounter rate and prey accessibility. It also highlights that using the duration of the foraging phase as a proxy of foraging success can be seriously misleading in SES. Finally, the need to use an energetic approach with bio-logging technology to study behavioural ecology is emphasized.

Southern Elephant Seals Replenish Their Lipid Reserves at Different Rates According to Foraging Habitat.

Assessing energy gain and expenditure in free ranging marine predators is difficult. However, such measurements are critical if we are to understand how variation in foraging efficiency, and in turn individual body condition, is impacted by environmentally driven changes in prey abundance and/or accessibility. To investigate the influence of oceanographic habitat type on foraging efficiency, ten post-breeding female southern elephant seals Mirounga leonina (SES) were equipped and tracked with bio-loggers to give continuous information of prey catch attempts, body density and body activity. Variations in these indices of foraging efficiency were then compared between three different oceanographic habitats, delineated by the main frontal structures of the Southern Ocean. Results show that changes in body density are related not only to the number of previous prey catch attempts and to the body activity (at a 6 day lag), but also foraging habitat type. For example, despite a lower daily prey catch attempt rate, SESs foraging north of the sub-Antarctic front improve their body density at a higher rate than individuals foraging south of the sub-Antarctic and polar fronts, suggesting that they may forage on easier to catch and/or more energetically rich prey in this area. Our study highlights a need to understand the influence of habitat type on top predator foraging behaviour and efficiency when attempting a better comprehension of marine ecosystems.

Glucansucrases of GH family 70: What are the determinants of their specifities?

Glucansucrases from family 70 of glycoside-hydrolases catalyse the synthesis of α-glucans with various types of osidic linkages from sucrose. Among these enzymes, alternansucrase (ASR) and dextransucrase E (DSR-E) catalyse the formation of unusual α-glucans. ASR catalyses the synthesis of linear glucan with α-1,3 and α-1,6 alternating linkages and DSR-E synthesizes a glucan containing α-1,6 linkages in the linear chain and α-1,2 branches. The sequence analysis of these enzymes enabled the identification of structural elements suspected to be involved in the enzyme specificities. Biochemical characterization of ASR and DSR-E variants obtained from gene truncations or site-directed mutagenesis experiments showed that the specificity of these enzymes to form different types of osidic linkage is controlled by two different approaches. For ASR, the double specificity is controlled by only one catalytic domain where important amino acids involved in the enzyme specificity have been identified. In the case of DSR-E, the double specificity is controlled by two different catalytic domains both belonging to family 70, each domain being specific of one type of linkage.

Cultural Transmission of Fine-Scale Fidelity to Feeding Sites May Shape Humpback Whale Genetic Diversity in Russian Pacific Waters

Mitochondrial DNA (mtDNA) differences between humpback whales on different feeding grounds can reflect the cultural transmission of migration destinations over generations, and therefore represent one of the very few cases of gene-culture coevolution identified in the animal kingdom. In Russian Pacific waters, photo-identification (photo-ID) studies have shown minimal interchange between whales feeding off the Commander Islands and those feeding in the Karaginsky Gulf, regions that are separated by only 500 km and have previously been lumped together as a single Russian feeding ground. Here, we assessed whether genetic differentiation exists between these 2 groups of humpback whales. We discovered a strong mtDNA differentiation between the 2 feeding sites (FST = 0.18, ΦST = 0.14, P < 0.001). In contrast, nuclear DNA (nuDNA) polymorphisms, determined at 8 microsatellite loci, did not reveal any differentiation. Comparing our mtDNA results with those from a previous ocean-basin study reinforced the differences between the 2 feeding sites. Humpback whales from the Commanders appeared most similar to those of the western Gulf of Alaska and the Aleutian feeding grounds, whereas Karaginsky differed from all other North Pacific feeding grounds. Comparison to breeding grounds suggests mixed origins for the 2 feeding sites; there are likely connections between Karaginsky and the Philippines and to a lesser extent to Okinawa, Japan, whereas the Commanders are linked to the Mexican breeding grounds. The mtDNA differentiation between the Commander Islands and Karaginsky Gulf suggests a case of gene-culture coevolution, correlated to fidelity to a specific feeding site within a particular feeding ground. From a conservation perspective, our findings emphasize the importance of considering these 2 feeding sites as separate management units.

Icelandic herring-eating killer whales feed at night

Abstract Herring-eating killer whales debilitate herring with underwater tail slaps and likely herd herring into tighter schools using a feeding-specific low-frequency pulsed call (‘herding’ call). Feeding on herring may be dependent upon daylight, as the whales use their white underside to help herd herring; however, feeding at night has not been investigated. The production of feeding-specific sounds provides an opportunity to use passive acoustic monitoring to investigate feeding behaviour at different times of day. We compared the acoustic behaviour of killer whales between day and night, using an autonomous recorder deployed in Iceland during winter. Based upon acoustic detection of underwater tail slaps used to feed upon herring we found that killer whales fed both at night and day: they spent 50% of their time at night and 73% of daytime feeding. Interestingly, there was a significant diel variation in acoustic behaviour. Herding calls were significantly associated with underwater tail slap rate and were recorded significantly more often at night, suggesting that in low-light conditions killer whales rely more on acoustics to herd herring. Communicative sounds were also related to underwater tail slap rate and produced at different rates during day and night. The capability to adapt feeding behaviour to different light conditions may be particularly relevant for predator species occurring in high latitudes during winter, when light availability is limited.