From In Situ to satellite observations of pelagic Sargassum distribution and aggregation in the Tropical North Atlantic Ocean

International audienceThe present study reports on observations carried out in the Tropical North Atlantic in summer and autumn 2017, documenting Sargassum aggregations using both ship-deck observations and satellite sensor observations at three resolutions (MSI-10 m, OLCI-300 m, VIIRS-750 m and MODIS-1 km). Both datasets reported that in summer, Sargassum aggre-gations were mainly observed off Brazil and near the Caribbean Islands, while they accumulated near the African coast in autumn. Based on in situ observations, we propose a five-class typology allowing standardisation of the description of in situ Sargassum raft shapes and sizes. The most commonly observed Sargassum raft type was windrows, but large rafts composed of a quasi-circular patch hundreds of meters wide were also observed. Satellite imagery showed that these rafts formed larger Sargassum aggregations over a wide range of scales, with smaller aggregations (of tens of m 2 area) nested within larger ones (of hundreds of km 2). Match-ups between different satellite sensors and in situ observations were limited for this dataset, mainly because of high cloud cover during the periods of observation. Nevertheless, comparisons between the two datasets showed that satellite sensors successfully detected Sargassum abundance and aggregation patterns consistent with in situ observations. MODIS and VIIRS sensors were better suited to describing the Sargas-sum aggregation distribution and dynamics at Atlantic scale, while the new sensors, OLCI and MSI, proved their ability to detect Sargassum aggregations and to describe their (sub-) mesoscale nested structure. The high variability in raft shape, size, thickness, depth and biomass density observed in situ means that caution is called for when using satellite maps of Sargassum distribution and biomass estimation. Improvements would require additional in situ and airborne observations or very high-resolution satellite imagery

Etude de la diversité spécifique des macroalgues de la Pointe de Bretagne et analyse des composés phénoliques des Phéophycées dominantes

A la pointe de la Bretagne, les fortes amplitudes de marée se traduisent par des durées d'émersion variant de façon marquée selon le niveau bathymétrique. Sur l'estran, chacune des espèces d'algues brunes structurantes est adaptée à un niveau particulier, résultant en un étagement de ceintures formées par ces espèces. Cette canopée abrite de nombreuses macroalgues et la faune caractéristique des estrans rocheux. La première partie de ce travail s'insère dans le contexte global d'étude de la biodiversité et plus particulièrement de la diversité spécifique des macroalgues intertidales d'une région tempérée. Ainsi, des différences dans la composition spécifique et la structure des peuplements ont été mises en évidence selon leur situation sur l'estran, la saison et l'hydrodynamisme. Ce dernier influence également les variations saisonnières qui dépendent du niveau hypsométrique. La deuxième partie de ce travail s'intéresse aux macroalgues brunes structurantes. Ces organismes produisent des composés phénoliques afin notamment de résister au rayonnement solaire direct lors des périodes d'émersion. Cette étude a permis de caractériser l'évolution des teneurs, de l'activité antioxydante et de la composition du pool de ces composés chez les Phéophycées étudiées. Ainsi, des variations intrathalles, horaires, saisonnières et interspécifiques de ces paramètres ont été mises en évidence. A l'opposé, pas ou peu de variations intersites ont été notées, suggérant une relative homogénéité interpopulationelle. L'influence des radiations solaires et de l'alternance émersion/immersion a été mise en évidence sur les teneurs et les activités des phlorotannins. Ce travail devrait servir de base à une étude plus approfondie de la nature du pool de composés phénoliques chez les macroalgues afin d'établir une correspondance entre structure et activité et de vérifier si la nature du pool de molécules photoprotectrices conditionne l'adaptabilité des espèces de Phéophycées à l'estran.In western Brittany, strong tidal ranges result in emersion durations which vary markly according to the bathymetric level. On the intertidal zone, each of the structuring brown algal species is adapted to a particular level, resulting in a zonation of belts formed by these species. This canopy shelters many seaweeds and the characteristic fauna of rocky shores. The first part of this work takes place in the global problematic of biodiversity and particularly of specific diversity of intertidal, temperate seaweeds. Thus, differences in both the specific composition and the structure of the communities have been demonstrated according to their location on the shore, to the season and to the hydrodynamism. Hydrodynamism also influences the seasonal variations which depend on the bathymetric level. The second part of this work is devoted to structuring brown macroalgae. These organisms produce phenolic compounds especially in order to resist the direct solar radiation during emersion periods. This study made it possible to characterise the evolution of contents, antioxidant activity and composition of the pool of these compounds in the studied Phaeophyceae. Thus, intra-thallus, hourly, seasonal and interspecific variations of these parameters have been demonstrated. At the opposite, no or few intersites variations were noted, suggesting a relative interpopulation homogeneity. The influence of solar radiations and of the emersion/immersion alternation has been shown on both the contents and the activities of the phlorotannins. This work could be used as a basis for a thorough study on the nature of the phenolic pool in seawweds in order to establish a correspondance between their structure and their activity and to check whether if the nature of the photo-protecting pool conditions the adaptability of phaeophycean species to the intertidal zone.BREST-BU Droit-Sciences-Sports (290192103) / SudocPLOUZANE-Bibl.La Pérouse (290195209) / SudocSudocFranceF

A new protocol using acidification for preserving DMSP in macroalgae and comparison with existing protocols

Dimethylsulfoniopropionate (DMSP) plays many important physiological and ecological roles in macroalgae. The most common method to measure DMSP is by gas chromatography analysis of the dimethylsulfide (DMS) produced after NaOH hydrolysis (pH > 12). Storage of DMS, however, is not recommended for more than a week. We investigated if acidification can be a suitable method to preserve DMSP in macroalgal samples over three months of storage, compared to widely used protocols such as drying and freezing at ‐20°C. The DMSP content of green (Ulva sp. and Ulva compressa), red (Chondrus crispus) and brown (Bifurcaria bifurcata) macroalgae were analyzed 24 h after NaOH addition (control values); and after acidification (0.2 mol · L HCl‐1) for 24 h of fresh material, followed by NaOH addition for 24 h. These values were compared to measurements after 3‐month storage of samples that had been either dried in a heater (60°C for a night, and storage at room temperature), or frozen at ‐20°C, or kept in 0.2 mol · L HCl‐1. There was no significant difference between DMSP measurements on freshly collected material and after acidification of the samples, whether 24 h later or after 3 months of storage. This was in contrast with 3‐month storage protocols involving overnight drying at 60°C (75‐98% DMSP loss), and to a lesser degree freezing at ‐20°C (37‐80% DMSP loss). We thus advise to acidify macroalgal samples for preservation over long periods of time rather than drying or freezing, when assaying DMSP content

Mycosporine-like Amino Acids in Palmaria palmata (Rhodophyta): Specific Implication of Usujirene in Photoprotection

International audienceThe effect of UV radiation on the accumulation of mycosporine-like amino acids (MAAs) and pigments was investigated on red macroalga Palmaria palmata cultivated for 21 days. The data were combined with the effect of NaNO3 to further investigate the synthesis of these nitrogenous compounds. A progressive decrease in both total MAA and pigment contents was observed, with a positive effect of nitrate supply. Usujirene was the only MAA exhibiting a significantly increasing content when exposed to UV radiation, changing from 9% to 24% of the total MAA’s contribution, with no variation observed with NaNO3. This suggests a specific induction or synthesis pathway of usujirene for photoprotection, while the synthesis of other MAAs could have been limited by an insufficient amount of UV radiation and/or irradiance. The photoprotective ability of some MAAs could have been impacted by nitrogen starvation over time, resulting in a limited synthesis and/or potential use of MAAs as a nitrogen source for red macroalgae. The data confirmed the multiple effects of environmental factors on the synthesis of MAAs while providing new insights into the specific synthesis of usujirene, which could find an application in the cosmetics sector as natural sunscreen or an anti-ageing agent

Influence of day-night and tidal cycles on phenol content and antioxidant capacity in three temperate intertidal brown seaweeds

International audienceThe daily variations of phenol contents and antioxidant capacities were surveyed in a 32-h field experiment in three temperate brown seaweeds belonging to Fucales - Pelvetia canaliculata, Ascophyllum nodosum and Bifurcaria bifurcata - living at different intertidal levels -- high-, mid- and low-tide level, respectively. Phenolic compounds of brown seaweeds are secondary metabolites involved in many different protection mechanisms, as for example against grazer and pathogen attack as well as UV damage. This study was thus aimed at understanding the influence of both day/night and tidal cycles on the brown seaweed phenol pool with respect to their bathymetric level on the shore. These cycles affect the quantity and quality of light received by intertidal seaweeds (protection via the water layer during immersion), and the photoprotective role of phlorotannins was thus evaluated. Phenol levels and antioxidant capacities were monitored every hour during a tidal cycle and a half at the equinox spring tide, in March 2003. The three species contained rather high phenol levels, i.e. about 3, 6 and 4% DW in P. canaliculata, A. nodosum and B. bifurcata, respectively. Antioxidant capacities globally paralleled phenol contents in the three species under study. Moreover, the measured antioxidant capacities and the phenol contents of the extracts were significantly and positively correlated in the three species. Significant effect of neither cycles nor their interaction was observed in any species despite a trend to follow day/night alternation for P. canaliculata, and emersion/immersion cycle for A. nodosum. No trend was observed for the third species. However, significant correlations between phenol levels of P. canaliculata and A. nodosum and measured air-temperature were found. Our results suggested an effect of aerial, aquatic and both conditions on the phenolic pool of P. canaliculata, B. bifurcata and A. nodosum, respectively, and a fast evolution of the phenolic pool on a day time scale

Temporal variation in pigment and mycosporine-like amino acid composition of the red macroalga Palmaria palmata from Brittany (France): hypothesis on the MAA biosynthesis pathway under high irradiance

WOS:000520805600001Changes in pigment (chlorophyll-a, carotenoids and phycobiliproteins) and mycosporine-like amino acid (MAA) composition were studied in the red edible macroalga Palmaria palmata, harvested between February 2018 and February 2019 at 2 sites from Brittany (France). The results occuring in natural conditions revealed a high seasonal variability that can be related to a combination of environmental parameters, including light, nutrients, temperature, salinity and pH, which may have synergistic or antagonistic effects. A decrease in pigments was observed in summer, matching with the increase in irradiance at the same period. On the contrary, the increase in light from February to May has caused an increase in the total MAA content, confirming the UV-photoprotective role attributed to these compounds. Analysis of the results coupled with the biosynthesis pathway suggests that some MAAs are favoured in the occurrence of high irradiance. In this sense, asterina-330 was synthetized only between March and July and not the rest of the year, supporting its importance in photoprotection. In addition, analysis revealed that nutrient availibility could have a significant influence on MAAs since their synthesis seemed to be limited in summer, despite significant exposure to light. However, this decrease could be due to another potential role of MAAs as a nitrogen source for macroalgae. Thus, the diversity of these compounds and the different synthesis pathways could be explained by different roles including photo-protection and nitrogen source, making MAA multi-purpose protective agents which could play a key role in the adaptation of this species to its changing environment

Phenology, TPC and size-fractioning phenolics variability in temperate Sargassaceae (Phaeophyceae, Fucales) from Western Brittany: Native versus introduced species

International audienceThe phenology of the introduced Sargassum muticum and two native species Bifurcaria bifurcata and Cystoseira baccata were monitored during eighteen months at two sites in Brittany. Density and length varied seasonally only in Sargassum. Larger individuals of Sargassum were observed in summer whereas in Cystoseira, they appeared almost in autumn. Peaks in maturity were delayed: in summer for Sargassum and in winter for Cystoseira and Bifurcaria. Phenolic contents increased before their respective reproductive period as a chemical defence. Moreover, size composition varied with site and season depending on species. In Sargassum, the quantity of small compounds decreased in summer together with an increase of 2000/5000 Da compounds. In Bifurcaria 2000/5000 Da compounds increased in summer (photoprotection) while in Cystoseira it increased in winter (protection for reproduction). Sargassum presented then a phenological plasticity not observed in native species. Moreover the three species possessed different chemical strategies to succeed in partitioning their vital space

Intra-thallus phlorotannin content and antioxidant activity in Phaeophyceae of temperate waters

International audienceThis study focuses on the intra-thallus variation in phlorotannin content and antioxidant activity in temperate brown seaweeds. Two species of Fucales (Ascophyllum nodosum and Sargassum muticum) and two species of Laminariales (Laminaria digitata and L. hyperborea) were collected from the shoreline of Brittany (France). Both Fucales contained high phenolic levels along their thallus (ca. 5% DW in A. nodosum and 6% DW in S. muticum). No significant differences were found between vegetative and reproductive parts of A. nodosum. In contrast, intra-thallus and inter-specific variations were observed in phenolic contents of the two species of Laminariales studied. In L. digitata, phenolic content was about 0.2% DW, with a maximum in the holdfast, whereas in L. hyperborea, it averaged about 2.6% DW, with the highest value measured in the basal part of the old blade. In both species, phlorotannin contents were lower in the meristematic region. Intra-thallus variations of the antioxidant activity measured by the DPPH (2,2-diphenyl-1-picrylhydrazyl) method were also detected. Antioxidant activity was correlated to the phenol contents of the different parts of thalli in three species. In L. digitata, no activity was found because of low phenol levels. The observed variations in both the antioxidant activity and the phenol content could result from qualitative features of the phenolic pool

Phlorotannin and Pigment Content of Native Canopy-Forming Sargassaceae Species Living in Intertidal Rockpools in Brittany (France): Any Relationship with Their Vertical Distribution and Phenology?

Five native Sargassaceae species from Brittany (France) living in rockpools were surveyed over time to investigate photoprotective strategies according to their tidal position. We gave evidences for the existence of a species distribution between pools along the shore, with the most dense and smallest individuals in the highest pools. Pigment contents were higher in lower pools, suggesting a photo-adaptive process by which the decreasing light irradiance toward the low shore was compensated by a high production of pigments to ensure efficient photosynthesis. Conversely, no xanthophyll cycle-related photoprotective mechanism was highlighted because high levels of zeaxanthin rarely occurred in the upper shore. Phlorotannins were not involved in photoprotection either; only some lower-shore species exhibited a seasonal trend in phlorotannin levels. The structural complexity of phlorotannins appears more to be a taxonomic than an ecological feature: Ericaria produced simple phloroglucinol while Cystoseira and Gongolaria species exhibited polymers. Consequently, tide pools could be considered as light-protected areas on the intertidal zone, in comparison with the exposed emerged substrata where photoprotective mechanisms are essential