The Effect of Thermal Stress on the Physiology and Bacterial Communities of Two Key Mediterranean Gorgonians.

Gorgonians are important habitat-providing species in the Mediterranean Sea, but their populations are declining due to microbial diseases and repeated mass mortality events caused by summer heat waves. Elevated seawater temperatures may impact the stress tolerance and disease resistance of gorgonians and lead to disturbances in their microbiota. However, our knowledge of the biological response of the gorgonian holobiont (i.e., the host and its microbiota) to thermal stress remains limited. Here, we investigated how the holobiont of two gorgonian species (Paramuricea clavata and Eunicella cavolini) are affected throughout a 7-week thermal stress event by following both the corals' physiology and the composition of their bacterial communities. We found that P. clavata was more sensitive to elevated seawater temperatures than E. cavolini, showing a greater loss in energy reserves, reduced feeding ability, and partial mortality. This lower thermotolerance may be linked to the ∼20× lower antioxidant defense capacity in P. clavata compared with E. cavolini. In the first 4 weeks of thermal stress, we also observed minor shifts in the microbiota of both species, suggesting that the microbiota likely plays a limited role in thermal acclimation of the holobiont. However, major stochastic changes occurred later on in some colonies, which were of a transient nature in E. cavolini, but were linked to partial colony mortality in P. clavata. Overall, our results show significant, but differential, effects of thermal stress on the holobionts of both E. cavolini and P. clavata and predict potentially severe impacts on gorgonian populations under future climate scenarios. IMPORTANCE In the Mediterranean Sea, the tree-shaped gorgonian corals form large forests that provide a place to live for many species. Because of this important ecological role, it is crucial to understand how common habitat-forming gorgonians, like Eunicella cavolini and Paramuricea clavata, are affected by high seawater temperatures that are expected in the future due to climate change. We found that both species lost biomass, but P. clavata was more affected, being also unable to feed and showing signs of mortality. The microbiota of both gorgonians also changed substantively under high temperatures. Although this could be linked to partial colony mortality in P. clavata, the changes were temporary in E. cavolini. The overall higher resistance of E. cavolini may be related to its much higher antioxidant defense levels than P. clavata. Climate change may thus have severe impacts on gorgonian populations and the habitats they provide.

From seascape ecology to population genomics and back. Spatial and ecological differentiation among cryptic species of the red algae Lithophyllum stictiforme/L. cabiochiae, main bioconstructors of coralligenous habitats.

Ecosystem engineering species alter the physical structure of their environment and can create or modify habitats, having a massive impact on local biodiversity. Coralligenous reefs are highly diverse habitats endemic to the Mediterranean Sea built by calcareous benthic organisms among which Crustose Coralline Algae are the main engineering species. We analyzed the diversity of Lithophyllum stictiforme or L. cabiochiae in coralligenous habitats combining a multiple barcode and a population genomics approach with seascape features. Population genomics allowed disentangling pure spatial effects from environmental effects. We found that these taxa form a complex of eight highly divergent cryptic species that are easily identifiable using classic barcode markers (psbA, LSU, COI). Three factors have a significant effect on the relative abundances of these cryptic species: the location along the French Mediterranean coast, depth and Photosynthetic Active Radiation (PAR). The analysis of around 5000 SNPs for the most abundant species revealed genetic differentiation among localities in the Bay of Marseille but no differentiation between depths within locality. Thus, the effect of depth and PAR on cryptic species communities is not a consequence of restricted connectivity but rather due to differential settlement or survival among cryptic species. This differential is more likely driven by irradiance levels rather than by pressure or temperature. Both the genetic and species diversity patterns are congruent with the main patterns of currents in the Bay. Ecological differentiation among these engineering cryptic species, sensitive to ocean warming and acidification, could have important consequences on the diversity and structure of the coralligenous communities.

An efficient protocol for extracting thylakoid membranes and total leaf proteins from Posidonia oceanica and other polyphenol-rich plants.

BACKGROUND: The extraction of thylakoids is an essential step in studying the structure of photosynthetic complexes and several other aspects of the photosynthetic process in plants. Conventional protocols have been developed for selected land plants grown in controlled conditions. Plants accumulate defensive chemical compounds such as polyphenols to cope with environmental stresses. When the polyphenol levels are high, their oxidation and cross-linking properties prevent thylakoid extraction. RESULTS: In this study, we developed a method to counteract the hindering effects of polyphenols by modifying the grinding buffer with the addition of both vitamin C (VitC) and polyethylene glycol (PEG4000). This protocol was first applied to the marine plant Posidonia oceanica and then extended to other plants synthesizing substantial amounts of polyphenols, such as Quercus pubescens (oak) and Vitis vinifera (grapevine). Native gel analysis showed that photosynthetic complexes (PSII, PSI, and LHCII) can be extracted from purified membranes and fractionated comparably to those extracted from the model plant Arabidopsis thaliana. Moreover, total protein extraction from frozen P. oceanica leaves was also efficiently carried out using a denaturing buffer containing PEG and VitC. CONCLUSIONS: Our work shows that the use of PEG and VitC significantly improves the isolation of native thylakoids, native photosynthetic complexes, and total proteins from plants containing high amounts of polyphenols and thus enables studies on photosynthesis in various plant species grown in natural conditions.

Lessons from photo analyses of Autonomous Reef Monitoring Structures as tools to detect (bio-)geographical, spatial, and environmental effects.

We investigated the validity of Autonomous Reef Monitoring Structures (ARMS) as monitoring tools for hard bottoms across a wide geographic and environmental range. We deployed 36 ARMS in the northeast Atlantic, northwest Mediterranean, Adriatic and Red Sea at 7-17 m depth. After 12-16 months, community composition was inferred from photographs, in six plate-faces for each ARMS. Overall, we found a highly significant effect of sea region, site (within seas), and plate-face on community composition. Plate-faces thus represent distinct micro-habitats and provide pseudo-replicates, increasing statistical power. Within each sea region taken individually, there was also a highly significant effect of site and plate-face. Because strong effects were obtained despite the fusion of taxonomic categories at high taxonomic ranks (to ensure comparability among biogeographic provinces), ARMS photo-analysis appears a promising monitoring tool for each sea region. We recommend keeping three ARMS per site and analyzing more numerous sites within a sea region to investigate environmental effects.

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

The 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 aggregations 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 m2 area) nested within larger ones (of hundreds of km2). 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 Sargassum 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.