Isostasy of aseismic tectonic units in the South Atlantic Ocean and geodynamic implications

Mechanical isostasy of the lithosphere in the South Atlantic Ocean was studied using information on gravity anomalies and bathymetry with additional constraints imposed by the altimetric geoid. The isostatic responses (admittances) over the Walvis Ridge, Rio Grande Rise and Trindade Seamount Chain were computed using a three-dimensional algorithm. The eastern Walvis Ridge and the Rio Grande Rise have the same response, which is well explained by an Airy model of isostasy. The other features are regionally supported. A variation in the thickness of the elastic plate was found along the western Walvis Ridge. A high value of the elastic plate thickness (20 km) was found under the Trindade Chain. Geodynamic implications are discussed in the light of these results.     

Spatiotemporal variability of vegetation due to drought dynamics (2012–2017): a case study of the Upper Paraíba River basin,Brazil

Natural Hazards - Vegetation indices have been widely used for monitoring the spatiotemporal variables of vegetation and characterizing droughts, primarily in semiarid regions. Drought is a...     

Geographical ecology of the symbiotic crabs Libinia ferreirae and Libinia spinosa: Distributional patterns in south and south-eastern coast of Brazil

The Brazilian coast is home to two species of Majoid crabs: Libinia ferreirae and Libinia spinosa, with similar habits, as they live in symbiosis with the jellyfish Lychnorhiza lucerna during their juvenile stage and dwell on the bottom of the ocean near the coast when adults. This study investigated the spatial and temporal distribution of L. ferreirae and L. spinosa during different ontogenic phases on the south and south-eastern coast of Brazil, between latitudes 22º and 27ºS. Our results showed that both species have different distributions during their development in the three studied regions. Libinia ferreirae were more abundant associated with jellyfish, and L. spinosa were more abundant in the free-living phase. Environmental factors, especially temperature and salinity, for associated and free-living crabs, respectively, are the main factors that need to be supported for the establishment of these specimens. We were able to identify that the presence or absence of hosts (given the ecological interactions between the species) is determinant in finding the associated crabs. Geographical variations (morphology of the regions, marine currents and the strength of the tide) are linked directly to the movement of the jellyfish hosts to the coast, influencing the abundance of L. fereirae and L. spinosa populations and affecting their distribution patterns. We can infer that both species are connected with other populations because of the association with the jellyfish, crabs could be passively transported along the south and south-eastern coast of Brazil, maintaining the connection between metapopulations.     

Cold seep communities in the deep eastern Mediterranean Sea: composition, symbiosis and spatial distribution on mud volcanoes

Two mud volcano fields were explored during the French–Dutch MEDINAUT cruise (1998) with the submersible NAUTILE, one south of Crete along the Mediteranean Ridge at about 2000 m depth (Olimpi mud field) and the other south of Turkey between 1700 and 2000 m depth (Anaximander mud field) where high methane concentrations were measured. Chemosynthetic communities were observed and sampled on six mud volcanoes and along a fault scarp. The communities were dominated by bivalves of particularly small size, belonging to families commonly found at seeps (Mytilidae, Vesicomyidae, Thyasiridae) and to Lucinidae mostly encountered in littoral sulfide-rich sediments and at the shallowest seeps. Siboglinid polychaetes including a large vestimentiferan Lamellibrachia sp. were also associated. At least four bivalve species and one siboglinid are associated with symbiotic chemoautotrophic bacteria, as evidenced by Transmission Electronic Microscopy and isotopic ratio measurements. Among the bivalves, a mytilid harbors both methanotrophic and sulfide-oxidizing bacteria. Video spatial analysis of the community distribution on three volcanoes shows that dense bivalve shell accumulations (mainly lucinids) spread over large areas, from 10% to 38% of the explored areas (2500–15000 m²) on the different volcanoes. Lamellibrachia sp. had different spatial distribution and variable density in the two mud volcano fields, apparently related with higher methane fluxes in the Anaximander volcanoes and maybe with the instability due to brines in the Olimpi area. The abundance and richness of the observed chemosynthetic fauna and the size of some of the species contrast with the poverty of the deep eastern Mediterranean. The presence of a specialized fauna, with some mollusk genera and species shared with other reduced environments of the Mediterranean, but not dominated by the large bivalves usually found at seeps, is discussed.     

Comparative plant growth promoting traits and distribution of rhizobacteria associated with heavy metals in contaminated soils

The heavy metals at high concentration are generally toxic to the plants for their metabolism and growth; therefore, interactions among metals, rhizosphere microbes and plants have attracted attention because of the biotechnological potential of microorganisms for metal removal directly from contaminated soils or the possible transference of them to the plants. The aim of this study was to compare the relationships between the physiological in vitro characteristics of rhizobacteria isolated from plant metal accumulators and their distribution relating with the heavy metals content in contaminated soils. The results of this study showed that the heavy metals present in the rhizosphere of the plant species analyzed, decrease the microbial biomass and content of heavy metals caused a different distribution of rhizobacteria found. Gram negative rhizobacteria (90 %) and gram positive rhizobacteria (10 %) were isolated; all of them are metal-resistant rhizobacteria and 50 % of the isolated rhizobacteria possess both traits: higher indol acetic acid and siderophore producers. The inoculation with these rhizosphere microorganisms that possess metal-tolerating ability and plant growth promoting activities, can be recommended with a practical importance for both metal-contaminated environment and plant growth promotion.     

Freshwater seepages and ephemeral macroalgae proliferation in an intertidal bay: II. Effect on benthic biomass and metabolism

Intertidal soft-sediments biomass and metabolism are naturally heterogeneous in time and space at different scales. Particular perturbations such as freshwater seepages and seasonal proliferation of ephemeral macroalgae can intermittently and/or locally create additional variability in these systems. Since the impacts of such environmental stresses on natural processes are not well understood, the hypothesis that they would affect the functioning of the benthic system was tested. An intertidal bay whose structure and functioning has been previously described and where a carbon budget has been calculated, was chosen. The results showed that the metabolism of the intertidal sediments was greatly impacted by the above perturbations. Freshwater seepage increased meiofauna and microalgae biomasses and enhanced the total benthic metabolism (increasing community respiration and gross primary production until 4 and 2 fold respectively) without altering its seasonal trend. Ephemeral macroalgae proliferation had a more important effect on the total benthic metabolism, increasing community respiration and gross primary production 8 and 12 fold respectively and leading to a change in the seasonal trend.