β-N-methylamino-l-alanine (BMAA) and isomers: Distribution in different food web compartments of Thau lagoon, French Mediterranean Sea

The neurotoxin BMAA (β-N-methylamino-l-alanine) and its isomer DAB (2,4-diaminobutyric acid) have been detected in seafood worldwide, including in Thau lagoon (French Mediterranean Sea). A cluster of amyotrophic lateral sclerosis (ALS), a neurodegenerative disease associated with BMAA, has also been observed in this region. Mussels, periphyton (i.e. biofilms attached to mussels) and plankton were sampled between July 2013 and October 2014, and analyzed using HILIC-MS/MS. BMAA, DAB and AEG (N-(2-aminoethyl)glycine) were found in almost all the samples of the lagoon. BMAA and DAB were present at 0.58 and 0.83, 2.6 and 3.3, 4.0 and 7.2 μg g−1 dry weight in plankton collected with nets, periphyton and mussels, respectively. Synechococcus sp., Ostreococcus tauri, Alexandrium catenella and eight species of diatoms were cultured and screened for BMAA and analogs. While Synechococcus sp., O. tauri and A. catenella did not produce BMAA under our culture conditions, four diatoms species contained both BMAA and DAB. Hence, diatoms may be a source of BMAA for mussels. Unlike other toxins produced by microalgae, BMAA and DAB were detected in significant amounts in tissues other than digestive glands in mussels.

Food-Web Complexity in Guaymas Basin Hydrothermal Vents and Cold Seeps

In the Guaymas Basin, the presence of cold seeps and hydrothermal vents in close proximity, similar sedimentary settings and comparable depths offers a unique opportunity to assess and compare the functioning of these deep-sea chemosynthetic ecosystems. The food webs of five seep and four vent assemblages were studied using stable carbon and nitrogen isotope analyses. Although the two ecosystems shared similar potential basal sources, their food webs differed: seeps relied predominantly on methanotrophy and thiotrophy via the Calvin-Benson-Bassham (CBB) cycle and vents on petroleum-derived organic matter and thiotrophy via the CBB and reductive tricarboxylic acid (rTCA) cycles. In contrast to symbiotic species, the heterotrophic fauna exhibited high trophic flexibility among assemblages, suggesting weak trophic links to the metabolic diversity of chemosynthetic primary producers. At both ecosystems, food webs did not appear to be organised through predator-prey links but rather through weak trophic relationships among co-occurring species. Examples of trophic or spatial niche differentiation highlighted the importance of species-sorting processes within chemosynthetic ecosystems. Variability in food web structure, addressed through Bayesian metrics, revealed consistent trends across ecosystems. Food-web complexity significantly decreased with increasing methane concentrations, a common proxy for the intensity of seep and vent fluid fluxes. Although high fluid-fluxes have the potential to enhance primary productivity, they generate environmental constraints that may limit microbial diversity, colonisation of consumers and the structuring role of competitive interactions, leading to an overall reduction of food-web complexity and an increase in trophic redundancy. Heterogeneity provided by foundation species was identified as an additional structuring factor. According to their biological activities, foundation species may have the potential to partly release the competitive pressure within communities of low fluid-flux habitats. Finally, ecosystem functioning in vents and seeps was highly similar despite environmental differences (e.g. physico-chemistry, dominant basal sources) suggesting that ecological niches are not specifically linked to the nature of fluids. This comparison of seep and vent functioning in the Guaymas basin thus provides further supports to the hypothesis of continuity among deep-sea chemosynthetic ecosystems.