The arrival of large masses of driftingSargassumsince 2011 has caused changes in the natural dynamics of Caribbean coastal ecosystems. In the summer of 2015, unprecedented and massive mats ofS. fluitansandS. natanshave been observed throughout the Mexican Caribbean including exceptional accumulations ashore. This study uses stable isotopes to assess the impact ofSargassumblooms on the trophic dynamics of theDiadema antillarumsea urchin, a keystone herbivore on many Caribbean reefs. Bayesian models were used to estimate the variations in the relative proportions of carbon and nitrogen of assimilated algal resources. At three lagoon reef sites, the niche breadth ofD. antillarumwas analysed and compared under massive influx of driftingSargassumspp. vs. no influx ofSargassumblooms. The effects of the leachates generated by the decomposition ofSargassumled to hypoxic conditions on these reefs and reduced the taxonomic diversity of macroalgal food sources available toD. antillarum. Our trophic data support the hypothesis that processes of assimilation of carbon and nitrogen were modified underSargassumeffect. Isotopic signatures of macroalgae associated with the reef sites exhibited significantly lower values of δ¹⁵N altering the natural herbivory ofD.antillarum. The Stable Isotopes Analysis in R (SIAR) indicated that, under the influence ofSargassumblooms, certain algal resources (Dictyota,HalimedaandUdotea) were more assimilated due to a reduction in available algal resources. Despite being an abundant available resource, pelagicSargassumwas a negligible contributor to sea urchin diet. The Stable Isotope Bayesian Ellipses in R (SIBER) analysis displayed differences between sites, and suggests a reduction in trophic niche breadth, particularly in a protected reef lagoon. Our findings reveal thatSargassumblooms caused changes in trophic characteristics ofD. antillarumwith a negative impact by hypoxic conditions. These dynamics, coupled with the increase in organic matter in an oligotrophic system could lead to reduce coral reef ecosystem function.

Microbialites are highly diverse microbial communities that represent modern examples of the oldest life forms, stromatolites (dated >3.7 Ga). Bacalar Lagoon, in Mexico, harbors the largest freshwater microbialite occurrences of the world; yet diverse anthropogenic activities are changing the oligotrophic conditions of the lagoon. The objective of this work was to perform a spatial exploration of the microbialites of Bacalar Lagoon, analyze their prokaryote diversity, following a high throughput sequencing approach of the V4 region of the 16S rDNA, and correlate to the environmental parameters that influence the structure of these communities. The results indicate the presence of microbialites throughout the periphery of the lagoon. The microbiome of the microbialites is composed primarily of Proteobacteria (40–80%), Cyanobacteria (1–11%), Bacteroidetes (7–8%), Chloroflexi (8–14%), Firmicutes (1–23%), Planctomycetes (1–8%), and Verrucomicrobia (1–4%). Phylogenetic distance analyses suggests two distinct groups of microbialites associated with regions in the lagoon that have differences in their environmental parameters, including soluble reactive silicate (in the north), bicarbonates and available forms of nitrogen (ammonium, nitrates and nitrites) (in the south). These microbialite groups had differences in their microbiome composition associated to strong anthropogenic pressure on water quality (agriculture, landfill leachate, lack of water treatment infrastructure and intensive tourism), which were related to a loss of microbial diversity.