Centro de Investigación Científica y de Educación Superior de Ensenada, Baja California Doctorado en Ciencias en Ecología Marina Meiofaunal biodiversity of deep-sediments from the Gulf of Mexico: a metabarcoding and morphological approach for the establishment of a baseline

approved by: ______________________________ Dr. Axayácatl Rocha Olivares Thesis Director Meiofauna is an ecological group composed by organisms whose size ranges from 20m as a lower limit to 100m as the upper limit. It is composed of a high diversity of taxa of which Nematoda is dominant in both abundance and biomass. Meiofauna plays important roles in the ecosystem such as the exchange of nutrients by bioturbation of sediment, remineralization of organic matter, therefore playing an important role in biogeochemical cycles. It also occupies an intermediate trophic level between the microfauna and macrofauna. Despite its great ecological importance, meiofauna has been little studied, a situation that becomes more evident if we focus on the meiofauna of deep-sea sediments. In this research we aimed to evaluate the biodiversity of the meiofauna of the sediments from the Gulf of Mexico, under two approaches, "classical" morphological approach and through the metabarcoding of the 18S ribosomal gene, to answer the following questions (i) Is there an association between the environmental variability of the Gulf of Mexico and the meiofaunal communities? and (ii) What is the influence of dispersal on community structures? For this, a total of 128 samples were collected from three regions of the Gulf: deepsea, continental shelf of Yucatan and platform and northwest slope. First, the structure of the meiofaunal community and the nematofauna of gulf deep-sea sediments was evaluated under the morphological approach, in relation to a set of environmental variables. The results indicate that there is spatial and temporal heterogeneity in environmental conditions and that some of them, such as depth, inorganic carbon, carbon/nitrogen ratio, oxygen, and percentage of sand, have an influence on the abundance of the meiofaunal community. Nematodes were numerically dominant in all the sampling sites, and the majority of taxa shared bacterivory as feeding strategy. This trophic group also had the highest maturity index. The structure of the nematofauna was significantly related to environmental characteristics, but not the dispersal of the genera. Based on these results, we postulate that the meiofauna of the deep-sea sediments from Gulf of Mexico may represent a metacommunity following the species sorting model. Next, the structure of the meiofauna was evaluated in the three regions of the Gulf of Mexico under the molecular approach of metabarcoding. The results indicate that each geographical area shelters a different meiofaunal community and that these differences occur at the level of sequence variants (taxonomic list) and the proportions between them. On the other hand, phylogenetic analyzes suggest that the northwest region of the Gulf of Mexico is an intermediate evolutionary region between the platform of Yucatan and the deep sea and that, in addition, the lineages of these last two regions still have not diverged from the lineages found in the Northwest Gulf region. The general results of this research allow us to conclude that the structure of the meiofauna differs significantly at a regional scale (among the three regions analyzed here), but also at a habitat scale in deep-sea sediments. On the other hand, results are consistent with the existence of limited dispersal.