Biology subjects

De Santiago, A.

Publications and source records attributed to De Santiago, A..

5 recordsLinked to original sources

bioRxiv2026

Marine nematodes exhibit widespread symbiosis, novel chemoautotrophy, and evolutionary conservation of holobiont taxa

Microbial symbioses drive the evolutionary and functional diversification of eukaryotic clades, from single-celled protists to large invertebrates. However, our knowledge of host-associated assemblages (the "holobiont") is limited in microscopic animal phyla with a body size <1mm, due to practical challenges such as low biomass and difficult taxonomy of host species. Marine nematodes represent an ideal case study for rapidly advancing our knowledge of bacterial-animal symbioses, representing a globally abundant invertebrate group with strong links to terrestrial and model organism species within the same phylum. Here, we sequenced the holobionts of 220 marine nematodes and generated 815 metagenome-assembled genomes (MAGs) of host-associated bacteria/archaea. Our data indicates that 20-34% of marine nematodes harbor an obligate intracellular symbiont, often with multiple endosymbionts co-occurring within the same host. Three bacterial phyla (Pseudomonadota Bacteroidota, and Verrucomicrobiota) account for three-quarters of all nematode-associated MAGs, and the majority of these holobiont MAGs represent deeply divergent lineages in the prokaryotic tree of life. The Flavobacteriaceae (a core microbiome taxon in C. elegans and other terrestrial nematodes), were consistently recovered across phylogenetically diverse marine nematode lineages, suggesting evolutionary conservation of holobiont taxa across marine and terrestrial environments. We also report a novel chemoautotroph family (Ca. Thionematobacter) recovered from nematode hosts in both deep-sea and shallow-water habitats, and report the first confirmed instance of Cardinium endosymbionts from marine invertebrates. Finally, [~]65% of nematode-associated MAGs are able to degrade chitin, via hexosaminidase, implying that benthic invertebrate holobionts make significant contributions to global carbon cycling. These results underline the importance of evaluating symbiosis in microscopic marine invertebrates, and accelerating our understanding of animal evolution and ecosystem dynamics in vast benthic habitats.

microbiology↗

bioRxiv2026

MeioBIOME: A snakemake workflow for the parallel analysis of meiofaunal genomes and host-associated bacteria/archaea

Microbes closely interact with every living organism, including meiofauna (i.e., microbial eukaryotes 38 m - 1 mm in length), and influence the development, life cycle, and evolution of diverse metazoans. Together, meiofauna and their microbiomes, collectively referred to as the holobiont, underpin biogeochemical cycles and drive decomposition of organic matter. However, our understanding of the ecological and evolutionary dynamics of meiofauna microbiomes are limited, typically owed to low-resolution 16S rRNA surveys, which cannot accurately delineate bacterial taxa. Single-specimen holobiont sequencing can help overcome the limitations of metabarcoding approaches by 1) generating metagenome-assembled genomes (MAGs) of the host microbiome and 2) recovering host single-copy genes (SCGs) to phylogenetically confirm the identity of the host organism. However, most bioinformatics pipelines for the assembly of metagenomic datasets have been developed for the assembly of high-complexity microbial communities of bulk sediment or soil samples (and cannot be used for the assembly of host genomes), rely on co-assembly approaches (which collapses strain-level genomic information of bacterial taxa), and focus on binning either prokaryotic or eukaryotic taxa. Therefore, there is a tremendous need for a computational workflow for the dual analysis of host genomes and their microbiomes. Here, we developed MeioBIOME, a modular Snakemake pipeline for the reproducible analysis of holobiont metagenomes obtained from individually sequenced microbial metazoa. We analyze publicly available single-specimen metagenomics datasets to show the utility of MeioBIOME and recover host-associated symbiont MAGs and host SCGs. Additionally, we integrate state-of-the-art binning algorithms which generate more MAGs than the DOE Joint Genome Institute metagenomic pipeline. We anticipate that MeioBIOME will facilitate studies of phylosymbiosis by generating high-quality host genome skims (to build well-supported host phylogenetic trees) and host-associated prokaryotic MAGs obtained from single specimens.

bioinformatics↗

bioRxiv2026

Environmental gradients and habitat specificity structure benthic microbial assemblages in a temperate seagrass ecosystem

Seagrass meadows are globally distributed coastal ecosystems that provide habitats for diverse species and facilitate ecosystem services, such as carbon storage and nutrient cycling. However, factors linked to human activities and climate change are causing seagrass decline worldwide, thus impacting benthic community structure and ecosystem functioning. Although macrofaunal communities in seagrass habitats are well studied, the structure and function of bacteria/archaea and microeukaryote assemblages remains underexplored. In this study, we used environmental DNA (eDNA) metabarcoding (16S and 18S rRNA genes) to analyze bacteria/archaea (16S), microeukaryotes (18S raw sediment), and meiofauna (18S Ludox) communities in seagrass and bare sediment habitats across three sites along an estuarine gradient in Bodega Harbor, California. We found that microbial and microeukaryote communities were primarily structured by site as a response to the environmental gradient (e.g., sediment properties), whereas meiofauna responded more strongly to the presence of seagrass. Beta diversity showed strong differentiation among estuarine sites, while alpha diversity varied within sites, with bare sediments often displaying higher diversity. Network analysis revealed significant co-occurrences between bacteria and nematodes, including chemosynthetic symbiont-bearing lineages in mudflats and stress-tolerant bioindicators. Our findings underscore the importance of environmental gradients and microhabitat features in shaping benthic biodiversity in temperate seagrass ecosystems. Furthermore, it highlights potential microbial-nematode associations, likely a response to environmental variation. By integrating multiple benthic components, our study advances understanding of biodiversity drivers in temperate estuarine seagrass habitats and provides new insights on microbial community structure in these ecosystems. ImportanceSeagrass meadows, which sustain a wide variety of plant and animal life, are vital coastal habitats. These submerged ecosystems support biodiversity, protection of coasts, nitrogen cycling, and carbon storage. However, they are declining due to pressures from climate change and human activity. This study characterizes the microbial and microeukaryote communities, including meiofauna, that inhabit and surround temperate seagrass meadows. By analyzing these communities across different areas of Bodega Harbor, California, we demonstrate how seagrass and environmental factors affect the resilience and overall health of the ecosystem. Our findings highlight the importance of seagrass in maintaining biodiversity and suggest that microscale interactions and habitat variation are important factors shaping these temperate coastal ecosystems. A comprehensive understanding of the variation within benthic communities associated with seagrass habitats can significantly enhance conservation and restoration efforts for seagrass meadows..

ecology↗

bioRxiv2025

Pseudoalteromonas is a novel symbiont of marine invertebrates that exhibits broad patterns of phylosymbiosis

Despite growing insights into the composition of marine invertebrate microbiomes, our understanding of their ecological and evolutionary patterns remains poor, owing to limited sampling depth and low-resolution datasets. Previous studies have provided mixed results when evaluating patterns of phylosymbiosis between marine invertebrates and marine bacteria. Here, we investigated potential animal-microbe symbioses in Pseudoalteromonas, an overlooked bacterial genus consistently identified as a core microbiome taxon in diverse invertebrates. Using a pangenomic analysis of 236 free-living and invertebrate-associated bacterial strains (including two new nematode-associated isolates generated in this study), we confirm that Pseudoalteromonas is a novel symbiont with substantial evidence of phylosymbiosis across at least three marine invertebrate phyla (e.g., Nematoda, Mollusca, and Cnidaria). Patterns of symbiosis were consistent irrespective of geography (including in Antarctica), with FISH images from nematodes indicating that bacterial symbionts form biofilms in the mouth and esophagus. The evolutionary history of Pseudoalteromonas is marked by substantial host-switching and lifestyle transitions, and host-associated genomes suggest that these bacteria are facultative symbionts involved in nutritional mutualisms. In marine environments, we hypothesize that horizontally-acquired symbionts may have co-evolved with invertebrates, using host mucus as a physical niche and food source, while providing their animal hosts with Vitamin B, amino acids, and bioavailable carbon compounds in return.

microbiology↗

bioRxiv2021

Dataset complexity impacts both MOTU delimitation and biodiversity estimates in eukaryotic 18S rRNA metabarcoding studies

How does the evolution of bioinformatics tools impact the biological interpretation of high-throughput sequencing datasets? For eukaryotic metabarcoding studies, in particular, researchers often rely on tools originally developed for the analysis of 16S ribosomal RNA (rRNA) datasets. Such tools do not adequately account for the complexity of eukaryotic genomes, the ubiquity of intragenomic variation in eukaryotic metabarcoding loci, or the differential evolutionary rates observed across eukaryotic genes and taxa. Recently, metabarcoding workflows have shifted away from the use of Operational Taxonomic Units (OTUs) towards delimitation of Amplicon Sequence Variants (ASVs). We assessed how the choice of bioinformatics algorithm impacts the downstream biological conclusions that are drawn from eukaryotic 18S rRNA metabarcoding studies. We focused on four workflows including UCLUST and VSearch algorithms for OTU clustering, and DADA2 and Deblur algorithms for ASV delimitation. We used two 18S rRNA datasets to further evaluate whether dataset complexity had a major impact on the statistical trends and ecological metrics: a "high complexity" (HC) environmental dataset generated from community DNA in Arctic marine sediments, and a "low complexity" (LC) dataset representing individually-barcoded nematodes. Our results indicate that ASV algorithms produce more biologically realistic metabarcoding outputs, with DADA2 being the most consistent and accurate pipeline regardless of dataset complexity. In contrast, OTU clustering algorithms inflate the metabarcoding-derived estimates of biodiversity, consistently returning a high proportion of "rare" Molecular Operational Taxonomic Units (MOTUs) that appear to represent computational artifacts and sequencing errors. However, species-specific MOTUs with high relative abundance are often recovered regardless of the bioinformatics approach. We also found high concordance across pipelines for downstream ecological analysis based on beta-diversity and alpha-diversity comparisons that utilize taxonomic assignment information. Analyses of LC datasets and rare MOTUs are especially sensitive to the choice of algorithms and better software tools may be needed to address these scenarios.

ecology↗