Description: Host-microbe interactions play crucial roles in marine ecosystems. However, we still have very little understanding of the mechanisms that govern these relationships, the evolutionary processes that shape them, and their ecological consequences. The holobiont concept is a renewed paradigm in biology that can help to describe and understand these complex systems. It posits that a host and its associated microbiota with which it interacts, form a holobiont, and have to be studied together as a coherent biological and functional unit to understand its biology, ecology, and evolution. Here we discuss critical concepts and opportunities in marine holobiont research and identify key challenges in the field. We highlight the potential economic, sociological, and environmental impacts of the holobiont concept in marine biological, evolutionary, and environmental sciences. Given the connectivity and the unexplored biodiversity specific to marine ecosystems, a deeper understanding of such complex systems requires further technological and conceptual advances, e.g., the development of controlled experimental model systems for holobionts from all major lineages and the modeling of (info)chemical-mediated interactions between organisms. Here we propose that one significant challenge is to bridge cross-disciplinary research on tractable model systems in order to address key ecological and evolutionary questions. This first step is crucial to decipher the main drivers of the dynamics and evolution of holobionts and to account for the holobiont concept in applied areas, such as the conservation, management, and exploitation of marine ecosystems and resources, where practical solutions to predict and mitigate the impact of human activities are more important than ever.

Description: Red seaweeds are key components of coastal ecosystems and are economically important as food and as a source of gelling agents, but their genes and genomes have hitherto received little attention. Here we report the sequencing of the 105 Mbp genome of the florideophyte Chondrus crispus (Irish Moss) and the annotation of the 9,606 genes. The genome features an unusual structure, characterized by gene-dense regions surrounded by repeat-rich regions dominated by transposable elements. Despite its fairly large size, this genome shows features typical of compact genomes, e.g. on average only 0.3 introns per gene, short introns, low median distance between genes, small gene families, and no indication of large-scale genome duplication. The genome also gives insights into the metabolism of marine red algae, as well as adaptations to the marine environment, including genes related to halogen metabolism, oxylipins, and multicellularity (miRNA processing and transcription factors). Particularly interesting are features related to carbohydrate metabolism, including a minimalistic gene set for starch biosynthesis, the presence of cellulose synthases acquired before the primary endosymbiosis, showing the polyphyly of cellulose synthesis in Archaeplastida and cellulases absent in terrestrial plants as well as the occurrence of a mannosylglycerate synthase potentially originating from a marine bacterium. To explain the observations on genome structure and gene content we propose an evolutionary scenario which involves an ancestral red alga that was driven by early ecological forces to lose genes, introns and intergenetic DNA; this was followed by an expansion of genome size as a consequence of activity of transposable elements.

Description: Host-microbe interactions play crucial roles in marine ecosystems. However, we still have very little understanding of the mechanisms that govern these relationships, the evolutionary processes that shape them, and their ecological consequences. The holobiont concept is a renewed paradigm in biology that can help to describe and understand these complex systems. It posits that a host and its associated microbiota with which it interacts, form a holobiont, and have to be studied together as a coherent biological and functional unit to understand its biology, ecology, and evolution. Here we discuss critical concepts and opportunities in marine holobiont research and identify key challenges in the field. We highlight the potential economic, sociological, and environmental impacts of the holobiont concept in marine biological, evolutionary, and environmental sciences. Given the connectivity and the unexplored biodiversity specific to marine ecosystems, a deeper understanding of such complex systems requires further technological and conceptual advances, e.g., the development of controlled experimental model systems for holobionts from all major lineages and the modeling of (info)chemical-mediated interactions between organisms. Here we propose that one significant challenge is to bridge cross-disciplinary research on tractable model systems in order to address key ecological and evolutionary questions. This first step is crucial to decipher the main drivers of the dynamics and evolution of holobionts and to account for the holobiont concept in applied areas, such as the conservation, management, and exploitation of marine ecosystems and resources, where practical solutions to predict and mitigate the impact of human activities are more important than ever.

Description: Pseudochattonella farcimen (Eikrem, Edvardsen, et Throndsen) is an ichthyotoxic alga within the Dictyochophyceae (Heterokonta), which has been shown to form blooms in Scandinavian waters every year since 1998. To improve our understanding of the biology of this alga and to facilitate future genomic studies, we report the sequencing and analysis of 〉10,000 expressed sequence tags (ESTs) corresponding to 8149 gene models from this species. A direct comparison with EST libraries from other heterokonts revealed several functional categories to be significantly overrepresented among the P. facimen ESTs, such as genes involved in cell communication, transporters, or genes targeted to cell organelles. Interestingly, P. farcimen ESTs also code for a high proportion (1.4%) of proteins related to fatty acid metabolism, including eight fatty acid desaturases and two phospholipase A2 genes. Three of the desaturases belong to a family of delta-4 desaturases, known so far only from haptophytes, where they catalyze the conversion of n3-docosapentaenoic (n3-DPA) acid to docosahexaenoic acid (DHA). These findings may partially explain the unusual fatty acid profiles observed in P. farcimen and are discussed both from an evolutionary point of view and in relation the ichthyotoxic effects of this alga