Was the chlamydial adaptive strategy to tryptophan starvation an early determinant of plastid endosymbiosis?
Professor Steven Ball, University of Lille, France
Chlamydiales were recently proposed to have sheltered the future cyanobacterial ancestor of plastids in a common inclusion. The intracellular pathogens are thought to have donated those critical transporters that triggered the efflux of photosynthetic carbon and the consequent onset of symbiosis. Chlamydiales are also suspected to have encoded glycogen metabolism TTS (Type Three Secretion) effectors responsible for photosynthetic carbon assimilation in the eukaryotic cytosol.
We now review the reasons underlying other chlamydial lateral gene transfers evidenced in the descendants of plastid endosymbiosis. In particular, we show that half of the genes encoding enzymes of tryptophan synthesis in Archaeplastida are of chlamydial origin. Tryptophan is known to define an essential cue triggering two alternative modes of replication in Chlamydiales. In addition, sophisticated tryptophan starvation mechanisms are known to have been implemented as antibacterial defences by their eukaryotic hosts. We propose that Chlamydiales have donated their tryptophan operon to the emerging plastid to ensure increased synthesis of tryptophan by the plastid ancestor. This would have allowed massive expression of the tryptophan rich chlamydial transporters responsible for symbiosis. It would also have allowed possible export of this valuable amino-acid in the inclusion of the tryptophan hungry pathogens. Free-living single cell cyanobacteria are devoid of proteins able to transport this amino-acid. We therefore investigated the phylogeny of the E.coli Tyr/Trp transporters and found yet another LGT from Chlamydiales to Archaeplastida thereby considerably strengthening our proposal.
The Use of Comparative Genomics and the GreenCut Assemblage of Proteins to Identify Novel Photosynthetic Functions
Professor Arthur Grossman, Carnegie Institution for Science, USA
Using powerful bioinformatics tools and comparative genomics is allowing us to identify novel components of photosynthesis. This information will help elucidate new photosynthetic functions and provide opportunities for engineering plants and algae for efficient solar energy utilization, increased agricultural outputs and improved resiliency to changing global environments. The GreenCut represents an informatics assemblage of nuclear-encoded proteins that are conserved among photosynthetic organisms of the green lineage (Viridiplantae), but that are either not present, or poorly conserved in heterotrophic (nonphotosynthetic) organisms. Many uncharacterized GreenCut proteins (unknown specific functions) appear to have regulatory functions or roles in the biogenesis of the photosynthetic apparatus based on specific domains in their protein sequences. Other GreenCut proteins have no domains that are currently informative with respect to function. Generating insertional mutants of genes encoding GreenCut proteins in Chlamydomonas reinhardtii has allowed for the characterization of mutant phenotypes, suggesting roles for a number of these proteins with respect to photosynthetic activities. One GreenCut protein was recently shown to be involved in stabilizing the assembly of photosystem I (PSI) under oxic conditions, and is potentially involved in protecting oxygen sensitive PSI cofactors (e.g. Fe-S clusters) during the assembly process; as the Earth became oxygenated mechanisms must have evolved that protect existing oxygen sensitive cofactors from disruption. Other novel proteins of the GreenCut appear to be important for the biogenesis/stability of the cytochrome b6f complex. I will discuss the GreenCut and the ways in which it is being used to examine photosynthetic function and evolution.
Seaweed microbiomes: a new age of discovery
Professor Juliet Brodie, Natural History Museum, UK
Marine macroalgae are host to a wide range of prokaryotic and eukaryotic life, creating a dynamic and complex community of specialists or generalists that can be beneficial (mutualistic), neutral (commensal) or harmful (parasitic) organisms. Bacteria are the dominant active group that make up the microbiome, and macroalgal-bacterial studies suggest that there is a core microbiome at the phylum level consisting of the Gammaproteobacteria, Bacteroidetes, Alphaproteobacteria, Firmicutes and Actinobacteria. Until relatively recently, studies of bacteria associated with various macroalgal hosts were undertaken using cultures and electron microscopy. Next generation sequencing (NGS) is now revolutionising the subject and revealing the extent of bacterial diversity in these microbiomes. This talk will review what is known about microbiomes of marine macroalgae with a particular focus on the red algae. It will explore the notion of a core microbiome for different host groups and also the spatial and temporal ecological impact on host photosynthesis of different types of epiphytes. Focussing in particular on the prokaryote component of the microbiome, the nature of the relationships between the bacteria and host and the implications for ecosystem function and environmental change including ocean acidification and increasing sea surface temperatures will be explored. Drawing upon our results of the microbiome of Corallina officinalis – the first for a geniculate coralline alga – differences in microbiome composition will be compared both within and between fleshy and calcified species. Evidence that the epiphytic bacteria provide important services to hosts that are vital to their health, performance and resilience will be discussed. Ways forward to identify prokaryote and eukaryote diversity, to understand their roles in productivity, and the overall nature of these relationships will be explored.
The "virocell" metabolism-metabolic innovations during algal-virus interactions in the ocean
Professor Assaf Vardi, Weizmann Institute of Science, Israel
Marine viruses that infect marine microorganisms are recognized as major ecological and evolutionary driving forces, shaping community structure and controlling cycling of nutrient energy in the marine environment. A major challenge in our current understanding of host-virus interactions in the marine environment is to decode the wealth of genomic and metagenomic data and translate it into cellular mechanisms that mediate host susceptibility and resistance to viral infection. Nevertheless, the cellular mechanisms that govern these host-virus dynamics are largely underexplored. Recent reports highlighted a novel genomic inventory found in marine viruses which can encode for auxiliary metabolic genes previously thought to be restricted to their host genomes. Thus, these genes can expand the metabolic capabilities of the infected host cell (Virocell) and the flux of the nutrients and metabolites between the cell and its micro-environment.
Emiliania huxleyi is a globally important coccolithophore forming massive algal blooms in the North Atlantic Ocean that are routinely infected and terminated by large DNA viruses (EhVs) belong to the coccolithoviruses. We explore the molecular and metabolic basis for these host-virus dynamics and the signal transduction pathways that mediate host-virus interactions. By combining genome-enabled technologies, analytical chemistry and advance cell imaging approaches, we were able to identify several fundamental metabolic pathways that mediate these host-virus interactions. We revealed the role of viral-encoded sphingolipid biosynthesis, redox and DMS metabolism and their function in determining host cell fate (e.g. PCD and autophagy) and viral replication strategies.
Pythium porphyrae: a plant pathogen seeing red?
Dr Claire Gachon, Scottish Marine Institute, UK
Pythium porphyrae is responsible for devastating outbreaks in seaweed farms of Pyropia, the most valuable seaweed worldwide. While the genus Pythium contains many well studied plant and animal pathogens, the infection strategies and genome content of P. porphyrae remains to be elucidated. Recent reports also indicated the ability of P.porphyrae to infect, colonize and reproduce on a great variety of land plants, begging the question of its potential land origin, as well as the molecular mechanisms underpinning its host specificity. Here, we used RNA sequencing to provide the first description of P. porphyrae gene repertoire and assess its similarity to fully sequenced Pythiums. Using ab-initio detection strategies, similarity based and manual annotation, we found that the P.porphyrae gene repertoire is strikingly similar to the ones described for classical phytopathogenic Pythium species, including Crinklers, elicitins, cellulases, CBEL-like proteins and a total absence of RxLR effectors. Comparative genomics revealed that 1507 genes, including CBEL-like proteins and elicitins, have orthologs in some plant infecting Pythiums but not in the animal pathogen P.insidiosum. Despite 34% of the P.porphyrae proteome having no orthologs in other sequenced Pythiums, we could not identify any enzyme involved in the degradation of red algal-specific cell wall components. Complementary infection experiments indicated that P.porphyrae is specific of Bangiales, contrasting with the general broad host range observed on land plants, perhaps suggesting a recent adaptation linked with the development of Pyropia cultivation.
Chemical interaction between seaweeds and their epibacterial ‘friends’ and ‘foes’
Dr Mahasweta Saha, Helmholtz Center for Ocean Research, GEOMAR, Germany
Fouling is a paramount phenomenon in the marine environment. However, surfaces of certain seaweeds like the brown alga Fucus vesciculosus, remain relatively free from heavy fouling and are covered by a thin film of epibiotic microorganisms. We found that Fucus in a given habitat harbors a distinct epibacterial community. Fucus also produces defence compounds like fucoxanthin, DMSP reducing bacterial settlement. These compounds have also been found to be strain specific in their action, thus probably assisting Fucus in ‘gardening’ a distinct bacterial community on its surface. This idea was further supported by a study that correlated the surface concentration of the defence compounds with the presence or absence of different bacterial clades. Several bacterial groups were found to be positively or negatively affected by the compounds present on surfaces of Fucus individuals. Seaweeds also account for a substantial proportion of all introduced species. The East Asian red macroalga Gracilaria vermiculophylla has successfully invaded several temperate areas of the Northern hemisphere. Although foulers have the potential to determine invasion success or failure of invasive seaweeds, this perspective has been ignored so far. We tested whether the impressive invasion success of Gracilaria may be enhanced by a rapid adaptation of defence against potentially facultative new target microfoulers in the invaded range. Native and invasive Gracilaria populations were equally well defended against presently co-occurring bacterial foulers. However, native populations were weakly defended against bacteria from the invaded range, while invasive populations were weakly defended against bacteria from the native range. Thus, the invasive populations exhibited an adaptation of their defence capacity to cope with the new foes, but have lost capacity to fend off old foes. These results provide the first evidence that confrontation by new foulers can trigger a rapid defence adaptation of aquatic weeds, which could be necessary for algal invasiveness.