Saccharina latissima
, known as sugar kelp, is a brown macroalga with huge ecological and economic values. In marine intertidal environment,
S. latissima
has to cope with both biotic and abiotic ...stress, which can cause the reduction of the yield during cultivation. To better understand the physiological responses of
S. latissima
under different stress conditions, large-scale transcriptomic analyses are useful to explore global metabolic pathway regulations. In addition, real-time quantitative polymerase chain reaction (RT-qPCR) is a powerful and rapid method for further quantifying changes in gene expression, and for targeting specific defense-related gene pathways. However, its level of accuracy is highly related to the expression stability of reference genes used for normalization and those still need to be evaluated in
S. latissima
. In this study, we therefore experimentally tested eight candidate reference genes identified from in silico screening of public transcriptomic datasets of
S. latissima
from different abiotic and biotic stress treatments. The stability analysis using complementary statistical approaches showed that
EIF5B
and
ATPase
are the most stable reference genes under biotic stress, whereas, under temperature and light stress, their combination with
NDH
gene is the best choice for RT-qPCR normalization. The validated reference genes were used to monitor the expression of target genes, related to oxidative responses, such as those involved in oxylipin pathways, in
S. latissima
plantlets submitted to different stress in laboratory-controlled conditions.
Brown algae (Phaeophyceae) are complex photosynthetic organisms with a very different evolutionary history to green plants, to which they are only distantly related. These seaweeds are the dominant ...species in rocky coastal ecosystems and they exhibit many interesting adaptations to these, often harsh, environments. Brown algae are also one of only a small number of eukaryotic lineages that have evolved complex multicellularity (Fig. 1). We report the 214 million base pair (Mbp) genome sequence of the filamentous seaweed Ectocarpus siliculosus (Dillwyn) Lyngbye, a model organism for brown algae, closely related to the kelps (Fig. 1). Genome features such as the presence of an extended set of light-harvesting and pigment biosynthesis genes and new metabolic processes such as halide metabolism help explain the ability of this organism to cope with the highly variable tidal environment. The evolution of multicellularity in this lineage is correlated with the presence of a rich array of signal transduction genes. Of particular interest is the presence of a family of receptor kinases, as the independent evolution of related molecules has been linked with the emergence of multicellularity in both the animal and green plant lineages. The Ectocarpus genome sequence represents an important step towards developing this organism as a model species, providing the possibility to combine genomic and genetic approaches to explore these and other aspects of brown algal biology further.
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 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 and adaptations to the marine environment, including genes related to halogen metabolism, oxylipins, and multicellularity (microRNA processing and transcription factors). Particularly interesting are features related to carbohydrate metabolism, which include 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 involving an ancestral red alga that was driven by early ecological forces to lose genes, introns, and intergenetic DNA; this loss was followed by an expansion of genome size as a consequence of activity of transposable elements.
is a canopy-forming species of brown algae and, as such, is considered an ecosystem engineer. Several populations of this alga are exploited worldwide, and a decrease in the abundance of
at its ...southern distributional range limits has been observed. Despite its economic and ecological interest, only a few data are available on the composition of microbiota associated with
and its role in algal physiologyn.
We studied the whole bacterial community composition associated with
samples from three locations (Brittany, Helgoland, and Skagerrak) by 16S metabarcoding analyses at different scales: algal blade part, regions, season (at one site), and algal physiologic state.
We have shown that the difference in bacterial composition is driven by factors of decreasing importance: (i) the algal tissues (apex/meristem), (ii) the geographical area, (iii) the seasons (at the Roscoff site), and (iv) the algal host's condition (healthy vs. symptoms). Overall,
,
, and
dominated the general bacterial communities. Almost all individuals hosted bacteria of the genus
, accounting for 12% of the total sequences, and eight additional core genera were identified. Our results also highlight a microbial signature characteristic for algae in poor health independent of the disease symptoms. Thus, our study provides a comprehensive overview of the
microbiome, forming a basis for understanding holobiont functioning.
Colonizations of freshwater by marine species are rare events, and little information is known about the underlying mechanisms. Brown algae are an independent lineage of photosynthetic and ...multicellular organisms from which few species inhabit freshwater. As a marine alga that is also found in freshwater, Ectocarpus is of particular interest for studying the transition between these habitats. To gain insights into mechanisms of the transition, we examined salinity tolerance and adaptations to low salinities in a freshwater strain of Ectocarpus on physiological and molecular levels. We show that this isolate belongs to a widely distributed and highly stress‐resistant clade, and differed from the genome‐sequenced marine strain in its tolerance of low salinities. It also exhibited profound, but reversible, morphological, physiological, and transcriptomic changes when transferred to seawater. Although gene expression profiles were similar in both strains under identical conditions, metabolite and ion profiles differed strongly, the freshwater strain exhibiting e.g. higher cellular contents of amino acids and nitrate, higher contents of n‐3 fatty acids, and lower intracellular mannitol and sodium concentrations. Moreover, several stress markers were noted in the freshwater isolate in seawater. This finding suggests that, while high stress tolerance and plasticity may be prerequisites for the colonization of freshwater, genomic alterations have occurred that produced permanent changes in the metabolite profiles to stabilize the transition.
Brown macroalgae are colonized by diverse microorganisms influencing the physiology of their host. However, cell-cell interactions within the surface microbiome (epimicrobiome) are largely ...unexplored, despite the significance of specific chemical mediators in maintaining host-microbiome homeostasis. In this study, by combining liquid chromatography coupled to mass spectrometry (LC-MS) analysis and bioassays, we demonstrated that the widely diverse fungal epimicrobiota of the brown alga Saccharina latissima can affect quorum sensing (QS), a type of cell-cell interaction, as well as bacterial biofilm formation. We also showed the ability of the bacterial epimicrobiota to form and inhibit biofilm growth, as well as to activate or inhibit QS pathways. Overall, we demonstrate that QS and anti-QS compounds produced by the epimicrobiota are key metabolites in these brown algal epimicrobiota communities and highlight the importance of exploring this epimicrobiome for the discovery of new bioactive compounds, including potentially anti-QS molecules with antifouling properties.
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•The epimicrobiome of brown alga S. latissima shows high microbial and chemical diversity•The algal epimicrobiome hosts intense and multidirectional cell-cell communication•QS and anti-QS compounds are crucial for cell-cell communication in fungi and bacteria•Algal epimicrobiome yields bioactive compounds, potential anti-QS antifouling agents
Microbiology; Microbiofilms
Chondrus crispus is a common red macroalga living on the rocky shores of the North Atlantic Ocean. It has a long research history, being a major source of carrageenan, a thickener widely used in the ...food industry, but also for physiological and ecological studies. To establish it as a model for red algae, its genome has been sequenced, allowing the development of molecular tools such as quantification of gene expression, including RNAseq and RT-qPCR. To determine appropriate genes for RT-qPCR normalization, the expression of 14 genes was monitored in 18 conditions using two sets of algal samples: samples from the sequenced strain, cultured and stressed in laboratory conditions and C. crispus collected on the shore and stressed in situ. The expression stability of the genes between the samples was evaluated by comparing the Ct range and using the programs geNorm and NormFinder. The candidate genes encoded translation related proteins (initiation factors IF4A-1 and IF4A-2, elongation factor EF1α and eRF3, an eukaryotic polypeptide chain release factor), cytoskeleton proteins (two β-tubulins, α-tubulin and actin), enzymes involved in the pentose phosphate pathway (glucose 6-phosphate deshydrogenase), protein recycling process (ubiquitin and ubiquitin-conjugating enzyme) and glycolysis (isocitrate dehydrogenase). The two sets of samples showed different expression patterns. Most of the genes were stable in the algae cultivated in the laboratory, whereas environmental samples showed a more important variation in gene expression. When analyzing the two sets separately, the ranking of the most stables genes were different from one method to another. When considering all samples, the two statistical methods were concordant, revealing translation initiation factor 4A-2 and eukaryotic polypeptide chain release factor 3 as pertinent normalization genes. This study highlights thus the importance of testing reference genes according to the experiments as well as the genetic and physiological background of the organism.
Mannitol represents a major end product of photosynthesis in brown algae (Phaeophyceae), and is, with the β-1,3-glucan laminarin, the main form of carbon storage for these organisms. Despite its ...importance, little is known about the genes and enzymes responsible for the metabolism of mannitol in these seaweeds. Taking benefit of the sequencing of the Ectocarpus siliculosus genome, we focussed our attention on the first step of the synthesis of mannitol (reduction of the photo-assimilate fructose-6-phosphate), catalysed by the mannitol-1-phosphate dehydrogenase (M1PDH). This activity was measured in algal extracts, and was shown to be regulated by NaCl concentration in the reaction medium. Genomic analysis revealed the presence of three putative M1PDH genes (named EsM1PHD1, EsM1PDH2 and EsM1PDH3). Sequence comparison with orthologs demonstrates the modular architecture of EsM1PHD1 and EsM1PDH2, with an additional N-terminal domain of unknown function. In addition, gene expression experiments carried out on samples harvested through the diurnal cycle, and after several short-term saline and oxidative stress treatments, showed that EsM1PDH1 is the most highly expressed of these genes, whatever the conditions tested. In order to assess the activity of the corresponding protein, this gene was expressed in Escherichia coli. Cell-free extracts prepared from bacteria containing EsM1PDH1 displayed higher M1PDH activity than bacteria transformed with an empty plasmid. Further characterisation of recombinant EsM1PDH1 activity revealed its very narrow substrate specificity, salt regulation, and sensitivity towards an inhibitor of SH-enzymes.
The filamentous algal endophyte
Laminarionema elsbetiae
is highly prevalent in European populations of the brown alga
Saccharina latissima
, but has also been found occasionally in the other kelp ...species
Laminaria digitata
. The presence of
L. elsbetiae
coincides with morphological changes in the hosts such as twisted stipes and deformed blades, however, little is known about the molecular bases of these algal host-endophyte interactions. Using a co-cultivation experiment, we showed that physiological and gene regulation responses, and later endophyte prevalences are different between the main and the occasional host. The contact with the endophyte
L. elsbetiae
induced a stronger and faster transcriptomic regulation in the occasional host
L. digitata
after 24 h, from which growth rate was later affected. During the first two days of co-cultivation, only 21 differentially expressed genes (DEGs) were common in both kelps, indicating a crucial difference between the molecular responses of the two hosts. By functional annotation, we identified DEGs related to host-endophyte recognition, defense response and cell wall modification. Our results suggest that expression pattern differences between the two kelps related to the recognition of the endophyte and later defense reactions could explain the variability of observed physiological responses and host-endophyte specificity in kelp natural populations.
The sugar kelp Saccharina latissima is cultivated in Europe for food, feed and ultimately the production of chemical commodities and bioenergy. Being cultivated in the open sea, S. latissima is ...exposed to potentially harmful organisms, such as Laminarionema elsbetiae, a filamentous brown algal endophyte with a very high prevalence in wild populations of European S. latissima. As it was shown previously that S. latissima sporophytes get infected by L. elsbetiae very early in their life, seeding the spores on collectors and keeping them under controlled conditions during the critical time of a possible infection with filamentous endophytes could be advantageous over direct seeding techniques, where the ropes are deployed within days after seeding. We used a qPCR-assay to assess the prevalence of the endophyte L. elsbetiae in S. latissima cultivated during winter in Northern Brittany, comparing individuals from direct-seeded ropes and collector-seeded lines that were kept in laboratory conditions for different time spans. No DNA of the endophyte was detected in the samples, suggesting that either the kelps were not infected or the amount of endophytic filaments were below the detection rate of the qPCR assay. Furthermore, L. elsbetiae could not be detected in the seawater surrounding the kelp farm, indicating that L. elsbetiae is not fertile or disperses at a very small scale in Northern Brittany during the deployment time of young kelps. Our results suggest that infections of cultivated S. latissima with the endophyte L. elsbetiae might be a minor problem in kelp farms in Northern Brittany if the seeding production is kept under controlled conditions without external contamination.
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