ALBERT

Description: The genus Ulva is has broadly negative connotations because of its ability to form harmful “green tides” and the problems it causes with precise species identification, due to its morphological plasticity. During recent years, tides of unattached Ulva compressa U. Linneus 1753 with an atypical sheet-like morphology were for the first time observed in the German Baltic. Here we report that this nuisance alga is conspecific with the type strain of U. mutabilis Föyn 1958 from Faro in Portugal, an important model organism to study morphogenesis, morphogenetics and mutualistic interactions. Different approaches were used to examine conspecificity: (1) Comparisons on vegetative and reproductive features of cultured material of Ulva mutabilis and German Ulva compressa resulted in congruent results proving that a certain morphogenetic mutation pattern is shared. Spontaneous mutations of “slender-like” thalli are appearing whilst the common form exhibits a “leaf-like” wildtype morphology. (2) Interbreeding experiments of gametes of Ulva compressa and Ulva mutabilis were successful and showed a fertile first-generation offspring exhibiting the typical wildtype morphology similar to the phenotype of the parental generation. (3) Phylogenetic and species delimitation analyses were carried out on 128 tufA sequences of Ulva compressa specimens sampled in 2014–2016 in Germany and on tufA sequences of two clones of the strains Ulva mutabilis (sl-G[mt+]) and Ulva mutabilis (wt-[mt-]) to identify Molecular Operational Taxonomic Units (MOTUs). The Generalized Mixed Yule-Coalescent (GMYC) method comprises one major MOTU containing all included sequences of Ulva compressa and Ulva mutabilis, while reference sequences included in the analysis clustered outside this MOTU. This highly supports the monophyly of Ulva compressa and Ulva mutabilis, which can be treated as the same species. As a consequence, U. mutabilis is also a suitable model for future studies of green tides and their molecular and morphogenetic basis in the Baltic Sea.

Description: Invasive species are one of the principal components of global change along with ocean warming at the global scale, or overfishing and deoxygenation at the regional scale. Seaweeds represent up to 40% of all introduced marine species and some seaweeds can significantly affect the composition and functioning of marine benthic communities. Within ten years of its first discovery in the Kiel Fjord in 2005 the East Asian red seaweed Gracilaria vermiculophylla has spread approximately 100 km eastward and 120 km northward along the German Baltic Sea coast, now inhabiting many lagoons and sheltered bays between the German-Danish border and Neustadt. During the first two years after its discovery Gracilaria vermiculophylla increased its biomass in the Kiel Fjord massively. However, this was followed by a sudden decline in late summer 2008, when the alga decayed in nearly all inhabited parts of the bay within few weeks. Co-cultivation of healthy Gracilaria from unaffected environments with small amounts of decaying material from the Kiel Fjord in laboratory assays demonstrated that the decay was apparently caused by an infectious disease. Thus, 59 different species of epibacteria isolated from Gracilaria were tested for their capacity to induce decay in a bleaching assay. Out of these, three were found to induce the disease, while 19 others significantly reduced the risk of decay and were thus protective. When protectors and pathogens were tested together, the protective strains fully prevented the negative impact of the bleachers, hinting at the presence of an associational defence offered by Gracilaria’s epibacteria. Presence of such an associational resistance was also supported in a follow- up bioassay where surface extract of Gracilaria and its associated microbiome attracted the beneficial strains, but deterred the detrimental ones. Thus, we suggest that the breakdown in 2008 was due to a collapse of such associational resistance provided by bacterial partners.

Description: Fouling is a stressor that might determine the fate of seaweeds, but reports of algal adaptation to epibiosis are scarce. Previous comparisons have shown resistance to epibionts can be higher in non-native than in resident seaweed species, but we do not know whether it is an intrinsic trait of the non-natives or it has been acquired during the invasion process. We here compared native and non-native populations of the same algal species to elucidate this question. Resistance against two groups of epiphytes was assessed in living thalli and in artificial substrata coated with surface extracts, both gained from four Asian (native) and four European (non-native) populations of the red alga Gracilaria vermiculophylla. Two diatom species and two filamentous macroalgae were used as micro- and macro-epiphytes, and one of each type was collected in Asia, while the other came from Europe. Laboratory assays were done in both distributional ranges of G. vermiculophylla and in different seasons. We used a fully crossed design with the factors (i) ‘Origin of Gracilaria’, (ii) ‘Origin of epiphytes’, (iii) ‘Season’ and (iv) ‘Solvent used for extraction’. Both groups of epiphytes, regardless of their origin, attached less to living thalli and to surface extracts from non-native G. vermiculophylla. Fewer diatoms attached to hexane-based extracts, while fewer Ceramium filaments settled on extracts gained with dichloromethane. Our results show for the first time that non-native individuals of a seaweed are better defended against epiphytes than native conspecifics. Furthermore, we found evidence that at least a part of the defence is based on extractable secondary metabolites. We suggest that an enhanced defence against epiphytes after introduction is one reason for G. vermiculophylla’s invasion success. Our observation may also apply to other basibiont–epibiont interactions and could be a key feature of seaweed bioinvasions.

Description: To evaluate the importance of anti-herbivore resistance for algal invasion success we compared resistance traits among specimens of the red macroalga Gracilaria vermiculophylla from six native populations in Korea and China and eight invasive populations in Europe and Mexico that were maintained under identical conditions in the laboratory. Herbivorous snails both from the native range (Littorina brevicula) and from the invaded range (Littorina littorea) consumed significantly less of seaweed specimens originating from non-native populations. Metabolome profiling revealed that this preference was correlated with an increased woundactivated production of deterring prostaglandins and hydroxyeicosatetraenoic acids. Thus, invasive populations of G. vermiculophylla are more strongly defended against challenge by herbivores and other biological enemies that cause local tissue or cell disruption and activate oxylipin production. Anthropogenic distribution of genotypes adapted to resist elevated feeding pressure probably contributed to the invasion success of this species.

Description: Grazing by the isopod Idotea baltica induces chemical defenses in the brown seaweed Fucus vesiculosus. A combination of a 33 day induction experiment, feeding choice assays and functional genomic analyses was used to investigate temporal defense patterns and to correlate changes in palatability to changes in gene expression. Despite permanent grazing, seaweed palatability varied over time. Controls were significantly more consumed than grazed pieces only after 18 and 27 days of grazing. Relative to controls, 562/402 genes were up-/down-regulated in seaweed pieces that were grazed for 18 days, i.e. when defense induction was detected. Reprogramming of the regulative expression orchestra (translation, transcription), up-regulation of genes involved in lipid and carbohydrate metabolism, intracellular trafficking, defense and stress response, as well as downregulation of photosynthesis was found in grazed seaweed. These findings indicate short-term temporal variation in defenses and that modified gene expression patterns arise at the same time when grazed seaweed pieces show reduced palatability. Several genes with putative defensive functions and cellular processes potentially involved in defence, such as reallocation of resources from primary to secondary metabolism, were revealed

Description: Plants rely on both mechanical and chemical defence mechanisms to protect their surfaces against microorganisms. The recently completed genome of the eelgrass Zostera marina, a marine angiosperm with fundamental importance for coastal ecosystems, showed that its re-adaptation from land to the sea has led to the loss of essential genes (for chemical communication and defence) and structural features (stomata and thick cuticle) that are typical of terrestrial plants. This study was designed to understand the molecular nature of surface protection and fouling-control strategy of eelgrass against marine epiphytic yeasts. Different surface extraction methods and comparative metabolomics by tandem mass spectrometry (LC-MS/MS) were used for targeted and untargeted identification of the metabolite profiles of the leaf surface and the whole tissue extracts. Desorption electrospray ionization-imaging mass spectrometry (DESI-IMS) coupled with traditional bioassays revealed, for the first time, the unique spatial distribution of the eelgrass surface-associated phenolics and fatty acids, as well as their differential bioactivity against the growth and settlement of epiphytic yeasts. This study provides insights into the complex chemical defence system of the eelgrass leaf surface. It suggests that surface-associated metabolites modulate biotic interactions and provide chemical defence and structural protection to eelgrass in its marine environment.

Description: Invasive species are co-introduced with microbiota from their native range and also interact with microbiota found in the novel environment to which they are introduced. Host flexibility toward microbiota, or host promiscuity, is an important trait underlying terrestrial plant invasions. To test whether host promiscuity may be important in macroalgal invasions, we experimentally simulated an invasion in a common garden setting, using the widespread invasive macroalga Agarophyton vermiculophyllum as a model invasive seaweed holobiont. After disturbing the microbiota of individuals from native and non-native populations with antibiotics, we monitored the microbial succession trajectories in the presence of a new source of microbes. Microbial communities were strongly impacted by the treatment and changed compositionally and in terms of diversity but recovered functionally by the end of the experiment in most respects. Beta-diversity in disturbed holobionts strongly decreased, indicating that different populations configure more similar –or more common– microbial communities when exposed to the same conditions. This decline in beta-diversity occurred not only more rapidly, but was also more pronounced in non-native populations, while individuals from native populations retained communities more similar to those observed in the field. This study demonstrates that microbial communities of non-native A. vermiculophyllum are more flexibly adjusted to the environment and suggests that an intraspecific increase in host promiscuity has promoted the invasion process of A. vermiculophyllum. This phenomenon may be important among invasive macroalgal holobionts in general.