Description: Originating from East Asia, the perennial red macroalga Gracilaria vermiculophylla (Ohmi) Papenfuss has successfully invaded several temperate areas of the Northern hemisphere and continues to spread. In its new range, the seaweed tends to form local mass appearances and to dominate the native community. A high tolerance towards both abiotic and biotic environmental stressors could explain the invasion success of this species. I therefore compared the stress resistance of G. vermiculophylla from six native populations from South Korea and China and eight invasive populations from Europe and NW-Mexico. In short-term experiments G. vermiculophylla individuals were exposed to 1) heat shock, 2) UV-C-radiation and 3) elevated copper concentrations in the water. In a long-term experiment the seaweed had to cope with depletion stress (darkness in combination with low temperature and dryness) for several months. All experiments were carried out twice - one time in the native range in Qingdao, China and one time in the invaded range in Kiel, Germany - to rule out local acclimation effects. In order to compare the resistance against herbivory individuals of native and invasive G. vermiculophylla populations were fed to snails from the native (Littorina brevicula) and the invasive (Littorina littorea) range. In the Baltic Sea, G. vermiculophylla might threaten the habitat-forming native brown alga Fucus vesiculosus through direct competition for resources and by providing a shelter for mesograzers, which prefer to feed on F. vesiculosus. Mesocosm-experiments were conducted over one year in the Kiel Fjord in order to test the direct and indirect effects of G. vermiculophylla on F. vesiculosus.

Description: The number of introduced species, also called non-native or invasive species, has substantially increased in both terrestrial and aquatic ecosystems worldwide in past years. One possible reason for invasion success, i.e. the permanent establishment and spread after introduction, could be a high resistance towards biotic stressors such as herbivory. In the marine environment epibiosis (the colonization of living surfaces or exoskeletons by sessile organisms) is a common stressor and a high resistance towards fouling could explain invasion success in introduced species. This the first study that compares the susceptibility to eukaryote microfouling, macrofouling and natural fouling between native and non-native populations of an aquatic species. Further, this work gives the first example that non-native individuals of an aquatic species are better defended against fouling than native conspecifics and suggests that an enhanced defence against fouling after introduction could explain – at least in parts - the invasion success of G. vermiculophylla. Additionally, this work demonstrates that the chemical antifouling defence in G. vermiculophylla varies with season and is based on multiple compounds that have different polarities. Finally, the new technique of enclosing macroalgae in dialysis tubes represents a simple, efficient and accurate way to test for the presence of chemical antifouling defences in these organisms and could possibly be applied to other algal species. The study thus provides new insights into the invasion ecology of macroalgae.

Description: In the dissertation, we described the existence of the chemical antifouling defense of Zostera marina. In general, the surface compounds on Z. marina are able to repel bacterial foulers in a non-toxic way. Three compounds were separated from the surface of Z. marina and rosmarinic acid was the most abundant and active compounds on the surface. Meanwhile, the seasonal fluctuation of chemical defense on Z. marina was observed, the peak of defense strength appeared in summer when the bacterial fouling pressures were the highest. In addition, we observed the significant correlations between the seasonality of antifouling defense and bacterial fouling pressure, suggesting Z. marina possibly regulate their defense base on the change of surrounded bacterial fouling pressure. In the end, we proved that Z. marina tends to upregulate their defense activity when they were facing high-temperature threats.

Description: Summary Bacteria are omnipresent in the marine environment. For example, in the Kiel Fjord (Baltic Sea) 0.7 to 2.24 x 106 bacterial cells are present in one ml of seawater. Marine organisms are thus continuously exposed to high densities of bacteria, some of which tend to settle and colonise living surfaces. Some associations between host and bacteria could be neutral or mutualistic, for example algae associated bacteria may protect their host from fouling. But mostly, bacterial epibionts have a number of negative impacts on their hosts, such as providing positive settlement cues to macrofoulers. Although macroalgae constitute a potential substratum for surface colonisation, many of them remain largely free from heavy fouling, presumably due to surface properties or release of antifouling chemicals. In the Baltic Sea this has also been observed in the ecologically important alga Fucus vesiculosus (Phaeophyceae), which remains free from heavy fouling during most of the year. So far there was no indication of a mechanical or structural defence against epibionts in this alga, which suggested that one or several chemical defence mechanisms may exist. The chemical defence of macroalgae has been a theme of literature over two decades. But so far, most of these effects have been demonstrated in studies investigating total tissue extracts instead of testing ecologically relevant surface extracts or surface metabolites at natural concentrations, which actually affect the fouling organisms in nature. Further, only very few among the studies which so far tested surface based compounds at ecologically relevant concentrations targeted the first phase of fouling, i.e. bacterial settlement. It was a main aim of my work to study the effect of surface-associated metabolites on microbial settlement, using the rockweed Fucus vesiculosus as a model organism. Since the control of bacterial fouling on macroalgae is of substantial ecological importance, it is highly relevant to characterize the nature and dynamics of potential bacterial settlement inhibitors - the first line of defensive compounds against microfoulers. In F. vesiculosus, polyphenols have been suggested as fouling deterrents. However, a bioassay-guided structure elucidation of the alga’s metabolites deployed in defence against bacterial settlement has been realized for the first time in my study, and a role of polyphenols in antimicrobial defence of F. vesiculosus was not detected. Using different chromatographic techniques followed by bioassays, I was instead able to isolate and identify an active lipophilic metabolite: fucoxanthin (Chapter I). Subsequently, I proved its surface presence. This was followed by an investigation of the metabolic provenance through segregation of fucoxanthin originating from Fucus and fucoxanthin originating from surface associated diatoms. Further I quantified the compound on algal tips and on whole individuals, in order to know whether it is sufficiently concentrated on the algal surface to warrant an inhibitory effect on bacterial settlers. With the help of this comprehensive study, I was able to report a novel defence strategy of Fucus. In chapter II, I proceeded to investigate the active polar metabolites on Fucus surfaces and reported DMSP and proline to be the hydrophilic metabolites contributing to an inhibition of bacterial settlement. This was the first study reporting the role of DMSP and of the amino acid proline as algal antifoulants. The combined results of chapter I and II provides a picture of multiple chemical defence strategies of Fucus vesiculosus in an ecological context. In chapter III, I show how the algal defence based on these inhibitors may be affected by environmental factors/ shifts such as high temperature and low light. Based on the surface concentrations of the three settlement inhibitors the defence capacity of F. vesiculosus appears to be only moderately affected by potential stressors. Finally, I report on the seasonal and geographical variation of antibacterial defence of the alga (Chapter IV). The anti-settlement defence showed a temporal variation with a peak activity in late summer/ autumn and also showed a strong and consistent difference between sites throughout the year. In summary this thesis highlights the capability of the Baltic foundation species Fucus vesiculosus for chemical defence against microfoulers, as well as the natural defence variability with site and season and the alga’s responses toward simulated environmental conditions. The study thus provides numerous new insights into algae-bacteria interactions and their dynamics and it helps in better understanding of the alga’s reaction towards bacterial epibiosis in response to potential abiotic stressors.

Description: In this doctoral project, I investigated the recent inventory, distribution and phylogenetic relationships of Ulva sensu lato in northern Germany, including sampling sites at the Baltic Sea, Wadden Sea and on Helgoland. Furthermore, I compared the recent results with historic findings. Therfore, this thesis constitutes a complete revision of the species inventory of Ulva sensu lato in northern Germany. Assessments of biodiversity were based on both the analysis of classical morphological characters and DNA barcoding. Phylogenetic analysis of more than 370 sequences of the tufA marker gene revealed the presence of 20 different species in German waters.

Description: In this doctoral project, I investigated the effects of abiotic factors on the composition and cell density of epibacterial communities associated with the ecologically important brown macroalga Fucus vesiculosus in laboratory and outdoor experiments. During the experiments, specimens of the macroalgal host and their microbial biofilm were exposed to different levels of either temperature, irradiance, or salinity. Subsequently, bacteria were harvested from the macroalga, and bacterial community composition was analyzed by culture-independent analyses of 16S rRNA gene sequences. Cell densities were enumerated via epifluorescence microscopy directly on the algal surface. Furthermore, I studied the effects of natural microbial assemblages on the attachment rates of barnacle larvae. For this purpose, the microbial biofilm community was harvested from the macroalgal surface and exposed to barnacle larvae during their peak settlement season in the Kiel Fjord in a specially constructed bioassay apparatus. As revealed by redundancy analyses, epibacterial communities significantly clustered along the temperature (5, 10, 15, 20, 25°C) and salinity (5, 19, 25) gradients but not along the gradient of irradiance (100%, 44%, 23%, 12%, 5%, and 0% of sunlight reaching the algal surface). Temperature and salinity are important factors in structuring epibacterial communities since they each accounted for about 20% of the variation in the bacterial community composition. In general, 〈10% of bacterial OTUs (operational taxonomic units grouped at the 97% sequence similarity level) siginificantly correlated with the abiotic factors. The differences in community composition due to temperature were largely attributed to the Rhodobacteraceae, the bacterial family whose mean relative abundance was twice as high at the high temperatures than at the low temperatures. Members of the subphylum Betaproteobacteria made up about a third of the bacterial community at the low salinity level (5) on average and were nearly absent at the higher salinity levels. Members of the Gammaproteobacteria and of the Actinobacteria were on average at least twice as abundant at the higher salinities than at the low salinity. Furthermore, the type of substrate (stones were included in the salinity experiment as non-living reference substrate) had a significant effect on the composition of epibacteria. Epiphytic and epilithic bacterial communities differed significantly due to a higher mean relative percentage of members of the phylum Cyanobacteria. They made up about 8% on average in the epilithic community, while they were nearly absent on F. vesiculosus. Furthermore, members of the subphylum Gammaproteobacteria were at least twice as abundant in epiphytic than in epilithic bacterial communities. Epibacterial richness (i.e., the number of OTUs) significantly differed between treatment levels, although variability between algal individuals of one treatment level was generally high. OUT richness was highest at 15°C, lowest at a salinity of 5, and lowest at 44% irradiance reaching the algal surface. The most even distribution of epibacterial groups (evenness expressed with the Inverse Simpson Index) showed similar patterns as richness, although in the temperature experiment this was a non-significant trend. Epibacterial densities did not significantly differ between levels of temperature but decreased with shading. However, variability between individuals of one treatment level was often larger than between replicates of one treatment level. Whether direct or indirect effects via altered biotic interactions may have caused the observed shifts is discussed in the appropriate sections. In bioassays conducted in the Kiel Fjord, barnacle cyprids attached significantly less to filters treated with microbial assemblages harvested from F. vesiculosus than to non-treated seawater filters. This indicates a repellent effect of the microbes. The most pronounced repellent effect was found with microbial assemblages that had been kept at 15°C for 14 days (54% fewer attached cyprids than on filters without microbes). My study contributes to the basic understanding of the ecology of host-associated bacterial communities and the interactions between host, epibacteria, and macrofoulers: with the tool of high-throughput sequencing, high bacterial diversity for this macroalgal host was confirmed and a detailed description of community composition under different abiotic conditions was achieved. The impact that abiotic factors can have on this highly complex epibacterial community has been elucidated and may become more important in the future taking into consideration predicted climate change scenarios. The suspected potential of the epibacterial biofilm of playing a mutualistic role for the macroalgal host by assisting the host in its own defense against macrofouling was confirmed, adding to the notion that microbial communities should be considered when studying macroalgae and other marine hosts.