ongoing:

Impact of MALV on species succession in bloom dominated plankton communities

In this project, I want to investigate the impact of parasites on species succession in plankton communities that are dominated by algal blooms. Bloom-forming microalgal species that are not infected by MALV could benefit from the presence of parasites, as these might control the abundance of grazers. I will follow species composition and interactions for two years in the English Channel and identify MALV hosts and the impact of parasites on Phaeocystis globosa blooms. By sampling a transect from the coast to off-shore repeatedly over one year, including two weeks with daily samples, I will be able to follow the temporal dynamics of the eukaryotic plankton community using amplicon sequencing. Environmental data will be available from the SOMLIT national network and enable identification of environmental drivers for changes in community composition in the English Channel.

Heterotrophic dinoflagellates are likely the most important grazers of P. globosa and their abundance might be significantly affected by parasites. Additionally, MALV might directly infect P. globosa colonies, although previous network analyses have not found any evidence for it. This project will, for the first time, comprehensively identify MALV hosts in plankton communities and assess potential top-down control on blooms. Using the same molecular approaches as in the Southern Ocean, namely epicPCR and FISH, I will assess the abundance of MALV and identify infected community members. Additionally, interactions between parasites and their hosts will be studied in great detail by establishing cultures from isolated single cells. Using these cultures, I will be able to describe life cycles of currently understudied parasitic species. I will also conduct infection experiments with different combinations of hosts and parasites to assess vulnerability and specificity.  These results will improve our understanding about ecological implications of parasitism on microbial succession occurring during blooms of P. globosa.

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Impact of parasitism on carbon cycling (part of MobyDick)

The biological carbon pump is an essential mechanism regulating the carbon balance on our planet, as it transports CO2 fixated through primary production by phytoplankton to the bottom of the ocean, where it is stored in the sediment. However, the efficiency of this carbon sink depends on the species interactions on different trophic levels in the foodweb. The effects of differently structured foodwebs can be observed around the Kerguelen Islands in the Southern Ocean, as there are areas with low biomass and low export (LBLE), as well as areas with high biomass and low export (HBLE). The MobyDick project will investigate the relationship between oceanic biodiversity and functioning of the biological carbon pump using a research cruise to the Southern Ocean to enable predictions about nutrient cycling in the future. 

Parasitic marine alveolates (MALV) are among the most common microorganisms in all aquatic ecosystems, but their impact on community composition and ecosystem functioning is poorly understood. Previous studies have suggested that MALV can terminate harmful algal blooms and control grazer communities. Additionally, parasitism might increase the complexity of planktonic trophic networks as infections on various trophic levels can extend food chains. Therefore, I will utilize sampling during the MobyDick cruise to assess the contribution of MALV to carbon cycling in the Southern Ocean. High-throughput molecular methods can nowadays provide a comprehensive picture of all microbial community members. By sequencing of ribosomal genes from environmental samples, I will assess diversity and distribution of MALV species in the two contrasting ecosystems (LBLE and HBLE) in the Southern Ocean. Studies of interactions between parasites and hosts have previously been limited to cultivation experiments in the lab. I will directly identify MALV hosts using a novel method called epicPCR and fluorescence in-situ hybridization (FISH) with probes for MALV. This approach will provide the first thorough identification of MALV hosts in an entire plankton community and significantly advance our understanding of host specificity in these parasites.  Furthermore, I will illuminate MALV  distribution in two contrasting foodwebs and their contribution to carbon cycling in the Southern Ocean.

 

Chloroplast evolution in raphidophytes

In an earlier study, I found unexpected pigments in the freshwater raphidophyte Gonyostomum semen. This pigment composition clearly distinguishes it from marine raphidophyte species and challenges monitoring and identification of this species based on maker pigments. Gonyostomum might have received its unusual pigments through strong local adaptation, horizontal gene transfer or endosymbiosis of other microalgae. To shed some light onto the evolutionary origin of these pigments, I am currently sequencing the chloroplast genome of Gonyostomum and other raphidophyte species.

 

 

finished:

Population genetic structure of Gambierdiscus in the Gulf of Mexico and the Caribbean

Several microalgal species produce potent toxins that can poison seafood and represent, therefore, a threat to humans. Gambierdiscus, a microalgal species associated with sea weed, causes Ciguatera Fish Poisoning in the Greater Caribbean Region and several locations there have a long history of outbreaks. However, outbreaks from new regions were recently reported, which suggests spreading of the microalgae Gambierdiscus. Therefore, I developed microsatellite markers for population genetic studies of Gambierdiscus caribaeus (Sassenhagen & Erdner 2017, J Appl Phycol), a common species at our sampling site in the US Virgin Islands. With these genetic markers, I am  going to examine the temporal and spatial population structure of this toxic dinoflagellate species in order to shed light onto its dispersal routes and intraspecific diversity. This study is part of a larger, multi-investigator project that seeks to understand the diversity, physiology, and ecology of Gambierdiscus in the Caribbean and Gulf of Mexico (http://www.fgcu.edu/CiguaHAB/).  

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Physical and biological dipersal barriers in invasive, bloom-forming microalgae

For a long time microorganisms were assumed to have unlimited dispersal due to their small size and high cell numbers. Being so small they can easily be transported in water droplets by wind and attached to other organisms like birds and insects. Only environmental conditions, which could be hostile for certain microbes, were supposed to affect the distribution of species and community composition. Surprisingly, several studies recently showed development of genetic differences between regional groups of microorganisms. These groups still belong to the same species, thus they are called “populations”. These findings indicate important dispersal barriers that prevent mixing of different populations.

Similar geographic patterns were also discovered in the freshwater microalga Gonyostomum semen. This species recently invaded several new lakes in Northern Europe and forms extensive, nuisance blooms in summer. Despite this recent expansion, all lake populations are genetically different from each other. In my PhD thesis I studied potential dispersal barriers in this species, which might explain the observed genetic differentiation of populations.

I assumed that populations in lakes, which are directly connected by a river or a stream, are very similar to each other. I also expected that populations, which are many kilometers apart from each other, would be genetically very different. However, I found that the majority of Gonyostomum cells do not disperse by rivers and streams, but get transported by other vectors that are independent of hydrological connections. Thus, hydrological connectivity does not influence the degree of differentiation between populations. Small distances between lakes slightly facilitate gene flow among populations, but on a larger scale geographic distance does not impact the population structure (Sassenhagen et al. 2015, Environmental Microbiology).

Evolutionary development of characteristic features, which are useful in a specific environment, is called local adaptation. It might provide a significant advantage for the local population in competition with invaders, prevent their establishment and drive differentiation. However, Gonyostomum populations are not adapted to the environmental conditions of their native lake. Instead, I showed that Gonyostomum cells can quickly adjust to all environmental conditions and establish in new habitats, which has probably facilitated the recent invasion (Sassenhagen et al. 2014, J Phycology; Sassenhagen et al. 2015, Hamful Algae; Sassenhagen et al. 2015, J Plankton Research). Nevertheless, one experiment showed that the succession of arrival in a habitat influences the later community composition. The first colonizers have a significant advantage over later invaders and dominate the habitat. This dominance might be due to higher cell number of the local population, resource exploitation or production of biochemical that suppress the growth of invaders (Sefbom et al. 2015, Biology Letters). This founder effect probably drives population differentiation in Gonyostomum.

 

Spatial and temporal genetic differentiation of an extensive algal bloom and its interdependence with the bacterial community

In the Baltic Sea, phytoplankton forms each spring an extensive bloom that propagates from coastal regions to the Baltic Proper. These blooms are dominated by the diatom species Skeletonema marinoi. It was often suggested that the blooms drift with the water currents and form one homogenous population. However, the bloom could also consist of different population that successively germinated from regional seed banks. To test both hypotheses, several researcher from the Scandinavian research network Prodiversa (http://prodiversa14.wix.com/prod) participated in four cruises from Helsinki (Finland) to Travemünde (Germany) to isolated Skeletonema cells from 9-10 stations along a 1132 km transect through the Baltic Sea. We analysed the population genetic structure and compared it with hydrographic measurements and oceanographic connectivity data. We found two significantly differentiated populations along the transect, which might experience limited gene flow due to  isolation by geographic distance and specific patterns of oceanographic connectivity. Additionally, environmental gradients like salinity and silica may select for specific populations (Godhe et al. 2016, Journal of Biogeography). 

The population genetic study was accompanied by an investigation of the bacterial community associated with the phytoplankton during two of the four cruises. We found that certain bacterial groups are strongly influenced by variables directly related to the phytoplankton bloom. Some taxa were strongly associated with phytoplankton biomass, diatom-dinoflagellate ratio and colored dissolved organic matter. Additionally, some bacterial groups were linked to specific populations of the diatom Skeletonema marinoi (Bunse et al. 2016, Frontiers in Microbiology). These findings indicate that intraspecific diversity in phytoplankton might differentially impact bacterial communities.

 

Taxonomy of filamentous cyanobacteria in the Darss-Zingst-Bodden chain

For my Master's thesis (German Diploma) I studied the taxonomy of filamentous cyanobacteria in an estuary of the southern Baltic Sea. This ecosystem is dominated the entire year by cyanobacteria. However, taxonomic identification of individual cyanobacteria was always a challenge due to very few morphological differences between the species and limited amount of cultures. Therefore, I established new cultures of both Nostocales and Oscillatorials and identified the species by sequencing of ribosomal genes (16S rRNA and ITS).