Doctor of Philosophy in Botany (PhD)
Microbial interactions with photosynthetic hosts
Dissertations completed in 2010 or later are listed below. Please note that there is a 6-12 month delay to add the latest dissertations.
Eelgrass (Zostera marina) is a coastal marine angiosperm found in the Northern Hemisphere and forms expansive meadows in sheltered estuaries and bays, providing habitat for a high diversity of organisms, including algae, invertebrates, microbes, fish, birds, and mammals. It also provides invaluable ecosystem services for humans, including coastal storm protection, carbon sequestration, and nutrient cycling. However, eelgrass meadows are threatened worldwide by numerous anthropogenic activities, including shoreline modification, overfishing, land-based nutrient loading, and climate change. In British Columbia, Canada, eelgrass ecosystems are vulnerable to human activities such as habitat destruction, fishing, and pollution, and warming waters, which can cumulatively lead to negative impacts on eelgrass and associated fauna. To understand how environmental variation and BC-specific human activities affect eelgrass ecosystems temporally and spatially, I used a three-year, monthly observational time series of two trophic groups, statistical modeling, and an experiment to study how eelgrass communities and species interactions vary at the micro and macro scales. I show that eelgrass and epiphytic algae biomass change seasonally, and the timing of their peak biomasses varies interannually and is driven by nutrient availability and mesograzer abundances. I also show that human activities have a negative effect on eelgrass biomass, epiphytic algae, and epifaunal invertebrate abundances and species richness. Lastly, I show the first experimental evidence that eelgrass leaves host a core microbiota, eelgrass leaf-associated microbial communities are primarily driven by their surrounding environment, yet there may be host influence because microbial communities are resistant to environmental change within short periods of time. This research provides new insight to how environmental variability and human activities shape eelgrass-associated epifauna communities. This information can be used to predict how eelgrass ecosystems may change over time, which can help inform management decisions for restoration and conservation practices. Implementing more regulations for coastal anthropogenic activities may help ensure that eelgrass, and the organisms and humans that depend on it, will be sustained well into the future.
The microbiome of marine organisms contributes to host health and ecosystem processes like nutrient cycling. Despite the importance to healthy ecosystems, little is known about the dynamics and drivers of microbiome variation on marine hosts. In this dissertation, I use longitudinal studies and field-based transplant experiments to assess the factors associated with microbiome change on four coastal foundation species. In the intertidal macroalga Fucus distichus, I characterize temporal dynamics and microbiome variation on host individuals. I show seasonal turnover is a highly significant predictor of microbiome change. Local environmental conditions and host developmental stage also explain some microbiome variation. I test microbiome fidelity to geographically and phenotypically differentiated F. distichus by exposing hosts to new abiotic conditions and microbial source pools. No immediate shifts in microbiome composition occur in five-day transplant experiments, suggesting the established microbiome is buffered against short-term environmental change. I test whether host filtering modulates the shell microbiome of the mussel, Mytilus californianus, and find the microbiome is not specific to living mussels. Instead, it is associated with abiotic conditions that vary across geographic locations and elevation in the intertidal zone. In cultivated kelps, I test if outplanting kelp from controlled hatcheries to open ocean sites alters the microbiome and if host and abiotic factors are correlated with microbiome variation at cultivation sites. Host-species specificity was evident throughout the cultivation process and outplanting is followed by high microbiome turnover. Microbiome variation is more strongly correlated to season than abiotic differences between cultivation sites. Altogether, my findings suggest abiotic factors and host identity influence selective microbiome assembly on coastal foundation species. Seasonal microbiome turnover occurs in multiple hosts and coastal habitats, indicating microbes associated with the prevailing conditions may commonly replace existing members of the microbiome over weeks to months. Within host species, local abiotic conditions and host physiological state are correlated to microbiome variation. This research broadens our understanding of the tempo of microbiome turnover and factors predicting microbial community variation in marine foundation species. It provides necessary foundational knowledge for a holistic understanding of host and ecosystem response to changing oceans.
Animals live with a symbiotic community of bacteria, and some of these bacteria affect host biology. Generally, these bacterial communities are diverse and complex. A key question in microbiome research is whether bacteria are specific to a host and affect host biology. In my dissertation, I use observational and experimental approaches to 1) understand the factors that contribute to composition of host-microbiota, 2) determine the core bacteria of a host by comparing the abundance of host bacteria to the bacteria in environmental bacterial communities, 3) determine the phylogenetic and environmental distribution of core bacteria and their close relatives to better understand their natural history, and 4) test whether core bacteria are correlated with aspects of host biology. Across three host species, I found bacterial communities that were populated by a few common and prevalent core bacteria that are absent from the environment; in most cases, these core bacteria belong to host-associated clades of bacteria. In chapter 2, I characterize the microbiota of the keystone sea star Pisaster ochraceus, identify core bacteria, and use phylogenetic trees to assess the distribution of their relatives. In chapter 3, I study the interactions between the skin microbiota and host innate immunity of Columbia spotted frogs and to determine whether they explain the occurrence of the amphibian pathogen, Batrachochytrium dendrobatidis (Bd). In chapter 4, I characterize the microbiota of wild and captive endangered Oregon spotted frogs (OSF) and whether their skin bacteria are associated with Bd intensity. Lastly, I experimentally test the hypothesis environmental reservoirs of bacteria influence the bacteria on the skins of frogs. Understanding the factors that structure captive communities has substantial conservation implications for captive breeding and head start programs. My results differ from previous studies by comparing the host to their environment and focusing on specific core bacteria.
Theses completed in 2010 or later are listed below. Please note that there is a 6-12 month delay to add the latest theses.
Community dynamics and structure are greatly affected by climate change through warming. Temperature directly affects the rates of biochemical reactions that in return affect growth, resource use, organismal abundance, species interactions, and, therefore, communities and biodiversity. In addition, low connectivity can limit dispersal between communities, reducing the potential for demographic rescue effects. Therefore, the effects of temperature on diversity and community structure in patchy landscapes can depend on the degree of connectivity among landscapes. We tested whether the effects of temperature on communities contingent on the degree of connectivity using experimental pond metacommunities, each comprised of four-1000L mesocosms spanning a 4.5°C spatial temperature gradient and connected by one of three dispersal rates. This spatial temperature gradient was maintained, while also allowing the mesocosm temperatures to fluctuate temporally with seasonal weather variation. Bacterial communities in the mesocosms were sampled in the summer to evaluate whether dispersal rate at the metacommunity level affects local and regional community response to seasonal fluctuations in temperature. We predicted that higher levels of dispersal would raise local (alpha) diversity and decrease species turnover among ecosystems (beta diversity) and metacommunity-level (gamma) diversity. However, we found no effect of dispersal on local and regional diversity metrics. We also predicted that dispersal rates would differently affect species compositional differences along the thermal gradient. At low dispersal rates among communities, we observed differences in species composition associated with temperature. At higher dispersal rates, communities were not structured by temperature and composition was similar within a metacommunity, which was not observed in any other dispersal treatment. This emphasizes the homogenizing effect high dispersal has on bacterial community structure. Our findings demonstrate that bacterial diversity metrics do not follow metacommunity predictions about dispersal effects on diversity. However, we found support for the hypothesis of high dispersal homogenizing communities. This suggests there are other processes that influence bacterial community diversity patterns, but dispersal can erode the effect of the environment on bacterial community structure.
Macroalgae (seaweeds) have an intimate relationship with their microbial symbionts. Microbial communities associated with macroalgal surfaces (epibiota) are generally host-specific and, historically, there has been great interest in the role of biological compounds and chemical warfare in microbial community assembly on seaweeds. However, the interaction between seaweeds and their environment may also influence community assembly of their microbiota. In my thesis, I conduct two experiments that ask how factors not related to seaweed chemistry influence microbial community assembly. First, I ask whether the interaction between flow and seaweed morphology affects the settlement and structure of microbial biofilms. In this project, I test whether three common algal morphologies select for differential biofilm communities using artificial macroalgae units (AM units) made out of latex. I find that morphology does affect initial microbial settlement and community structure, but that eventual dominance of substrate specialists (in our case a latex degrader) swamps the influence of morphology in long-term biofilms. The second chapter of my thesis asks whether macroalgae affect the microbial epibiota of each other. To test this, I co-incubate Nereocystis leutkeana meristem fragments with different species of mature macroalgae. I find that although water column communities change significantly when incubated with mature macroalgae, seaweed surface communities are far more resistant to change. Overall, these results support the idea that the seaweed surfaces are highly selective, and demonstrate that modulations on seaweed microbiota operate within an overarching paradigm of species specificity. With these experiments, I hope to contribute to the larger body of knowledge on seaweed-microbe associations and improve understanding of how, and why, we find the observed microbiota on seaweed surfaces.
A fundamental goal of microbial ecology is to understand how microbial diversity is distributed over space and time. At broad scales, abiotic factors drive microbial community composition, and salinity plays a particularly important role. Do all microbes respond in the same way? Here I investigate the diversity of two domains encompassed within the blanket term ‘microbes’ the bacteria and protists (unicellular eukaryotes). I examined their diversity and composition across a river to ocean transect and compare the shifts in diversity that occur in these two domains. I collected 122 samples from the Fraser River, its plume and the adjacent coastal water at three time points covering the spring freshet. I used a combined approach of high-throughput sequencing and inverted microscopy to investigate the changes in diversity and community composition. As expected, there was a strong turnover in the community composition of bacteria and protists across the salinity gradient. I found that diversity of both the bacteria and protists was highest in the freshwater and brackish water with low salinity, and decrease with increasing salinity, and observed a high degree of separation between marine and freshwater community composition. However, the shift from communities with predominately fresh to marine water taxa occurs at the higher taxonomic level for bacteria (phylum/class) and lower taxonomic level (family/genus) for protists. Moreover, the inflection point for taxonomic distribution tends to occur around at 20 salinity for bacteria and 10 salinity for protists. Overall, bacteria are distributed more broadly across the salinity gradient than protists and more likely to be present across time points. In contrast, protists tend to be found in a narrower salinity range and are more often restricted to either marine or freshwater, and found at a single time point.