Doctor of Philosophy in Botany (PhD)
Microbial interactions with photosynthetic hosts
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.