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Abstract
Microbes affect many important ecological processes that operate at different scales, influencing everything from the health of individual macroorganisms and the interactions between competing species, to the global carbon cycle. However, many aspects of their spatial ecology are poorly understood, particularly how some environmental drivers can affect microbial communities at multiple spatial scales. This understanding is necessary to explain variation in natural microbial communities, and their effects on the functions of, and spatial patterning in, ecosystems.
One system in which microbes play an outsized role is coral reefs, which are experiencing increasing disturbances that stimulate the proliferation of algae. This dissertation investigates how algae can affect microbial communities on reefs, focusing on the responses of prokaryotic microbes in the microbiomes of corals, as well as several genera of toxic dinoflagellates (eukaryotic microalgae) that are associated with ciguatera poisoning in humans.
I investigated how local algal contact and regional (site-level) algal density affected coral microbiomes. Local and regional effects of algae interacted antagonistically to affect microbiome diversity. Coral microbiomes became more similar to macroalgal microbiomes with increasing regional macroalgal density and with algal contact.Because microbes mediate many of the negative effects that algae have on corals, I then used theoretical models to investigate how positive feedbacks between algae and microbes with harmful effects on corals can influence regime shifts—abrupt transitions in reefs from coral to algal-dominated states. My results illustrate that algae-associated microbes increase the likelihood of regime shifts in reefs, and that sufficiently strong single positive feedbacks, or multiple combined positive feedbacks, only produce algal-dominated reefs.
Other microbes that may contribute to regime shifts in reefs include toxic epiphytes that grow on macroalgae and reduce herbivory on it (e.g., ciguatoxic dinoflagellates). I found that local effects of algae (patch area and species composition) could not explain most of the heterogeneity in ciguatoxic dinoflagellate densities, suggesting that high regional algal densities might have affected dinoflagellates. Future research disentangling local and regional effects of algae on these dinoflagellates will help determine whether increases in algae in reefs could amplify ciguatera poisoning risk and facilitate regime shifts from coral- to algal-dominance.
One system in which microbes play an outsized role is coral reefs, which are experiencing increasing disturbances that stimulate the proliferation of algae. This dissertation investigates how algae can affect microbial communities on reefs, focusing on the responses of prokaryotic microbes in the microbiomes of corals, as well as several genera of toxic dinoflagellates (eukaryotic microalgae) that are associated with ciguatera poisoning in humans.
I investigated how local algal contact and regional (site-level) algal density affected coral microbiomes. Local and regional effects of algae interacted antagonistically to affect microbiome diversity. Coral microbiomes became more similar to macroalgal microbiomes with increasing regional macroalgal density and with algal contact.Because microbes mediate many of the negative effects that algae have on corals, I then used theoretical models to investigate how positive feedbacks between algae and microbes with harmful effects on corals can influence regime shifts—abrupt transitions in reefs from coral to algal-dominated states. My results illustrate that algae-associated microbes increase the likelihood of regime shifts in reefs, and that sufficiently strong single positive feedbacks, or multiple combined positive feedbacks, only produce algal-dominated reefs.
Other microbes that may contribute to regime shifts in reefs include toxic epiphytes that grow on macroalgae and reduce herbivory on it (e.g., ciguatoxic dinoflagellates). I found that local effects of algae (patch area and species composition) could not explain most of the heterogeneity in ciguatoxic dinoflagellate densities, suggesting that high regional algal densities might have affected dinoflagellates. Future research disentangling local and regional effects of algae on these dinoflagellates will help determine whether increases in algae in reefs could amplify ciguatera poisoning risk and facilitate regime shifts from coral- to algal-dominance.