UC San Diego

In the pelagic environment, microbes act as the base of the food web (photosynthetic autotrophs), recycle nutrients (microbial loop), and perform other crucial ecosystem processes and services (such as carbon sequestration). The relative scale of these different process is driven by changes in marine microbiome community structure, diversity, and function. Over the last two decades, meta-omic sampling has provided a pathway forward with which to observe the community structure and function of the marine microbiome at a previously inaccessible resolution. However, with this increase in data complexity (large numbers of identified species and genes), it can be challenging to synthesize results across the multitude of observed taxonomic and functional groups. The goal of this thesis is to provide a general framework for understanding marine microbiome community responses (structure, diversity, and function) to environmental perturbations at previously unresolvable scales. The first study (Chapter 2) identifies the mechanisms that shape patterns in marine microbiome community structure and diversity across space and time within a coastal upwelling region. While traditional methods (such as microscopy and flow cytometry) have highlighted general patterns for broad taxonomic groups and or conspicuous taxa, this study represents a comprehensive examination of the mechanisms that shape all types of marine microbial groups, and in particular, highlights cryptic groups that could not be identified through more traditional means. The second study (Chapter 3) takes a more species-centric approach and asks, what is the rate of habitat specificity within marine microbes. Terrestrial systems often contain many species that are endemic to habitats or locales. Within the marine environment, habitats are constantly in motion, moving dynamically across space in time. The dynamic marine environment, coupled with the fast generation times of most microbes is thought by many to lead to less habitat specificity and more cosmopolitan (universally distributed) species. By identifying water mases (with internally consistent physical and chemical environments) we present a view of habitat specificity within the marine microbiome in a way that is comparable to terrestrial studies. The third study (Chapter 4) shifts to look at regional metatranscriptomic data and asks what are the mechanisms that shape the function and distribution of active marine microbes. Metatranscriptomics provides a framework to identify which taxa and their associated functions are active within a community in response to changing environmental conditions. In targeting the active community, we identify how environmental conditions can lead to in-situ functional traits within the microbial community—a crucial next step to better understanding the links between environmental conditions and the local to global magnitude of key ecological functions such as primary productivity, nutrient recycling, and carbon sequestration in the pelagic ocean.