About

Catherine Pfister is a Professor of Ecology and Evolution at The University of Chicago. Her research focuses on the dynamics of species in marine systems and their roles at ecosystem scales, with recurring themes including the implications of species interactions for community and ecosystem dynamics, and the contribution of climate change to species dynamics. Her interests are broadly grouped into four areas: the interplay between species and productivity in coastal marine ecosystems, the implications of ocean acidification in these ecosystems, the causes and consequences of variability in marine populations, and understanding the relative impacts of genetic and demographic factors on extinction risk. She trains PhD students interested in testing ecological and evolutionary concepts and pursuing discoveries in marine ecosystems, employing diverse methods such as mathematical models, genetics and -omics, biogeochemistry, and microscopy. Her work is connected with a broad network of faculty across multiple departments and the Marine Biological Laboratory at Woods Hole. Pfister holds a PhD in Zoology from the University of Washington Seattle, an MS in Marine Sciences from the University of North Carolina Chapel Hill, and a BS in Biology from the University of Illinois Champaign-Urbana.

Research topics

• Ecology
• Biology
• Genetics
• Geology
• Chemistry
• Environmental science
• Computational biology
• Oceanography
• Environmental chemistry
• Evolutionary biology

Selected publications

• <b>Genetic Consequences of Adaptive Traits across Heterogenous Environments in a Broadcast Spawning Species</b>

  Figshare · 2026-04-21

  datasetOpen access

  <b>ABSTRACT</b>In marine broadcast-spawning species, the interplay between selection and migration determines the genetic structure and adaptive potential of natural populations. Theory predicts that across spatially heterogeneous environments, strong selection can favor local adaptation over phenotypic plasticity, leading to the evolution of tightly linked genomic regions or structural variants with large effects on organismal fitness. In contrast, spatially homogeneous environments are expected to favor phenotypic plasticity and lead to polygenic trait architectures where many alleles have a small effect on fitness. To test these hypotheses, we compared genetic and phenotypic responses across <i>Mytilus californianus</i> populations distributed along 181 km spanning from the western end of the Strait of Juan de Fuca, Washington, USA to Cattle Point, San Juan Island, a strong environmental gradient. We combined whole-genome sequencing with ocean manipulation experiments to assess metabolic responses of individuals exposed to decreased pH, elevated temperature, and decreased pH combined with increased temperature conditions. Populations on the outer coast experience frequent low-pH conditions due to upwelling, while inner-coastal populations endure elevated sea surface and aerial exposure temperatures. Associated with these intense selective pressures, mussels from the inner coast of Washington exhibit increased structural genetic variation in heat shock proteins and decreased plasticity in increased temperature environments. Conversely, outer coastal populations, despite exhibiting reduced plasticity under low pH conditions, showed limited structural genetic variation in fitness-related proteins, likely due to their larger effective population sizes limiting adaptive structural variation. Notably, centrally located populations within the Strait of Juan de Fuca, which experience increased abiotic stress alongside higher migration rates, exhibited elevated plasticity across all treatments, greater structural variation, and higher genome-wide nucleotide diversity, suggesting that gene flow between outer and inner coastal populations, coupled with increased selection, enhances the adaptive potential for these populations. These findings support the hypothesis that strong selection across heterogenous environments favors local adaptation over plasticity in smaller populations who persist in stressful environments, even with high population connectivity. As ocean conditions continue to change, understanding the balance between local adaptation and phenotypic plasticity will be critical for predicting the resilience of marine species to environmental stressors.

  PublisherDOI

• Severity and certainty metric for kelp stressors: A tool for research, management, and conservation

  Ecological Indicators · 2026-05-15

  articleOpen access

  Foundation species face multiple threats, and many are in decline worldwide. Kelps are no exception, with declines documented throughout their range. Stressors such as warming, eutrophication, and reduced water quality—often linked to human activities—are well recognized, yet their relative severity and certainty have not been systematically evaluated. Such a framework may aid scientific, regulatory, and conservation organizations in prioritizing actions and decision making. Here we build on a conceptual model of stressors to kelp in the Salish Sea by rating stressor severity and scientific certainty. We convened a panel of regional kelp experts to perform the rating exercise for two canopy taxa, Nereocystis luetkeana and Macrocystis tenuifolia , as well as a grouping for understory species, including both the sporophyte (adult) and gametophyte (juvenile) life stages for these algae. Our process produced a rich dataset which can be separated by stressor, taxa, and life stage allowing for broad applications. The panel identified temperature, low nutrients, and substrate change/benthic sedimentation as the most severe stressors to kelp, with relatively high certainty for most taxa. Certainty scores were relatively high for Nereocystis and Macrocystis sporophytes but were considerably lower for understory species and the gametophyte life stage, for most stressors, highlighting research gaps. Our data set could be applied for management prioritization, setting ecological baselines, and spatial modeling exercises. We encourage scientific, regulatory, and conservation organizations to consider both our data and our process when evaluating their needs. As foundation species continue to face threats, exercises such as this can provide scientists and managers with actionable indices that support strategies to stem degradation. • An expert panel rated stressor severity and scientific certainty of kelp stressors. • Temperature, low nutrients and substrate change were identified as severe stressors. • Gametophyte life stages received low certainty scores, identifying research gaps. • Results have local and global applications to research, management and conservation.

  PublisherDOI

• <b>Genetic Consequences of Adaptive Traits across Heterogenous Environments in a Broadcast Spawning Species</b>

  Figshare · 2026-04-21

  datasetOpen access

  <b>ABSTRACT</b>In marine broadcast-spawning species, the interplay between selection and migration determines the genetic structure and adaptive potential of natural populations. Theory predicts that across spatially heterogeneous environments, strong selection can favor local adaptation over phenotypic plasticity, leading to the evolution of tightly linked genomic regions or structural variants with large effects on organismal fitness. In contrast, spatially homogeneous environments are expected to favor phenotypic plasticity and lead to polygenic trait architectures where many alleles have a small effect on fitness. To test these hypotheses, we compared genetic and phenotypic responses across <i>Mytilus californianus</i> populations distributed along 181 km spanning from the western end of the Strait of Juan de Fuca, Washington, USA to Cattle Point, San Juan Island, a strong environmental gradient. We combined whole-genome sequencing with ocean manipulation experiments to assess metabolic responses of individuals exposed to decreased pH, elevated temperature, and decreased pH combined with increased temperature conditions. Populations on the outer coast experience frequent low-pH conditions due to upwelling, while inner-coastal populations endure elevated sea surface and aerial exposure temperatures. Associated with these intense selective pressures, mussels from the inner coast of Washington exhibit increased structural genetic variation in heat shock proteins and decreased plasticity in increased temperature environments. Conversely, outer coastal populations, despite exhibiting reduced plasticity under low pH conditions, showed limited structural genetic variation in fitness-related proteins, likely due to their larger effective population sizes limiting adaptive structural variation. Notably, centrally located populations within the Strait of Juan de Fuca, which experience increased abiotic stress alongside higher migration rates, exhibited elevated plasticity across all treatments, greater structural variation, and higher genome-wide nucleotide diversity, suggesting that gene flow between outer and inner coastal populations, coupled with increased selection, enhances the adaptive potential for these populations. These findings support the hypothesis that strong selection across heterogenous environments favors local adaptation over plasticity in smaller populations who persist in stressful environments, even with high population connectivity. As ocean conditions continue to change, understanding the balance between local adaptation and phenotypic plasticity will be critical for predicting the resilience of marine species to environmental stressors.

  PublisherDOI

• Foundational kelp species reveal links between host traits, the environment and the associated microbial community

  Royal Society Open Science · 2025-10-01 · 1 citations

  articleOpen access1st authorCorresponding

  Canopy kelp are foundational species in coastal ecosystems and host diverse bacterial communities. Here, we test the association between bull kelp ( Nereocystis luetkeana ) host traits, blade-associated bacterial taxa and seawater environmental features across nine sites spanning more than 200 km in Washington state. Traits related to kelp fitness, environmental features and microbial community structure differed geographically. Kelp carbon fixation and tissue nitrogen content were greater at outer coast locales, compared with more inland locales in central and south Puget Sound. Geographic differences in carbon fixation rates, tissue nitrogen and bulb diameter were positively correlated with seawater nutrients and negatively correlated with sea surface temperature. Bacterial taxa showed differentiation among sites, and blade-associated bacterial densities were higher at the outer coast site compared with the most inland site. Yet, 11 bacterial genera were present in at least 80% of the samples; these taxa probably serve as core members of the N. luetkeana microbiome and show both positive and negative correlations with host health and environmental features. We show that there are strong interrelationships between kelp traits, seawater features and bacterial community composition with implications for the health of this highly productive foundational species in coastal ecosystems.

  PublisherDOI

• Evolutionary history and association with seaweeds shape the genomes and metabolisms of marine bacteria

  mSphere · 2025-06-02 · 5 citations

  articleOpen access1st authorCorresponding

  ABSTRACT Seaweeds harbor a rich diversity of bacteria, providing them with metabolic resources and a surface for attachment and biofilm development. The host’s unique environment potentially shapes the bacterial genomes and promotes adaptations for a symbiotic lifestyle. To investigate whether the genomes of seaweed-associated bacteria are genetically and metabolically distinct from their close free-living relatives in seawater, we compared both the seaweed-associated and free-living counterparts of 72 bacterial genera across 16 seaweed hosts using whole-genome sequences or high-quality metagenome-assembled genomes. While taxonomic affiliation strongly influenced genome characteristics such as GC content, gene number, and size, host association had a lower effect overall. A reduced genome size was suggested only in Nereocystis luetkeana -associated microbes, while only Ascophyllum nodosum -associated bacteria had an increased GC content. Metabolic adaptations were indicated from the genomes of seaweed-associated bacteria, including enriched pathways for B vitamin production, complex carbohydrate utilization, and amino acid biosynthesis. In particular, Flavobacteriia showed the most pronounced differences between host-associated and free-living strains. We further hypothesized that bacteria associated with seaweed might have evolved to complement their host’s metabolism and tested this inference by analyzing the genomes of both the seaweed Ectocarpus subulatus and its 28 bacterial associates but found no evidence for such complementarity. Our analyses of 72 paired bacterial genomes highlighted significant metabolic differences in seaweed-associated strains with implications for carbon, nitrogen, and sulfur cycling in the coastal ocean. IMPORTANCE We hypothesized that the unique environment of seaweeds in coastal oceans shapes bacterial genomes and promotes a symbiotic lifestyle. We compared the genomes of bacteria isolated from seaweed with bacteria from the same genus found free-living in seawater. For genome features that included the number of genes, the size of the genome, and the GC content, taxonomy was of greater importance than bacterial lifestyle. When we compared metabolic abilities, we again found a strong effect of taxonomy in determining metabolism. Although several metabolic pathways differed between free-living and host-associated bacteria, this was especially prominent for Flavobacteriia in the phylum Bacteroidota . Notably, bacteria living on seaweeds had an increased occurrence of genes for B vitamin synthesis, complex carbohydrate use, and nitrogen uptake, indicating that bacterial genomes reflect both their evolutionary history and the current environment they inhabit.

  PublisherDOI

• Author response for "Foundational kelp species reveal links between host traits, the environment and the associated microbial community"

  2025-08-06

  peer-review1st authorCorresponding
  PublisherDOI

• Warming Seawater Temperature and Nutrient Depletion Alters Microbial Community Composition on a Foundational Canopy Kelp Species

  Environmental Microbiology · 2025-03-01 · 9 citations

  articleOpen accessSenior authorCorresponding

  Warming seawater temperatures and low dissolved inorganic nitrogen (DIN) levels are environmental stressors that affect the health and abundance of marine macroalgae and their microbiomes. Nereocystis luetkeana, a canopy-forming species of brown algae that forms critical habitat along the Pacific coast, has declined in regions impacted by these synergistic stressors. Little is known about how these environmental factors affect the microbiome of N. luetkeana, which could affect nutrient availability, vitamin production, and stress response for the host. We experimentally tested the interactive effects of three seawater temperatures (13°C, 16°C, 21°C) crossed with abundant and replete DIN levels on the diversity and composition of blade-associated microbiomes from two spatially separated kelp host populations. We hypothesised that kelp microbiomes exposed to high temperatures and low DIN would experience the lowest diversity. Contrary to our hypothesis, the highest temperature treatment resulted in the largest increase in microbial diversity, and microbiomes in all temperature treatments experienced a decrease in previously dominant taxa. Temperature had a larger effect than DIN on the kelp microbiome in all cases. The disruption to the kelp microbiome across all temperatures, especially at the highest temperature, suggests that the effects of warming on N. luetkeana extend to the microbiome.

  PublisherOA PDFDOI

• Spatially Varying Selection Amplifies Intrapopulation Differentiation Among Phenotypic Traits in the Rocky‐Shore Mussel, <i>Mytilus californianus</i>

  Ecology and Evolution · 2025-06-30 · 1 citations

  articleOpen accessSenior author

  ABSTRACT Strong ecological gradients along heterogeneous environments play an important role in shaping population differentiation across species ranges. Thus, the selective pressure of environmental variation on phenotypic variation strongly affects an organism's ability to persist across diverse or new environments. We investigated the spatial variability of biological responses in the intertidal bivalve Mytilus californianus to highlight the costs and trade‐offs of local adaptation and phenotypic plasticity across various functional traits in a dynamic environment, the marine intertidal. To test this, we performed a reciprocal transplant experiment with M. californianus individuals originating from the upper and lower intertidal measuring relevant phenotypic traits, followed by whole genome sequencing (WGS). We determined that morphological traits in individuals demonstrated phenotypic plasticity when moved to new environments, whereas physiological traits such as metabolism exhibited constraints in plasticity. Additionally, mussels from high intertidal zones, which experience greater heat and aerial exposure stress, maintained lower metabolic rates and showed increased frequencies of non‐synonymous mutations in functionally relevant heat shock proteins when compared to low intertidal mussels. These results suggest that morphological and physiological traits responded differently to spatially varying selection within the marine intertidal.

  PublisherOA PDFDOI

• Bacteria on the foundational kelp in kelp forest ecosystems: Insights from culturing, whole genome sequencing and metabolic assays

  Environmental Microbiology Reports · 2024-05-22 · 11 citations

  articleOpen accessSenior author

  In coastal marine ecosystems, kelp forests serve as a vital habitat for numerous species and significantly influence local nutrient cycles. Bull kelp, or Nereocystis luetkeana, is a foundational species in the iconic kelp forests of the northeast Pacific Ocean and harbours a complex microbial community with potential implications for kelp health. Here, we report the isolation and functional characterisation of 16 Nereocystis-associated bacterial species, comprising 13 Gammaproteobacteria, 2 Flavobacteriia and 1 Actinomycetia. Genome analyses of these isolates highlight metabolisms potentially beneficial to the host, such as B vitamin synthesis and nitrogen retention. Assays revealed that kelp-associated bacteria thrive on amino acids found in high concentrations in the ocean and in the kelp (glutamine and asparagine), generating ammonium that may facilitate host nitrogen acquisition. Multiple isolates have genes indicative of interactions with key elemental cycles in the ocean, including carbon, nitrogen and sulphur. We thus report a collection of kelp-associated microbial isolates that provide functional insight for the future study of kelp-microbe interactions.

  PublisherOA PDFDOI

• Ammonification by kelp associated microbes increases ammonium availability

  PLoS ONE · 2024-03-29 · 13 citations

  articleOpen accessSenior authorCorresponding

  Microbes contribute biologically available nitrogen to the ocean by fixing nitrogen gas from the atmosphere and by mineralizing organic nitrogen into bioavailable dissolved inorganic nitrogen (DIN). Although the large concentration of plants and algae in marine coastal environments provides ample habitat and reliable resources for microbial communities, the role of the microbiome in host-microbe nitrogen cycling remains poorly understood. We tested whether ammonification by epiphytic microbes increased water column ammonium and improved host access to nitrogen resources by converting organic nitrogen into inorganic nitrogen that is available for assimilation by hosts. When bull kelp (Nereocystis luetkeana) in the northeast Pacific was incubated with 15N labelled amino acid tracers, there was accumulation of 15N in kelp tissue, as well as accumulation of 15NH4 in seawater, all consistent with the conversion of dissolved organic nitrogen to ammonium. Metagenomic analysis of surface microbes from two populations of Nereocystis indicated relative similarity in the percentage of genes related to ammonification between the two locations, though the stressed kelp population that had lower tissue nitrogen and a sparser microbiome had greater ammonification rates. Microbial communities on coastal macrophytes may contribute to the nitrogen requirements of their hosts through metabolisms that make ammonium available.

  PublisherOA PDFDOI

Recent grants

• Networking Microbiome Research: A Symposium for a Microbiome Center Consortium

  NSF · $39k · 2020–2022

• Variability and the Population Biology of Marine Organisms

  NSF · $295k · 1998–2002

• Collaborative Research: The Role of Regenerated Nitrogen for Rocky Shore Productivity

  NSF · $283k · 2009–2013

Frequent coauthors

• J. Timothy Wootton

  University of Chicago

  30 shared

• Jean‐Pierre Gattuso

  Sorbonne Université

  19 shared

• Brooke L. Weigel

  Western Washington University

  16 shared

• Mark A. Altabet

  University of Massachusetts Dartmouth

  15 shared

• Sophie J. McCoy

  University of North Carolina at Chapel Hill

  12 shared

• Robert T. Paine

  11 shared

• Jack A. Gilbert

  University of California, San Diego

  10 shared

• Mary E. Power

  University of California, Berkeley

  9 shared

Labs

• Pfister LabPI

Education

• Ph.D., Zoology

  University of Washington

  1993

• M.S., Marine Sciences

  University of North Carolina

  1987

• B.S., Biology

  University of Illinois

  1984

Awards & honors

• Frances Straus Mentorship Award (2018)
• Miller Postdoctoral Fellow at University of California at Be…