The Earth has entered a time of rapid, unprecedented environmental change, including increases in global mean temperature of roughly 0.2 degrees Celsius per decade over the last 40 years with a projected further increase of up to 5 degrees by 2100. This warming is expected to continue to alter the function of biological systems because living organisms are constrained by their thermal environments. For example, intracellular enzymes function more rapidly at warmer temperatures, leading to increases in core metabolic reactions. This proposal tests the effects of temperature on marine microbial mixotrophs, organisms that combine two fundamental forms of metabolism--photosynthesis and phagotrophic heterotrophy--within one cell. Existing evidence suggests that mixotrophs should become more heterotrophic with rising temperatures because rates of aerobic respiration increase more rapidly than photosynthesis per degree of warming. However, as small cells with rapid generation times and large populations, mixotrophs are likely to experience evolutionary changes in response to altered environmental conditions. Here, we describe a plan to experimentally evolve members of a globally distributed mixotroph genus by exposing them to a gradient of temperatures. We will also manipulate the availability of light and food in a fully factorial design to impose selection pressures on phototrophy and heterotrophy, respectively. We will then use our experimental results to parameterize a global upper ocean biogeochemistry model in order to quantify the impact of mixotroph evolution on future nutrient and carbon cycling.
Our proposed work unites two areas of growing interest: experimental evolution of marine microbes, and the study of mixotrophs. In recent years, mixotrophs have gained renewed attention as omnipresent members of marine ecosystems that may regulate nutrient recycling and carbon export. These organisms are also noteworthy for their dominance in oligotrophic waters, which are predicted to expand with increasing oceanic stratification. Thus, mixotrophs may mediate an important potential climate feedback loop: Increased rates of heterotrophy under warmer future ocean conditions could accelerate warming by reducing carbon uptake and export via the biological pump. This proposal combines empirical and theoretical approaches to assess the impacts mixotroph evolution will have on this climate feedback. The evolution experiment quantifies the extent to which adaptive changes to mixotroph physiology can ameliorate the effects of warming by focusing on changes to the biogeochemically relevant traits of: relative phototrophy, cell size, and cellular stoichiometry. The modeling work introduces mixotrophy for the first time into the COBALT upper ocean biogeochemistry model, allowing extension of laboratory results to the global scale. Collectively, our approach works toward a predictive understanding of how mixotrophs in future oceans will alter the planet's carbon cycle.
The proposed work has implications beyond biological oceanography to the fields of climate science, microbial ecology, and evolutionary biology. In addition, this research provides training opportunities for undergraduates (recruited through UCSB's California Alliance for Minority Participation and McNair Scholars programs to engage first-generation and underrepresented minority students), a doctoral student, and a postdoctoral investigator. This proposal also funds a continued collaboration between the Moeller Lab and the Santa Barbara Museum of Natural History's Sea Center, a non-profit educational center with ~100,000 visitors per year. Through this collaboration, we will (1) develop a "Meet the Plankton" science cart for permanent use by Sea Center interpreters; (2) develop, launch, and evaluate a Portal to the Planet program on "Climate Feedback Loops," to be shared with other regional zoos and aquariums; and (3) participate in the annual "Underwater Parks Day" open house. In addition, researchers supported by this project will engage in other outreach activities, including participation in the National Network of Ocean and Climate Change Interpreters regional climate science training for zoo and aquarium educators (Moeller) and public lectures such as the Santa Barbara Museum of Natural History's Science Pub Night (postdoctoral investigator).