Award: OCE-1638804

Phytoplankton are plant-like microorganisms that support nearly all of the life in the ocean, and remove large amounts of carbon dioxide from the air through their photosynthesis. It is therefore vitally important to understand how these tiny but essential primary producers will respond to future climate changes. This project investigated how key groups of phytoplankton may evolve in response to warmer temperatures, and how this adaptation process could be affected by expected future changes in the availability of required nutrients like nitrogen and phosphorus. We worked to understand the diverse ways that various species of phytoplankton can respond to environmental shifts in both temperature and nutrients together, conditions that are expected to be widespread in the future ocean. The project particularly focused on studying the variety of responses to climate change that can be found within individual species of phytoplankton, since these 'intra-specific' differences are potentially very important, yet are hard to detect using our current DNA-based methods of measuring biological diversity. The overall aim of this project was to uncover this hidden temperature-and nutrient-related diversity within phytoplankton species, and to predict how it will shape their responses to a future changing ocean Much of the field work for this project was carried out in Narragansett Bay (NB), Rhode Island. This is an ideal environment to study the sensitivity of phytoplankton to climate change, due the large seasonal temperature range in this temperate estuary, as well as the availability of a decades-long survey by local scientists studying the phytoplankton community in NB that provides us with needed long-term history and background information for our experiments. We collected whole mixed-species phytoplankton communities from NB in different seasons, and grew them in the laboratory under controlled combinations of temperature and nutrients. At the end of these incubation experiments, we recorded how the species composition changed, as well as measuring important properties like growth and photosynthesis rates. In some cases we obtained single-species phytoplankton cultures from these incubation experiments that were then used for further in-depth studies in the laboratory. Our results showed that the effects of temperature increases on the phytoplankton community were very different, depending on the amount of nutrients present. Warming had positive effects on NB phytoplankton growth at high nutrient levels, but at low, growth-limiting nutrient concentrations higher temperatures instead magnified these negative effects. We also saw that rising temperatures led to large shifts in the species composition of the community, an outcome that could change how the food chain works in the future ocean. Experiments with single-species phytoplankton cultures that we isolated from the NB incubation experiments allowed us to use a different set of scientific tools to delve more deeply into the diversity of intra-specific climate change responses. We found that populations of NB cyanobacteria contained individuals that were clearly better adapted to either warm or cool temperatures. Despite having very different responses to temperature, these warm and cool strains were virtually identical in every other way. Traditional molecular biology methods were unable to find any differences in the genetics (DNA sequences) of these two strains. We finally solved the puzzle of this "hidden" temperature diversity by showing that minor, reversible modifications to the DNA (epigenetics) were responsible for their very distinct temperature responses. It therefore seems that scientists may need to measure and consider these epigenetic mechanisms in order to accurately predict how phytoplankton populations will be affected by a warmer ocean. Overall, this project increased our understanding of how temperature and nutrients interact together to determine phytoplankton growth. It seems likely that in the future ocean, nutrient limitation will modify phytoplankton responses to temperature, making many species more vulnerable to warming. This will have large implications for marine food webs and the ocean carbon cycle. This project at USC fully supported two Ph.D. students, and one Ph.D. student was partially supported. Three undergraduate students received research experience and training. Results from this project were also presented at a science course in a public high school in Los Angeles with an underrepresented minority-dominated student body, and in graduate courses taught at USC. Our research results were presented to the scientific community through 22 published journal papers, and in a number of presentations at professional meetings. Last Modified: 01/11/2023 Submitted by: David A Hutchins