The Genetics and Epigenetics Governing the Transition from Plasticity to Adaptation During Long-term Growth at High CO2 in the Globally Distributed Marine Diazotroph Trichodesmium

Chair: Philip Munday

Nathan G. Walworth (1), Eric A. Webb (1), Fei-Xue Fu (1), Michael D. Lee (1), & David A. Hutchins (1)
1) University of Southern California, Los Angeles, California, 90089, USA

Background
Increasing anthropogenic carbon dioxide (CO 2) emissions have galvanized broad research attempts to assess both the rapid (plastic) and evolutionary (adaptive) responses to ocean acidification, particularly by biogeochemically-critical photosynthesizing and nitrogen-fixing microbes like Trichodesmium. A plastic response to an external stressor involves rapidly adjusting the current phenotype using standing genetic variation, while an adaptive response occurs through natural selection-mediated changes in genetic composition to alter the phenotype. Hence, the direction and magnitude of the plastic phenotype during an adaptive walk can heavily influence the resulting adaptive one. Additionally, studies assessing the impacts of DNA modifications (e.g. epigenetics) on environmental adaptation suggest populations can employ epigenetics to explore different plastic phenotypes prior to genetic mutation (adaptation). However, no marine microbial studies to date have assessed the genetic and/or epigenetic profiles associated with adaptive walks.

Methods
To address this, we investigated the global transcriptional and epigenetic underpinnings of a 7-year adaptive walk under CO2 selection in Trichodesmium as its phenotypic response transitioned from plastic to adaptive.

Findings
Both the plastic and adaptive phenotypes exhibited analogous increased growth and N 2 fixation rates in the selection environment (high CO2), but surprisingly, the high CO2-adapted cell lines also exhibited a 44% increase in the ancestral CO2 condition. These high CO2-adapted cell lines exhibited broad transcriptional changes in widespread metabolic pathways potentially mediated by transposition and upstream transcriptional regulators, while plastic transcriptional profiles retained a much narrower range of metabolic changes. Global epigenetic profiles showed differential methylation between the high and low CO2 conditions, implicating their influence in the adaptive walk.

Conclusions
Metabolic pathways affected by both levels of cellular regulation relative to the high-CO2 phenotypes will be discussed. By assessing subcellular processes underpinning adaptive walks to global change, high-resolution molecular diagnostics relating to evolutionary potential of important microbial functional groups can be determined.