Understanding the impact of warming on organisms, communities and ecosystems is a central problem in ecology. Although species responses to warming are well documented, our ability to scale up to predict community and ecosystem properties is limited. Improving understanding of the mechanisms that link patterns and processes over multiple levels of organisation and across spatial and temporal scales promises to enhance our ability to predict whether the biosphere will exacerbate, or mitigate, climate warming. In this thesis, I combine ideas from metabolic theory with a variety of experimental approaches to further our understanding of how warming will impact photosynthesis and respiration across scales. Firstly, I show how phytoplankton can rapidly evolve increased thermal tolerance by downregulating rates of respiration more than photosynthesis. This increased carbon-use efficiency meant that evolved populations allocated more fixed carbon to growth. I then explore how constraints on individual physiology and community size structure influence phytoplankton community metabolism. Using metabolic theory, I link community primary production and respiration to the size- and temperature- dependence of individual physiology and the distribution of abundance and body size. Finally, I show that selection on photosynthetic traits within and across taxa dampens the effects of temperature on ecosystem-level gross primary production in a set of geothermal streams. Across the thermal-gradient, autotrophs from cold streams had higher photosynthetic rates than autotrophs from warm streams. At the ecosystem-level, the temperature-dependence of gross primary productivity was similar to that of organism-level photosynthesis. However, this was due to covariance between biomass and stream temperature; after accounting for the effects of biomass, gross primary productivity was independent of temperature. Collectively, this work emphasises the importance of ecological, evolutionary and physiological mechanisms that shape how metabolism responds to warming over multiple levels of organisation. Incorporating both the direct and indirect effects of warming on metabolism into predictions of the biosphere to climate futures should be considered a priority
