Dylan Padilla

Metabolic theory posits that metabolism governs the rate at which organisms transform energy into biological work and growth. Thus, it constitutes the main mechanism driving the evolution of organismal growth and size across almost all domains of life. One general prediction of metabolic theory suggests that populations of larger organisms grow more slowly than populations of smaller organisms. However, increasing evidence show that prokaryotes seem to be the exception for such a trend. Larger prokaryotes appear to grow faster, challenging the standard theory and stimulating a further reevaluation of the current evidence. Here, I report a broad comparative analysis of the evolution of growth rate and cell size in prokaryotes. As opposed to previous investigations, my analysis relies on the concept of the thermal performance curve and the effects of its parameters on growth rate and cell size. Based on such approach, I found that prokaryotes evolved larger sizes at relatively high grow temperatures but their growth rates do not scale with size. At the optimum temperature for growth, the relationship between growth rate and cell size becomes unclear. These results call for a careful interpretation of the current evidence and highlight the importance of understanding the thermal sensitivity of the biochemical reactions that take place in the cells. Specifically, the metabolic reactions that regulate the protein synthesis in the cell, which are controlled by the translation machinery. In this regard, this study describes how different components of this machinery, such as the rRNA genes and the tRNA genes, interact to shape the evolution of growth rate and cell size across prokaryotes. Overall, I present more accurate results compared to previous evidence and suggest new hypotheses that can be applied to a wide range of taxa, paving the way for others to validate them at the intraspecific level.