Growth rate and yield are not constrained by a trade-off in microbes

Abstract

Ecological theory traditionally posits widespread trade-offs in organismal growth, survival, and reproduction. One core trade-off is the relation between maximum intrinsic growth rate (r) and carrying capacity (K), proposed as a key mechanism explaining species coexistence and the maintenance of biodiversity. To quantify how r relates to K in microbes, we analyzed more than 70,000 microbial growth curves in dozens of media conditions. In this set, we observed a strong positive correlation between r and K, challenging classic life-history expectations of a trade-off. This unexpected "trade-up" can be recovered by a model assuming a non-zero maintenance cost in cells. The model predicts a stronger trade-up under stress conditions that reduce growth rate, consistent with empirical data showing that salinity stress strengthens the trade-up. We also found that community assembly weakens but does not eliminate this positive r-K relationship. The robust growth-yield trade-up between species contrasts with patterns within species. Single-gene knockout datasets in Escherichia coli and budding yeast show no or only weak r-K relationships. Together, our results suggest that the lack of strong trade-offs between growth rate and carrying capacity may be widespread across species, emphasizing the importance of alternative trade-offs and coexistence mechanisms beyond the classical r-K framework.