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

Traditional ecological theory suggests widespread trade-offs between organisms' growth, survival, and reproduction. Among these, the relationship between maximum intrinsic growth rate (r) and carrying capacity (K) is often highlighted as a crucial mechanism for species coexistence and biodiversity maintenance. To understand how r correlates with K in microbes, researchers analyzed over 70,000 microbial growth curves across numerous media conditions.

The results revealed a strong positive correlation between r and K, contradicting the conventional expectation of a trade-off in life histories. This surprising phenomenon, referred to as a "trade-up," can be explained by a model that assumes a non-zero maintenance cost for cells. The model predicts that this trade-up becomes more pronounced under stress conditions that lower growth rates.

Empirical data supports this, showing that salinity stress intensifies the trade-up. The researchers also discovered that community assembly weakens but does not eliminate the positive r-K relationship. Interestingly, the study found that while there is a robust growth-yield trade-up between species, there is little to no such relationship within species.

This is evident in single-gene knockout datasets from Escherichia coli and budding yeast. Overall, the findings suggest that the absence of a strong trade-off between growth rate and carrying capacity might be common across species, highlighting the significance of alternative trade-offs and coexistence mechanisms beyond the traditional r-K framework.