The coexistence of r and K strategy in a unicellular microalga Haematococcus lacustris.

Understanding how organisms balance growth and defence is a general goal in biology. r (high reproductivity but stress-sensitive offspring) and K (low reproductivity but high stress-resistant offspring) selection are two classical life history strategy theories. Contrary to the common belief that an organism can only be classified as either r or K strategist, by integrating physiological, microscopic and transcriptomic data, we present experimental evidence for the coexistence of both strategies in the unicellular microalga Haematococcus lacustris. Under standard conditions, swimming vegetative cells (SVCs) normally grow and reproduce via binary fission (r strategy) and gradually transform into non-swimming cells (NSCs) over time. Intriguingly, unlike the prevailing notion that NSCs cannot propagate, they are found to reproduce via multiple fission at a barely detectable growth rate, resulting in stress-resistant, non_mobile daughter cells, demonstrating a complete life history (thus are defined as K strategy). Collectively, our findings suggest that H. lacustris adopts both r and K strategies, enhancing the understanding of the adaptation and survival of microbial populations in challenging environments.