A shift between r and K strategies in the life cycle of a unicellular microalga

r (high reproductivity but stress-sensitive offspring) and K (low reproductivity but high stress-resistant offspring) strategies are two classical life history theories. Contrary to the belief that one species only has a single strategy (i.e., either r or K), we present experimental evidence for the coexistence of both strategies in the isogenic microalga Haematococcus lacustris . Under standard conditions, swimming vegetative cells (SVC) normally grew and divided via binary fission (r strategy) and gradually shifted into non-swimming cysts (NSC) over time. Intriguingly, unlike the prevailing notion that NSC cannot propagate under stress, they were found to reproduce via multiple fission at a barely detectable growth rate, resulting in stress-resistant, non-mobile daughter cells and demonstrating a complete life history (K strategy). Collectively, our findings indicate that H. lacustris adopts both r and K strategies throughout its life history. This research enhances the understanding of the adaptation and survival of microbial populations in changing environments.