Expanding the upper-temperature boundary for the microalga Picochlorum sp. (BPE23) by adaptive laboratory evolution.

Closed photobioreactors reach temperatures that reduce microalgal production or even cause culture collapses. Cooling can maintain the temperature within tolerable boundaries, but cooling is energy-intensive and expensive. Thermotolerant microalgal strains can reduce dependence on such cooling. In this study, adaptive laboratory evolution was performed for 390 days to further increase the maximal tolerable temperature for the already thermotolerant microalgae Picochlorum sp. (BPE23). The parental wild-type strain of Picochlorum sp. (BPE23) exhibited a maximum mid-day growth temperature of 47.5°C, whereas the isolated clones grew up to 49°C. At a lower temperature of 40°C, the growth rate and absorption cross-sectional area were similar for the wild-type strain and the evolved clones. Interestingly, the clones showed a 46% increase in cell volume compared to the wild-type strain. The evolved clones with an expanded upper-temperature boundary can be applied for broader temperature control of 1.5°C, without trade-off effects at lower temperatures.