A genome-wide CRISPR screen reveals how diatoms thrive in dynamic light

Summary Diatoms are a highly diverse group of phytoplankton that have a large impact on global primary production and carbon sequestration in the ocean 1,2 . However, they are evolutionarily divergent from model phototrophs of the green lineage, and limited screening tools have hampered discovery of unique diatom biology. To address this challenge, we developed a genome-wide CRISPR/Cas9 screen in the model marine diatom, Phaeodactylum tricornutum. The screen was applied to identify genes required for survival in different light regimes, including both high light and fluctuating light. We identified a broad set of uncharacterized genes, providing the foundation for mechanistic studies of diatom adaptation to dynamic light. Among these genes, we demonstrated that the red lineage-exclusive gene STROBE1 is a new potentiator of cyclic electron flow (CEF) required for CEF to generate a trans-thylakoid proton gradient 3,4 . As dynamic light conditions are common in marine environments, STROBE1 and other genes identified in this screen may contribute to the broad ecological success of diatoms 5,6 . This genome-wide genetic screen in P. tricornutum will accelerate the unbiased discovery of novel gene functions in these ecologically important organisms.