Multiomics dissection of the CO ₂ -dependent fast growth of Picochlorum celeri

With a sub-3-µm cell and a 27-Mbp diploid genome, the small but mighty Picochlorum celeri doubles within three hours and exceeds 30 g m ⁻² d ⁻¹ outdoors with CO ₂ supplied. In climate-simulating photobioreactors, productivity declined from 39 g m ⁻² d ⁻¹ under 2.25% CO ₂ to near zero in air and resumed within hours of CO ₂ restoration with photosystem II photochemistry largely retained. We profiled the transcriptome, proteome, phosphoproteome, ubiquitinome, acetylome, metabolome and lipidome, including the first site-resolved green-algal ubiquitinome. In air, the Rubisco large subunit tripled to 7.4% of protein while its two small-subunit isoforms, 70% identical, exchanged near-reciprocally, a novel CO ₂ -dependent switch that we propose retunes Rubisco to low CO ₂ . Photorespiratory enzymes increased in abundance whereas proteins involved in external nitrogen assimilation decreased, consistent with increased reliance on internal nitrogen recycling as carbon fixation became limiting. Transcript and protein responses were nearly uncoupled, with cytosolic ribosomal proteins declining despite increased transcripts and plastid ribosomal proteins spared. Phosphorylation was extensively remodeled, as were the lysine modifications ubiquitination, acetylation and CO ₂ -dependent carbamylation, where a CO ₂ -competition assay identified 86 bicarbonate-protected lysines, including RbcL K252. CO ₂ limitation therefore imposes a reversible growth arrest that reduces cytosolic translational investment while preserving photosynthetic capacity for rapid recovery.